Electric Power / Controls. DC Machines. Student Manual

Transcription

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2 Electric Power / Controls DC Machines Student Manual AB

3 ELECTRIC POWER / CONTROLS DC MACHINES by Theodore Wildi and the Staff of Lab-Volt Ltd. Copyright 2002 Lab-Volt Ltd. All rights reserved. No part of this publication may be reproduced, in any form or by any means, without the prior written permission of Lab-Volt Ltd. Legal Deposit First Trimester 2002 ISBN ISBN (1 st Edition, 1982) SECOND EDITION, FEBRUARY 2002 Printed in Canada April 2010

4 Foreword Electricity has been used since more than a century and the number of applications requiring electricity is increasing constantly. As a result, the electrical power demand has been rising since the early use of electricity. Many reasons explain why electricity is so popular. One reason is the great versatility of electricity. We use it every day for cooling, heating, lighting, driving (through electric motors) etc. Furthermore, many apparatuses that are part of our everyday life, such as telephones, televisions, personal computers, etc., require electrical power. Another reason that explains the constantly rising demand for electricity lies in the fact that it is a highly reliable source of energy. The Lab-Volt 0.2-kW Electromechanical Training System and related courseware offer a comprehensive program in the field of electrical power technology. It is an ideal tool for preparing the students to the realities of the contemporary job market. The program was developed by educators to satisfy educational requirements that include industrial applications of electric power technology. The design objective was to develop a low-power educational system with equipment that operates like industrial equipment. The student manuals explain electrical principles as well as specific industrial applications of the phenomenon discussed in each exercise. Hands-on exercises carried out with the training system reinforce the student's knowledge of the theory being studied. The method of presentation is unique in its modular concept and places emphasis upon electrical laboratory procedures performed by the individual student. III

5 Symbols and Abbreviations The user of this Student Manual may find some unfamiliar symbols and abbreviations. In general, Lab-Volt Educational System has adopted the "Letter Symbols for Units" IEEE Standard Number 260/USA Standard Number Y10.19, dated October 18, The abbreviations have been adopted by Lab-Volt following a thorough study of available abbreviations and guidelines published by the Institute of Electrical and Electronic Engineers (IEEE) and are consistent in nearly all respects with the recommendations of the International Organization for Standardization (ISO) and with the current work of the International Electrotechnical Commission (IEC). The symbols and abbreviations used in this manual are listed below. Each symbol derived from a proper name has an initial capital letter. Singular and plural forms are identical. alternating current ac American wire gauge AW ampere A ampere-turn At applied voltage V A average avg British thermal unit BTU capacitance C capacitive reactance X C clockwise cw cosine cos coulomb C counterclockwise ccw counter electromotive force CEMF current I cycles per second Hz decibel db degree Celsius EC degree Fahrenheit EF degree (plane angle)...e direct current dc divide,/ effective value (ac) rms electromotive force EMF farad F foot ft, N frequency f greater than > ground gnd henry H hertz Hz horsepower hp hour h impedance Z inch in, O inductance L inductance - capacitance LC kilohertz khz kilohm kω kilovar kvar kilovolt kv kilovolt-ampere kva kilowatt kw kilowatthour kwh less than < load (resistance) R L logarithm log magnetomotive force MMF maximum max. megahertz MHz megavolt MV megawatt MW V

6 Symbols and Abbreviations (cont'd) megohm MΩ microampere µa microfarad µf microhenry µh microsecond µs microwatt µw mile mi milliampere ma millifarad mf millihenry mh milliohm mω millisecond ms millivolt mv milliwatt mw minimum min. minute (time) min minus! negative neg,! ohm Ω peak pk phase φ picofarad pf positive pos, + potential E pound-force lbf pound-force inch lbf@in pound-force foot lbf@ft power (active) P power (apparent) power (instantaneous) power (reactive) power factor reactance reactance (capacitance) reactance (inductance) reactive power resistance resistance-capacitance resistance-inductance revolutions per minute revolutions per second root-mean square second (time) sine source (current) source (voltage) tangent temperature time total current total power volt voltage (applied) volt-ampere watt watthour P S Q PF X X C X L var R RC RL r/min r/s rms s sin I S E S tan T t I T P T V V A VA W Wh VI

7 Table of Contents Introduction... IX Experiment 0 Safety and the Power Supply To learn the simple rules of safety. To learn how to use the AC/DC power supply. Experiment 1 Prime Mover and Torque Measurement To learn how to connect a three-phase synchronous motor. To learn how to connect the electrodynamometer. To learn how to use the Prony brake. Experiment 2 The Direct Current Motor Part I To examine the construction of a DC motor/generator. To measure the resistance of its windings. To study the nominal current capabilities of the various winding. Experiment 3 The Direct Current Motor Part II To locate the neutral brush position. To learn the basic motor wiring connections. To observe the operating characteristics of series and shunt connected motors. Experiment 4 The DC Shunt Motor To study the torque vs speed characteristics of a shunt wound DC motor. To calculate the efficiency of the shunt wound DC motor. Experiment 5 The DC Series Motor To study the torque vs speed characteristics of a series wound DC motor. To calculate the efficiency of the series wound DC motor. Experiment 6 The DC Compound Motor To study the toque vs speed characteristics of a compound wound DC motor. To calculate the efficiency of the compound wound DC motor. Experiment 7 The DC Separately Excited Shunt enerator To study the properties of the separately excited DC shund generator under no-load and full-load conditions. To obtain the saturation curve of the generator. To obtain the armature voltage vs armature current load curve of the generator. Experiment 8 The DC Self Excited Shunt enerator To study the properties of the self-excited DC shunt generator under no-load and full-load conditions. To learn how to connect the self-excited generator. To obtain the armature voltage vs armature current load curve of the generator. VII

8 Table of Contents (cont'd) Experiment 9 The DC Compound enerator To study the properties of compound DC generators under no-load and full-load conditions. To learn how to connect both the compound and the differential compound generators. To obtain the armature voltage vs armature current load curves for both generators. Experiment 10 DC Motor Starter To examine the construction of a DC motor starter. To observe the operation of a 3-point DC starter. To observe the operation of a 4-point DC starter. Experiment 11 Thyristor Speed Controller Completing this exercise will give you an introduction to thyristor speed controllers. You will learn how to control the speed of a DC motor by varying the armature voltage using a thyristor speed controller. Experiment 12 Thyristor Speed Controller with Regulation In this exercise, you will be introduced to thyristor speed controllers operating in the closed-loop mode of control. You will learn how the closed-loop mode of control regulates the motor speed by detecting the armature voltage and current. You will learn how to control the acceleration of the DC Motor/enerator. You will also learn how to limit the current and the torque of the DC Motor/enerator. Appendices A Equipment Utilization Chart B Impedance Table for the Load Modules C Performing the Electrical Power Technology Courseware Using the Lab-Volt Data Acquisition and Management System D SCR Speed Control Part I E SCR Speed Control Part II We Value Your Opinion! VIII

9 Introduction The subject matter in this manual, DC Machines, covers the study of direct current machines. The motor and generator characteristics, whether they are shunt, series or compound connected, are explained in details. The starting and speed control of the DC motors are some subjects also covered. The exercises in this manual provide a systematic and realistic means of learning the subject matter. Each exercise contains: an OBJECTIVE that clearly defines the objectives of the exercise; a DISCUSSION of the theory involved; a detailed step-by-step laboratory PROCEDURE in which the student observes and measures important phenomena. Schematic diagrams facilitate connecting the components; some REVIEW QUESTIONS to verify that the material has been well assimilated. The exercises can be carried out using either conventional instruments (AC/DC voltmeters and ammeters, power meters, oscilloscope, etc.), or the Lab-Volt Data Acquisition and Management (LVDAM) System. Appendix C of this manual provides useful guidelines to perform the exercises using the LVDAM system. As a reference manual, we suggest to consult Electrical Machines, Drives, and Power Systems written by Theodore Wildi and published by Prentice Hall. Note that the highlighted text in the manual only applies to the Imperial system of units. IX

10 Experiment 0 Safety and the Power Supply OBJECTIVE To learn the necessary safety rules when working with electricity. To learn how to use adequately the AC/DC power supply. DISCUSSION TO ALL STUDENTS AND TEACHERS Be sure to know the location of the first-aid kit in your class or lab at all times. Ensure that all cuts and burns receive immediate care, no matter how minor they may seem to be. Notify your instructor about every accident. He will know what to do. If students follow the instructions adequately, no serious accident will occur while using the Electro Mechanical Systems. There are many fatal shocks every year caused by ordinary electrical power found at home. A thorough safety program is a necessity for anyone working with electricity. Electricity can be dangerous and even fatal to those who do not understand and practice the simple rules of safety associated with it. There are many fatal electrical accidents caused by well trained technicians who, either through over-confidence or carelessness, disregard the basic rules of personal safety. The first rule of personal safety is always: THINK FIRST This rule applies to all industrial work as well as to electrical workers. Develop good work habits. Learn to use your tools correctly and safely. Always study experiments beforehand and carefully think through all of the required procedures and methods. Make sure you know how to use all of your tools, instruments and machines before proceeding with an experiment. Never let yourself be distracted from your work and never distract others around you. Do not joke around near moving machinery and electricity. There are generally three kinds of accidents which happen frequently to electrical students and technicians: electrical shocks, burns and mechanical injuries. Knowing how to avoid them by observing simple rules will make you a safe person to work with. Observing these precautions could save you from painful experiences (it could even save your life) and prevent expensive damage to the equipment. 0-1

11 Safety and the Power Supply Electric shock Are electric shocks fatal? The physiological effects of electric currents can generally be predicted by the chart shown in Figure 0-1. Figure 0-1. As you can see, it is the current that determines the intensity of an electrical shock. Currents above 100 ma are considered fatal. A person who has received a shock of over 2000 ma is in grave danger and needs immediate medical attention. Currents below 100 ma are still serious and painful. As a safety rule: Do not put yourself in a position where you could receive any kind of shock, no matter how low the current is. 0-2

12 Safety and the Power Supply What about VOLTAE? Current depends upon voltage and resistance. We will now measure the resistance of your body. Using your ohmmeter, measure your body resistance between these points: From right to left hand Ω From hand to foot Ω Now wet your fingers and repeat the measurements: From right to left hand Ω From hand to foot Ω As you can see, the actual resistance of your body varies greatly depending on the points of contact, the condition of your skin, and the contact area. Notice how your resistance varies as you squeeze the probes more or less tightly. Skin resistance may vary between 250 Ω for wet skin and large contact areas, to Ω for dry skin. Using the measured resistance of your body and considering 100 ma as a fatal current, calculate what voltages could be fatal to you: Use the formula: Volts =.1 x ohms. Contact between two hands (dry): V Contact between one hand and one foot (dry): V Contact between two hands (wet): V Contact between one hand and one foot (wet): V DO NOT ATTEMPT TO PROVE THIS! Here are nine rules to avoid electric shocks: 1. Be sure to check the condition of your equipment and the possible dangers before working on a piece of equipment. The same way someone can be accidentally killed by a supposedly unloaded gun, an electrical technician can receive shocks from a circuit that is supposedly turned off. 2. Never rely on safety devices such as fuses, relays and interlock systems to protect you. They may not be working correctly and may fail to protect you when most needed. 3. Never remove the grounding prong of a three wire input plug. Without ground, electrical equipments become much more dangerous shock hazards. 4. Do not work on a cluttered desk. A disorganized working area full of connecting leads, components and tools leads to careless thinking, short circuits, shocks and accidents. Develop systemized and organized work habits. 0-3

13 Safety and the Power Supply 5. Do not work on a wet floor. This would greatly reduce your contact resistance to the ground. Work on a rubber mat or an insulated floor. 6. Do not work alone. It s an additional safety measure to have someone around to shut off the power, give artificial respiration or make an emergency call in case of need. 7. Work with one hand behind you or in your pocket. A current between one hand to the other would pass through your heart and is thus more lethal than a current from hand to foot. 8. Never talk to anyone while working. Do not let yourself be distracted by your surroundings. Similarly, do not talk to someone working on dangerous equipment. 9. Always move slowly when working around electrical circuits. Fast and careless movements lead to accidental shocks and short circuits. Burns Accidents resulting in burns, although usually not fatal, can be painful and serious. They are generally caused by the production of heat due to electrical energy dissipation. Here are four rules to avoid electrical burns: 1. Resistors may get very hot, especially those that carry high currents. Watch out especially for five and ten watt resistors. They can severely burn your skin. Do not touch them until they cool off. 2. Be wary of capacitors: they may still retain a charge. Not only would you receive a dangerous and sometimes fatal shock, you could also be burned from the electrical discharge. If the rated voltage of electrolytic capacitors is exceeded or their polarities reversed, they may get very hot and actually burst. 3. Watch out for hot soldering irons or guns. Do not leave one where your arm might accidentally touch it. Never store a soldering iron away while it is still hot. Unknowing students could be burned by picking it up. 4. Hot solder can be painful when put in contact with naked skin. Wait for soldered joints to cool off. When desoldering joints, do not shake hot solder off. You or a neighbor could receive hot solder in the eye or on his body. Mechanical injuries This third class of safety rules concerns all students who work with tools and machinery. Here are five rules to avoid mechanical injury: 1. Metal corners and sharp edges on chassis and panels can cut and scratch. File them until they are smooth. 2. Improper tool selection for a job can result in damage to the equipment and personal injuries. 3. Use proper eye protection when grinding, chipping or working with hot metals which might splatter. 0-4

14 Safety and the Power Supply 4. Protect your hands and clothes when working with battery acids, etchants, and finishing fluids. These liquids can cause severe burns. 5. If you are unsure about something, ask your instructor. The Power Supply The Power Supply provides all of the necessary AC/DC power, both fixed and variable, single phase and three-phase, to perform all of the experiments presented in this manual. The module must be connected to a three-phase, 240 /415 V, four wire (with fifth wire ground) system. Power is brought in through a five prong, twist-lock connector located at the rear of the module. An input power cable with mating connector is provided for this purpose. The power supply provides the following outputs: 1. Fixed 24 V ac is made available for the use of accessory equipments such as meters. 2. Fixed 120 /208 V, 3φ power is brought out to four terminals, labeled 1, 2, 3 and N. Fixed 208 V 3φ may be obtained from terminals 1, 2 and 3. Fixed 208 V ac may be obtained between terminals 1 and 2, 2 and 3 or 1 and 3. Fixed 120 V ac may be obtained between any one of the 1, 2 or 3 terminals and the N terminal. The current rating of this supply is 15 A per phase. 3. Variable 120 /208 V, 3φ power is brought out to four terminals, labeled 4, 5, 6 and N. Variable 3φ V may be obtained from terminals 4, 5 and 6. Variable V ac may be obtained between terminals 4 ans 5, 5 and 6 or 4 and 6.Variable V ac may be obtained between any one of the 4, 5 or 6 terminals and the N terminal. The current rating of this supply is 5 A per phase. 4. Fixed 120 V dc is brought out to terminals labeled 8 and N. The current rating of this supply is 2 A. 5. Variable V dc is brought out to terminals labeled 7 and N. The current rating of this supply is 8 A. The full current rating of the various outputs cannot be used simultaneously. If more than one output is used at a time, reduced current must be drawn. The neutral N terminals are all connected together and joined to the neutral wire of the AC power line. All power is removed from the outputs when the on-off breaker is in the off position (breaker handle down). CAUTION! Power is still available behind the module face with the breaker off! Never remove the power supply from the console without first removing the input power cable from the rear of the module. The variable AC and DC outputs are controlled by the single control knob on the front of the module. The built-in voltmeter will indicate all the variable AC and the variable and fixed DC output voltages according to the position of the voltmeter 0-5

15 Safety and the Power Supply selector switch. The power supply is fully protected against overload or short circuit. Besides the main 15 A 3φ on-off circuit breaker on the front panel, all of the outputs have their own circuit breakers. They can be reset by a common button located on the front panel. The rated current output may be exceeded considerably for short periods of time without harming the supply or tripping the breakers. This feature is particularly useful in the study of DC motors under overload or starting conditions where high currents may be drawn. All of the power sources can be used simultaneously, providing that the total current drawn does not exceed the 15 A per phase input breaker rating. Your power supply, if handled properly, will provide years of reliable operation and will present no danger to you. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are used in this Experiment! Do not make or modify any banana jack connections with the power on unless otherwise specified! 1. Examine the construction of the Power Supply. On the front panel of the module, identify the following elements: a. The three-pole circuit breaker on-off switch. b. The three lamps indicating the operation of each phase. c. The AC/DC voltmeter. d. The AC/DC voltmeter selector switch. e. The variable output control knob. f. The fixed 24 V ac receptacle. g. The fixed 120 /208 V output terminals (labeled 1, 2, 3 and N). h. The variable /208 V output terminals (labeled 4, 5, 6 and N). i. The fixed DC output terminals (labeled 8 and N). j. The variable DC output terminals (labeled 7 and N). k. The common reset button. 0-6

16 Safety and the Power Supply 2. State the AC or DC voltage and the rated current available from each of the following terminals: a. Terminals 1 and N = V A b. Terminals 2 and N = V A c. Terminals 3 and N = V A d. Terminals 4 and N = V A e. Terminals 5 and N = V A f. Terminals 6 and N = V A g. Terminals 7 and N = V A h. Terminals 8 and N = V A i. Terminals 1, 2 & 3 = V A j. Terminals 4, 5 & 6 = V A k. The low power connector = V A 3. Examine the interior construction of the module. Identify the following items: a. The 3φ variable autotransformer. b. The filter capacitors. c. The thermal-magnetic circuit breakers. d. The solid state rectifier diodes. e. The five prong twist lock connector. 4. Insert the Power Supply into the console. Make sure that the on-off switch is in the off position and that the output control knob is turned fully counterclockwise for minimum output. Insert the power cable through the clearance hole in the rear of the console and into the twist-lock module connector. Connect the other end of the power cable into a source of 3φ 120 /208 V. 5. a. Set the voltmeter selector switch to its 7-N position and turn the power supply on by placing the on-off breaker switch in its up position. b. Turn the control knob of the 3φ autotransformer and note that the DC voltage increases. Measure and record the minimum and maximum DC output voltage as indicated by the built-in voltmeter. V dc minimum = V dc maximum = 0-7

17 Safety and the Power Supply c. Return the voltage to zero by turning the control knob to its full ccw position. 6. a. Place the voltmeter selector switch into its 4-N position. b. Turn the control knob and note that the AC voltage increases. Measure and record the minimum and maximum AC output voltage as indicated by the built- voltmeter. V ac minimum = V ac maximum = c. Return the voltage to zero and turn off the power supply by placing the on-off breaker switch in its down position. 7. What other AC voltages are affected by turning the control knob? Terminals and = V ac Terminals and = V ac Terminals and and = V ac 8. For each of the following conditions: a. Connect the 250 V ac meter across the specified terminals. b. Turn on the power supply. c. Measure and record the voltage. d. Turn off the power supply. Terminals 1 and 2 = Terminals 2 and 3 = Terminals 3 and 1 = Terminals 1 and N = Terminals 2 and N = Terminals 3 and N = V ac V ac V ac V ac V ac V ac e. Are any of these voltages affected by turning the control knob? Yes No 9. a. Set the voltmeter selector switch to its 8-N position. b. Turn on the power supply. c. Measure and record the voltage Terminals 8 and N = V dc 0-8

18 Safety and the Power Supply d. Is this voltage affected by turning the control knob? Yes No e. Turn off the power supply. 10. For each of the following positions of the voltmeter selector switch: a. Turn on the power supply and rotate the control knob to its full cw position. b. Measure and record the voltage. c. Return the voltage to zero and turn off the power supply. Terminals 4 and 5 = Terminals 5 and 6 = Terminals 6 and 4 = Terminals 4 and N = Terminals 5 and N = Terminals 6 and N = V ac V ac V ac V ac V ac V ac 0-9

19 Experiment 1 Prime Mover and Torque Measurement OBJECTIVE To learn how to connect a three-phase synchronous motor. To learn how to connect the electrodynamometer. To learn how to use the Prony brake. DISCUSSION The synchronous motor has the special property of maintaining a constant running speed under all conditions of load up to full load. This constant running speed can be maintained even under variable line voltage conditions. It is, therefore, a useful motor in applications where the running speed must be accurately known and unvarying. In this Experiment, you will learn how to use this motor as a stable driver for generators. The complete study of its characteristics will be taken up in later Experiments. It should be noted that, if a synchronous motor is severely overloaded, this operation (speed) will suddenly lose its synchronous properties and the motor will come to a halt. The synchronous speed of your motor is 1800 r/min. The load imposed on a motor can be measured by two different means; these two torque measuring devices are the Prony brake and the electrodynamometer. The Prony brake is an entirely passive device (no electrical power is required) while the electrodynamometer requires external power. The Prony Brake is a friction brake which is used to act as a load for any type of motor and to measure the torque developed by these motors. This brake is entirely mechanical, and consists of a spring balance mounted in a standard full size module. It is built to accurately measure the torque developed by any rotating machine placed on its left-hand side. A self-cooling friction wheel is slipped over the shaft of the machine under test, and attached to its output pulley by means of two screws. The friction belt of the Prony Brake is then removed from inside the module and slipped over the friction wheel. The braking torque applied to the machine can be varied by turning the knurled wheel (LOAD) in the upper left corner of the module. A second knurled wheel (TORQUE PRESET) in the upper center allows the spring balance to be brought back into equilibrium by aligning the red line in the right-hand window (ZERO) with the black line; the torque can then be read directly on the NAm [0-30 lbfain], 360E circular scale, in steps of 0.02 NAm [0.2 lbfain]. 1-1

20 Prime Mover and Torque Measurement The accuracy is better than 2% and the torque is continuously adjustable over the full range from no load to locked rotor. When one wants to apply a known torque to the machine, the TORQUE PRESET wheel must first be set so that the calibrated circular scale reads exactly the desired torque value and the LOAD wheel must then be turned in such a way that the red line in the ZERO window is aligned with the black line. The electrodynamometer is a device used to accurately measure the torque developed by motors of all kinds. It is actually an electrical brake in which the braking force can be varied electrically rather than by mechanical friction. The electrodynamometer is a more stable, easier to adjust, device than the mechanical friction brakes. The electrodynamometer consists of a stator and a squirrel-cage rotor. The stator, unlike other electromechanical devices, is free to turn, but its motion is restricted by a helical spring. In normal operation, DC current is applied to the stator winding. This sets up a magnetic field which passes through both the stator and the rotor. As the rotor turns (being belt-coupled to the driving motor), a voltage is induced in the rotor bars, and the resulting eddy currents react with the magnetic field causing the stator to turn in the same direction as the rotor. The stator rotation is limited by the helical spring and the amount that it turns is marked off on a scale attached to the external stator housing. The electrodynamometer is calibrated from!0.3 to 3 NAm [!3 to 27 pound-forceinches (lbfain)] which is more than adequate for the testing of 0.2 kw [1/4 hp] motors even when they are tested at overload conditions. The power output of a motor depends upon its speed and the torque it develops. This relationship is given by the following equation: (1) where: P out = Mechanical Output Power in watts (W) N = Speed in revolutions per minute (r/min) T = Torque in NewtonAmeter (NAm) (1) where: P out = Mechanical Output Power in horse power (hp) N = Speed in revolutions per minute (r/min) T = Torque in pound-force-inches (lbfain) EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. 1-2

21 Prime Mover and Torque Measurement PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! Synchronous Motor 1. Examine the front face of the Three-Phase Synchronous Motor/enerator. (The entire Three-Phase Synchronous Motor/enerator will be fully described in a later Experiment.) a. Note the three separate windings connected to terminals (1 and 4, 2 and 5, 3 and 6. These windings are identical and are actually located in the stator or stationary part of the motor. The three windings carry AC current and will connect to a three-phase power source. b. Identify these three windings and their corresponding connection terminals. c. The winding on the rotor or rotating part of the motor, is connected to connection terminals 7 and 8, through a 150 Ω rheostat and a toggle switch. This winding will carry DC current, whose value can be controlled by means of the rheostat. d. Identify the winding, the rheostat, the toggle switch and their corresponding connection terminals. 2. Using your Power Supply, and AC Ammeter, connect the circuit shown in Figure 1-1. Terminals 1, 2 and 3 on the power supply provide fixed three-phase power which is required for the three stator windings. (Three-phase power will be covered in later Experiments). Terminals 8 and N on the power supply provide fixed DC power which is required for the rotor winding. 1-3

22 Prime Mover and Torque Measurement Figure a. Adjust the rheostat for maximum resistance (control knob turned fully counterclockwise, ccw). b. The motor is supplied with DC current only when switch S is closed. Insure that switch S is open. c. Have your instructor check your completed circuit. 4. a. Turn on the Power Supply. The motor should start running immediately. b. Note the indications on the three current meters. c. Close switch S. 5. a. Vary the rheostat control for minimum stator current as indicated by the three current meters (The control knob should be close to its full ccw position). b. Measure and record the three stator winding currents (at minimum stator current). I 1 = A I 2 = A I 3 = A c. Increase the rotor DC excitation by adjusting the rheostat for minimum resistance (control knob turned fully clockwise cw). 1-4

23 Prime Mover and Torque Measurement d. Measure the three stator winding currents (at maximum rotor DC excitation). I 1 = A I 2 = A I 3 = A 6. Reduce the DC excitation until the stator currents are at their minimum values. Note and record the position of the rheostat control knob. (In all future procedures using the synchronous motor, the control knob should be set to this position for normal excitation.) Control knob scale position = 7. a. Using your hand tachometer measure the running speed of the motor as you vary the DC excitation. Speed-minimum excitation = Speed-midpoint excitation = Speed-maximum excitation = r/min r/min r/min Does the speed remain constant? Yes No b. Note whether the direction of rotation is clockwise or counterclockwise. Rotation = 8. a. Turn off the power supply and open switch S. Interchange any two of the AC connection leads at the power supply terminals. b. Turn on the power supply and note the direction of rotation: Rotation = c. Turn off the power supply. d. Disconnect the synchronous motor. Electrodynamometer Note: If your list of equipment does not include an electrodynamometer, proceed to procedure a. Examine the construction of the Electrodynamometer. b. Note the cradle construction for the electrodynamometer housing. (This is also called trunnion mounting.) 1-5

24 Prime Mover and Torque Measurement c. Note the helical spring at the rear of the machine. This spring has been accurately calibrated against the graduations marked on the front of the housing. d. Note the mechanical stops that limit the rotational travel of the stator housing. e. Identify the stator winding that is attached to the inside of the housing. (This winding carries DC current.) f. Identify the two wire leads that carry DC current to the stator winding. (They enter the housing through the center of the helical spring.) g. Identify the bridge rectifier located at the rear of the module. (This bridge furnishes DC power for the stator magnetic field.) h. Identify the variable autotransformer mounted on the front face of the module. The braking effect of the electrodynamometer is controlled by the strength of the stator magnetic field, which is proportional to the DC output of the bridge rectifier, which is varied by the variable autotransformer. i. Identify the two AC connections terminals mounted on the module face. 10. a. Connect the electrodynamometer tot the fixed AC output of the power source by connecting the two input terminals of the electrodynamometer to terminal 1 and N of the power source. DO NOT APPLY POWER AT THIS TIME! b. Set the electrodynamometer variable transformer control knob to its mid-position. c. Lower the front face of the module so that you may turn the pulley by hand. 11. a. Turn on the power supply. b. Keeping one hand in your pocket, for safety reasons, carefully reach in and try to turn the pulley. Caution is advised because there are several live terminals exposed when the front panel is dropped. Do you feel a drag when you turn the pulley? Yes No Does the stator housing tend to turn in the same direction as the pulley? Yes No 1-6

25 Prime Mover and Torque Measurement 12. a. Remove your hand from the inside of the module and advance the control knob, thereby increasing the magnetic stator field. b. Carefully reach in and turn the pulley. Did the drag increase? Yes No c. Repeat (a) but this time reduce the stator magnetic field. d. Carefully reach in and turn the pulley. Did the drag increase? Yes No e. Turn off the power supply. 13. a. Couple the synchronous motor and the electrodynamometer with the timing belt. b. Connect the motor as shown in Figure 1-1. c. Connect the electrodynamometer to terminals 1 and N of the power supply. (There should now be two connection leads at terminal 1 of the power supply one to the synchronous motor and one to the electrodynamometer). d. Set the dynamometer control knob at its full ccw position (to provide a minimum starting load for the motor). e. Set the synchronous motor rheostat control knob to its normal minimum stator current position. 14. Apply power and note if the motor revolves in a cw direction. If not, reverse its rotation (the dynamometer torque can only be measured for cw rotation). Close switch S. 15. Increase the load on the motor (the dynamometer braking action) by varying the control knob on the dynamometer until the scale marked on the stator housing indicates 1 NAm [9 lbfain]. (The numeral 1 [9]should be directly beneath the red vertical line on the window beneath the pulley). 16. a. Measure and record the three AC stator currents with a 1 NAm [9 lbfain] load on the motor. I 1 = A I 2 = A I 3 = A b. Measure and record the motor speed with a 1 NAm [9 lbfain] load. Speed with load = r/min 1-7

26 Prime Mover and Torque Measurement 17. a. Vary the DC excitation to the motor by turning the rheostat control knob, while under the 1 NAm [9 lbfain] load. What effect does varying the DC excitation have on the three stator currents? b. Measure the motor running speed at minimum and maximum DC excitation while under the 1 NAm [9 lbfain] load. Speed minimum excitation = Speed maximum excitation = r/min r/min c. Does the motor fall out of synchronism when the DC excitation is too low? Yes No 18. a. Increase the DC excitation to its maximum value. radually increase the load on the motor, by advancing the variable autotransformer control knob on the electrodynamometer, until the motor falls out of synchronization. Immediately turn off the power supply. Open switch S. b. Record the value of this breakdown torque. Breakdown torque = NAm [lbfain] Prony Brake Note: If your list of equipment does not include a Pony Brake, proceed to REVIEW QUESTIONS. CAUTION! The friction wheel may become very hot during this experiment. 19. a. Examine the construction of the Prony Brake. b. Remove the friction wheel from inside the module and note the holes in the web of the pulley; they are for cooling purposes. Slip the friction wheel over the synchronous motor shaft and secure it firmly to the motor pulley by tightening the two screws in the grooves of the motor pulley. c. Note the floating plate inside the module. In operation, this plate is pulled in one direction by the friction belt and in the other direction by a spring which is connected to the fixed frame by means of a rack gear. A pinion gear, driven by the rack, turns the front plastic disc which has 1-8

27 Prime Mover and Torque Measurement a red line to read the torque. The TORQUE PRESET wheel effectively increases the spring tension and consequently, the force which tends to turn the floating plate. d. The LOAD wheel effectively increases the tension on the friction belt, and tends to turn the floating plate in the opposite direction. e. Note the friction belt attached at one end to the floating plate and at the other end to the LOAD wheel screw. f. The complete system constitutes a spring balance. 20. a. Turn the LOAD wheel downwards to release the tension on the friction belt, and slip the belt over the friction pulley mounted on the synchronous motor. Leave the belt loose. b. Connect the synchronous motor as shown in Figure 1-1. c. Set the synchronous motor rheostat control knob to its normal minimum stator current position. Open switch S. d. Turn on the power supply and note if the motor revolves in a cw direction. If not, reverse its rotation (the Prony brake can only measure torque for cw rotation). Close switch S. e. Measure the three stator winding currents. I 1 = A I 2 = A I 3 = A f. Vary the TORQUE PRESET wheel until the circular scale indicates 1 NAm [9 lbfain]. This does not impose any torque on the motor and the currents should not change. You have just preset the balance to the indicated torque. g. Turn slowly the LOAD wheel upwards to tighten the belt over the friction pulley. Note the gradual increase in the motor currents. Keep turning the LOAD wheel until the hairlines in the right-hand window are aligned. The balance is now in equilibrium and is imposing a torque of 1 NAm [9 lbfain] on the synchronous motor. h. Measure and record the three AC stator currents with a 1 NAm [9 lbfain] load on the motor. I 1 = A I 2 = A I 3 = A i. Measure and record the motor speed with a 1 NAm [9 lbfain] load. Speed with load = r/min 21. a. Increase the DC excitation to its maximum value. radually increase the load on the motor by turning the LOAD wheel. Then bring the balance into equilibrium by turning the TORQUE PRESET wheel until the hairlines are aligned. Repeat successively these last two steps until the 1-9

28 Prime Mover and Torque Measurement motor falls out of synchronism. Immediately turn off the power supply. Open switch S. b. Record the value of the breakdown torque as indicated by the calibrated dial. Breakdown torque = NAm [lbfain] REVIEW QUESTIONS 1. How can you reverse the direction of rotation of a synchronous motor? 2. What effect does varying the DC incitation have on the stator currents of a synchronous motor? 3. How can you increase the power output of a synchronous motor? 4. Calculate the developed motor mechanical output power in procedure 16 b) or 20 i). Mechanical Output Power [hp]= 5. Calculate the developed motor mechanical output power in procedure 18 or 21 (just prior to breakdown ). Mechanical Output Power [hp] = 1-10

29 Prime Mover and Torque Measurement 6. Which torque measuring device is easier to use, the electrodynamometer or the Prony brake? 7. Where is the power (heat) dissipated in the electrodynamometer? 8. Where is the power (heat) dissipated in the Prony brake? 1-11

30 Experiment 2 The Direct Current Motor Part I OBJECTIVE To examine the construction of a DC motor / generator. To measure the resistance of its windings. To study the nominal current capabilities of the various windings. DISCUSSION Direct current motors are unsurpassed for adjustable-speed applications, and for applications with severe torque requirements. Uncounted millions of small power [fractional horsepower] DC motors are used by the transportation industries in automobiles, trains and aircraft where they drive fans and blowers for air conditioners, heaters and defrosters; they operate windshield wipers and raise and lower seats and windows. One of their most useful functions is for the starting of gasoline and Diesel engines in autos, trucks, buses, tractors and boats. The DC motor contains a stator and a rotor, the latter being more commonly called an armature. The stator contains one or more windings per pole, all of which are designed to carry direct current, thereby setting up a magnetic field. The armature and its winding are located in the path of this magnetic field, and when the winding also carries a current, a torque is developed, causing the motor to turn. A commutator associated with the armature winding is actually a mechanical device, to assure that the armature current under any given stator pole will always circulate in the same direction irrespective of position. If a commutator were not used, the motor could not make more than a fraction of a turn, before coming to a halt. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. 2-1

31 The Direct Current Motor Part I PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Examine the construction of the DC Motor/enerator paying particular attention to the motor, rheostat, connection terminals and wiring. Note that the motor housing has been designed to allow you to view the internal construction. Most commercial motors do not have this open construction. 2. Viewing the motor from the rear of the module: a. Identify the armature winding. b. Identify the stator poles. c. How many stator poles are there? d. The shunt field winding on each stator pole is composed of many turns of small diameter wire. Identify the shunt field winding. e. The series field winding, wound inside the shunt field winding on each stator pose, is composed of fewer turns of larger diameter wire. Identify the series field winding. 3. Viewing the motor from the front of the module: a. Identify the commutator. b. Approximately how many commutator bars (segments) are there? c. How many brushes are there? d. The neutral position of the brushes is indicated by a red line marked on the motor housing. Identify it. e. The brushes can be positioned on the commutator by moving the brush adjustment lever to the right or the left of the red indicator line. Move the lever both ways and then return it to the neutral position. 2-2

32 The Direct Current Motor Part I 4. Viewing the front face of the module: a. The shunt field winding (many turns of fine wire) is connected to terminals and. b. The series field winding (fewer turns of heavier wire) is connected to terminals and. c. The current rating for each winding is marked on the face of the module. Can you answer (a) and (b) having only this information? Explain. Yes No d. The brushes (commutator segments and armature winding) are connected to terminals and. 5. The rheostat, mounted on the module face, is designed to control (and safely carry) the shunt field current. a. It is connected to terminals and. b. What is its rated resistance value? R (field rheostat) Ω 6. You will now measure the resistance of each of the motor windings using the voltmeter-ammeter method. With this information you will calculate the power losses for each of the windings. Using your Power Supply, DC Voltmeter/Ammeter and DC Motor/enerator, connect the circuit shown in Figure 2-1. Figure Turn on the power supply. a. Slowly increase the DC voltage until the shunt field winding is carrying 0.3 A of current as indicated by the A dc meter (this is the nominal current value for the shunt field winding). 2-3

33 The Direct Current Motor Part I b. Measure and record the voltage across the shunt field winding. E (shunt field) = V dc c. Return the voltage to zero and turn off the power supply. d. Calculate the resistance of the shunt field winding. R (shunt field) = E/I = / = Ω e. Calculate the I 2 R (power) losses of the shunt field winding. P (shunt field) = I 2 R = X = W 8. Connect the circuit shown in Figure 2-2. Figure 2-2. a. This is the same circuit as shown in Figure 2-1 except that the series field winding has replaced the shunt field winding and that the 5 A dc meter has replaced the 500 ma dc meter. b. Turn on the power supply. Slowly increase de DC voltage until the series field winding is carrying 3 A of current as indicated by the 5 A dc meter, (this is the nominal current value for the series field winding). Warning! This only requires a few volts so advance the voltage control slowly. c. Measure and record the voltage across the series field winding. E (series field) = V dc d. Return the voltage to zero and turn off the power supply. e. Calculate the resistance of the series field winding. R (series field) = E/I = / = Ω f. Calculate the I 2 R losses of the series field winding. P (series field) = I 2 R = X = W 2-4

34 The Direct Current Motor Part I 9. Connect the circuit shown in Figure 2-3. Figure 2-3. a. This is the same circuit shown in Figure 2-2 except that the armature winding (plus the brushes) has replaced the series field winding. b. Turn on the power supply. Slowly increase the DC voltage until the armature winding is carrying 3 A of current as indicated by the 5 A dc meter (this is the nominal current value for the armature winding). c. Measure and record the voltage across the armature winding (plus brushes). E (armature) = V dc d. Return the voltage to zero and turn off the power supply. e. Calculate the resistance of the armature winding (plus brushes). R (armature) = E/I = / = Ω f. Calculate the I 2 R losses of the armature (plus brushes). P (armature) = I 2 R = x = W 10. Rotate the armature winding approximately 90E to the left. a. The brushes are now making contact with different commutator segments. b. Repeat procedure 9. c. E = V dc, R = Ω, P = W 2-5

35 The Direct Current Motor Part I 11. Rotate the armature 15E further to the left. a. Repeat procedure 9. b. E = V dc, R = Ω, P = W REVIEW QUESTIONS 1. What would be the shunt field current of your motor if the shunt field winding is excited by 120 V dc? 2. If a current of 3 A dc flows in the series field winding of your motor, what would the resultant voltage drop be? 3. If the rheostat were connected in series with the shunt field winding and the combination placed across a 120 V dc line, what shunt field current variations could be obtained from your motor? I minimum = A dc I maximum = A dc 4. All of the windings and even the commutator of your motor are made of copper. Why? 5. Why are the brushes or your motor made of carbon rather than copper? 6. If the series field winding of your motor was connected directly across the 120 V dc supply: a) What current would flow? b) What would the power loss be (in watts)? 2-6

36 The Direct Current Motor Part I c) Is this power loss entirely given up as heat? Yes No d) What do you think would happen to the winding if the current were sustained for a few minutes? 7. What is meant by a nominal current or nominal voltage? 8. If the armature winding and the series field winding of your motor were connected in series across a 120 V dc source, what would the starting current be? 9. In your motor, is the armature (plus brushes) resistance substantially the same for every rotational position of the armature? Explain. Yes No 2-7

37 Experiment 3 The Direct Current Motor Part II OBJECTIVE To locate the neutral brush position. To learn the basic motor wiring connections. To observe the operating characteristics of series and shunt connected motors. DISCUSSION In order of a DC motor to run, current must flow in the armature winding. The stator must develop a magnetic field (flux), either by means of a shunt winding or a series winding (or both). The torque developed by a DC motor is directly proportional to the armature current and the stator flux. On the other hand, motor speed is mainly determined by the armature voltage and the stator flux. Motor speed increases when the voltage applied to the armature increases. Motor speed will also increase when the stator flux is reduced. As a matter of fact, the speed can attain dangerous proportions if, accidentally, there is a complete loss of the stator field. DC motors have been known to fly apart under these overspeed conditions. However, your DC motor has been carefully designed to withstand possible overspeed condition. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! Finding the Neutral 1. You will now determine the neutral brush position for your DC motor by using alternating current. Using your Power Supply, AC Voltmeter and DC Motor/enerator, connect the circuit shown in Figure 3-1. Terminals 4 and N on the power supply will furnish variable V ac as the voltage output control is advanced. 3-1

38 The Direct Current Motor Part II Figure 3-1. DO NOT APPLY POWER AT THIS TIME! 2. Unlock the DC Motor/enerator and move it forward approximately 10 cm [4 in]. Reach behind the front face of the module and move the brush positioning lever to its maximum clockwise position. Do not slide the module back in place (you will later move the brushes again). 3. Turn on the power supply. Place the power supply voltmeter switch to its 4-N position. Slowly advance the voltage output control until the AC voltmeter connected across the shunt field winding indicates approximately 80 V ac. (The AC voltage across the shunt field is induced by the AC current through the armature. This will be covered in a later Experiment). 4. a. Carefully reach behind the front face of the module (preferably keeping one hand in your pocket) and move the brushes from one extreme position to another. You will notice that the induced AC voltage across the field drops to zero and then increases again as you approach the other extreme counter-clockwise position. b. Leave the brushes at the position where the induced voltage is zero. This is the neutral point of your DC Motor/enerator. Each time you use the DC Motor/enerator the brushes should be set at the neutral position. c. Return the voltage to zero and turn off the power supply. Slide your DC Motor/enerator back in place and disconnect your circuit. Series Motor Connections 5. Using your Power Supply, DC Voltmeter/Ammeter and DC Motor/enerator, connect the circuit shown in Figure 3-2. Notice that the armature is connected in series with the series field winding, across the input voltage. 3-2

39 The Direct Current Motor Part II Figure Turn on the power supply. Place the power supply voltmeter switch to its 7-N position. Adjust the output voltage to 120 V dc. 7. a. Does the motor turn fast? Yes No b. Using your hand tachometer, measure the motor speed in revolutions per minute. Series speed = r/min Note: The operating instructions are enclosed within the tachometer container. 8. a. Reduce the power supply voltage and note the effect on motor speed. Comments: b. Reduce the voltage until you can determine the direction of rotation (clockwise or counterclockwise). Rotation = c. Reduce the voltage to zero and turn off the power supply. 9. Reconnect your circuit as shown in Figure 3-3. (The only change made to the circuit of Figure 3-2 is that the connections to the armature have been reversed). 3-3

40 The Direct Current Motor Part II Figure Repeat procedures 6 through 8 (using the reversed armature connections shown in Figure 3-3). Series speed (reversed) = r/min Rotation = 11. State a rule for changing the direction of rotation of a series connected DC motor. Shunt Motor Connections 12. Connect the circuit shown in Figure 3-4. Notice that the rheostat is in series with the shunt field, and that this combination is in parallel with the armature, across the input voltage. Figure a. Adjust the rheostat for minimum resistance (approximately 0 Ω, when turned fully clockwise). 3-4

41 The Direct Current Motor Part II b. Turn on your power supply and adjust for 120 V dc. c. Using your tachometer measure the motor speed. Shunt speed (zero ohms) = r/min d. Adjust the rheostat for maximum resistance (approximately 500 Ω). e. Determine the direction of rotation. Rotation = 14. a. Return the voltage to zero and turn off the power supply. b. Reverse the polarity of the input voltage by interchanging the power supply connection leads only. 15. Repeat procedure 13 and compare your results: a. Did the rotation change direction? Yes No b. Did the speed change? Yes No c. Return the voltage to zero and turn off the power supply. 16. Interchange the connection leads to the power supply. Your circuit should be the same as the one shown in Figure 3-4. Now reverse the connections to the armature only. 17. Repeat procedure 13 and compare the direction of rotation to that found in procedure 13. Rotation = 18. a. While the motor is still running, momentarily open the shunt field circuit by removing the connection lead from one of the terminals of the shunt field winding (5 or 6). Be extremely careful not to touch any of the other terminal connections or any metal during this procedure. Be prepared to cut power to the motor by turning off the power supply within three seconds. 3-5

42 The Direct Current Motor Part II CAUTION! Do not leave power on the motor for more than three seconds when the shunt field circuit is open. Exceeding this time limit can result in serious damages to the DC motor. b. Explain what happens when a DC motor loses power to its shunt field. c. Could the same thing occur in a series field connected DC motor? Explain. Yes No 19. Connect the circuit shown in Figure 3-5. Note that the armature is connected to the variable V dc output (terminals 7 and N) while the shunt field is now connected to the fixed 120 V dc output (terminals 8 and N). Figure a. Turn on the power supply. Adjust the armature voltage to 30 V dc as indicated by the meter. b. Use your hand tachometer and measure the motor speed. Record your speed measurement in Table 3-1. (Wait until the motor speed stabilizes before you take your measurement). c. Repeat (b) for each of the voltage values listed in the Table. Return voltage to zero and turn off the power supply. 3-6

43 The Direct Current Motor Part II E (volts) SPEED (r/min) Table 3-1. d. Plot each of the points from Table 3-1 on the graph shown in Figure 3-6. Draw a smooth curve through your plotted points. Figure 3-6. e. Does varying the armature voltage (with the shunt field voltage held constant) offer a good method of speed control? Yes No REVIEW QUESTIONS 1. Explain how to locate the neutral brush position in a DC motor. 2. Would the motor turn if only the armature were excited (had voltage applied across it)? Yes No 3-7

44 The Direct Current Motor Part II 3. Why is it dangerous to supply power to an unloaded series connected DC motor? 4. In what two ways may the rotation of a shunt connected DC motor be reversed? 5. Why are field loss detectors necessary in large DC motors? 6. In procedure 20: a) Does the motor speed double when the armature voltage is doubled? Explain. Yes No b) Would it be correct to say with a fixed field voltage, the speed of a shunt motor is proportional to its armature voltage? Explain. Yes No 3-8

45 The Direct Current Motor Part II 7. Draw a circuit showing how you would connect: a) a shunt motor to a DC supply. b) a shunt motor to a DC supply, using a field rheostat. 3-9

46 The Direct Current Motor Part II c) a series motor to a DC supply. 8. In what two ways can the speed of DC motor be varied? a) b) 9. Of the two methods given in (8): a) which method gives the greatest speed range? b) which method is the most economical (uses fewer parts)? 3-10

47 Experiment 4 The DC Shunt Motor OBJECTIVE To study the torque vs speed characteristics of a shunt wound DC motor. To calculate the efficiency of the shunt wound DC motor. DISCUSSION The speed of any DC motor depends mainly upon its armature voltage and the strength of the magnetic field. In a shunt motor, the field winding, as well as the armature winding, is connected in parallel (shunt) directly to the DC supply lines. If the DC line voltage is constant, then the armature voltage and the field strength will be constant. It is, therefore, apparent that the shunt motor should run at a reasonably constant speed. The speed does tend to drop with an increasing load on the motor. This drop in speed is mainly due to the resistance of the armature winding. Shunt motors with low armature winding resistance run at nearly constant speeds. Just like most energy conversion devices, the DC shunt motor is not 100% efficient. In other words, all of the electric power which is supplied to the motor is not converted into mechanical power. The power difference between the input and output is dissipated in the form of heat, and constitutes what are known as the losses of the machine. These losses increase with load, with the result that the motor gets hot as it delivers mechanical power. In this Experiment you will investigate the efficiency of a DC shunt motor. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 4-1

48 The DC Shunt Motor 1. Using your Power Supply, DC Motor/enerator, DC Voltmeter/Ammeter and Electrodynamometer, connect the circuit shown in Figure 4-1. DO NOT APPLY POWER AT THIS TIME! Notice that the motor is wired for shunt field operation and is connected to the variable DC output of the power supply (terminals 7 and N). The electrodynamometer is connected to the fixed 120 V ac output of the power supply (terminals 1 and N). Figure 4-1. Couple the dynamometer to the DC motor/generator with the timing belt as shown in the photo. 2. Set the shunt field rheostat control knob at its full cw position (for maximum shunt field excitation). Make sure the brushes are in their neutral position. 3. Set the dynamometer control knob at its full ccw position (to provide a minimum starting load for the DC motor). 4. Turn on the power supply. Adjust the variable output voltage to 120 V dc as indicated by the meter. Note the direction of rotation; if it is not clockwise, turn off the power supply and interchange the shunt field connections. 5. a. Adjust the shunt field rheostat for a no-load motor speed of 1800 r/min as indicated on your hand tachometer. (Make sure that the voltmeter, connected across the input of your circuit, indicates exactly 120 V dc). 4-2

49 The DC Shunt Motor Note: Do not change the field rheostat adjustment for the remainder of the experiment. b. Measure the line current, as indicated by the ammeter, for a motor speed of 1800 r/min. Record this value in Table 4-1. Note: For an exact torque of 0 NAm [0 lbfain], uncouple the motor from the dynamometer. E (volts) I (amps) SPEED (r/min) TORQUE (NAm) Table 4-1. E (volts) I (amps) SPEED (r/min) TORQUE (lbfain) Table a. Apply a load to your DC motor by varying the dynamometer control knob until the scale marked on the stator housing indicates 0.3 NAm [3 lbfain]. (Readjust the power supply, if necessary, to maintain exactly 120 V dc). b. Measure the line current and motor speed. Record these values in Table 4-1. c. Repeat for each of the torque values listed in the Table, while maintaining a constant 120 V dc input. d. Return the voltage to zero and turn off the power supply. 7. a. Plot the recorded motor speed values from Table 4-1 on the graph of Figure 4-2. b. Draw a smooth curve through your plotted points. 4-3

50 The DC Shunt Motor c. The completed graph represents the speed vs torque characteristics of a typical DC shunt-wound motor. Similar graphs for series wound and compound wound DC motors will be constructed in the following two Experiments. The speed vs torque characteristics for each type of motor will then be compared and evaluated. Figure Calculate the speed vs torque regulation (full load = 1.2 NAm [9 lbfain]) using the equation: Speed regulation = % 9. Set the dynamometer control knob at its full cw position (to provide the maximum starting load for the shunt-wound motor). 10. a. Turn on the power supply and gradually increase the DC voltage until the motor is drawing 3 A of line current. The motor should turn very slowly or not at all. b. Measure and record the DC voltage and the torque developed. E = V Torque = NAm [lbfain] c. Return the voltage to zero and turn off the power supply. 4-4

51 The DC Shunt Motor 11. a. The line current in procedure 10 is limited only by the equivalent DC resistance of the shunt-wound motor. b. Calculate the value of the starting current if the full line voltage (120 V dc) were applied to the shunt-wound DC motor. Starting current = A REVIEW QUESTIONS 1. Calculate the mechanical output power by the shunt-wound DC motor when the torque is 1.2 NAm [9 lbfain]. Use the equation: where: P out = Mechanical Output Power in watts (W) N = Speed in revolutions per minute (r/min) T = Torque in NewtonAmeter (NAm) where: P out = Mechanical Output Power in horse power (hp) N = Speed in revolutions per minute (r/min) T = Torque in pound-force-inches (lbfain) P out = W [hp] 1. Knowing that 1 hp is equivalent to 746 W, what is the equivalent output power of the motor? Output power = W 2. What is the input power (in watts) of the motor in Question 1? Input power = W 4-5

52 The DC Shunt Motor 3. Knowing the input and output power in watts, calculate the efficiency of the motor in Question 1. Efficiency = (power out/power in) x 100% Efficiency = % 4. What are the losses (in watts of the motor in Question 1)? Losses = W 5. List where some of these losses occur. 6. Would the losses decrease if a cooling fan is mounted on the motor shaft? Explain. Yes No 7. ive two reasons why losses are undesirable. 8. How much larger is the starting current than the normal full load current? 4-6

53 Experiment 5 The DC Series Motor OBJECTIVE To study the torque vs speed characteristics of a series wound DC motor. To calculate the efficiency of the series wound DC motor. DISCUSSION The shunt wound DC motor was seen to have almost constant speed because its armature voltage and magnetic field remained substantially unchanged from no-load to full-load. The series motor behaves quite differently. In this motor, the magnetic field is produced by the current which flows through the armature winding, with the result that the magnetic field is weak when the motor load is light (the armature winding draws minimum current). The magnetic field is strong when the load is heavy (the armature winding draws maximum current). The armature voltage is nearly equal to the supply line voltage (just as in the shunt wound motor if we neglect the small drop in the series field). Consequently, the speed of the series wound motor is entirely determined by the load current. The speed is low at heavy loads, and very high at no load. In fact, many series motors will, if operated at no load, run so fast that they destroy themselves. The high forces, associated with high speeds, cause the rotor to fly apart, often with disastrous results to people and property nearby. The torque of any DC motor depends upon the product of the armature current and the magnetic field. For the series wound motor this relationship implies that the torque will be very large for high armature currents, such as occur during start-up. The series wound motor is, therefore, well adapted to start large heavy-inertia loads, and is particularly useful as a drive motor in electric buses, trains and heavy duty traction applications. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 5-1

54 The DC Series Motor 1. Using your Power Supply, DC Motor/enerator, DC Voltmeter/Ammeter and Electrodynamometer, connect the circuit shown in Figure 5-1. DO NOT APPLY POWER AT THIS TIME! Figure 5-1. Couple the dynamometer to the DC motor/generator with the timing belt. Notice that the motor is wired for series operation (the shunt field winding and the rheostat are not used) and is connected to the variable DC output of the power supply (terminals 7 and N). The electrodynamometer is connected to the fixed 120 V ac output of the power supply (terminals 1 and N). 2. Set the dynamometer control knob at its mid-range position (to provide a starting load for the DC motor). 3. a. Turn on the power supply. radually increase the DC voltage until the motor starts to turn. Note the direction of rotation. If it is not cw, turn off the power and interchange the series field connections. b. Adjust the variable voltage for exactly 120 V dc as indicated by the meter. 4. a. Adjust the loading of your DC series wound motor by varying the dynamometer control knob until the scale marked on the stator housing 5-2

55 The DC Series Motor indicates 1.2 NAm [12 lbfain]. (Readjust the power supply, if necessary, to maintain exactly 120 V dc). b. Measure the line current and motor speed (use your hand tachometer). Record these values in Table 5-1. c. Repeat for each of the torque values listed in the Table, while maintaining a constant 120 V dc input. d. Return the voltage to zero and turn off the power supply. E (volts) I (amps) SPEED (r/min) TORQUE (NAm) Table 5-1. E (volts) I (amps) SPEED (r/min) TORQUE (lbfain) Table 5-1. Note: For an exact torque of 0 NAm [lbfain], uncouple the motor from the dynamometer. 5. a. Plot the recorded motor speed values from Table 5-1 on the graph of Figure 5-2. b. Draw a smooth curve through your plotted points. c. The completed graph represents the speed vs torque characteristics of a typical DC series wound motor. A similar graph for the compound wound DC motor will be constructed in the next Experiment. the speed vs torque characteristics for each type of motor will then be compared and evaluated. 5-3

56 The DC Series Motor Figure Calculate the speed vs torque regulation (full load = 1.2 NAm [9 lbfain] using the equation: Speed regulation = % 7. Set the dynamometer control knob at its full cw position (to provide the maximum starting load for the series wound motor). 8. a. Turn on the power supply and gradually increase the DC voltage until the motor is drawing 3 A of line current. The motor should turn slowly. b. Measure and record the DC voltage and the torque developed. E = V Torque = NAm [lbfain] c. Return the voltage to zero and turn off the power supply. 9. a. The line current in procedure 8 is limited by the equivalent DC resistance of the series wound motor. b. Calculate the value of the starting current if the full line voltage (120 V dc) were applied to the series wound DC motor. Starting current = A 5-4

57 The DC Series Motor REVIEW QUESTIONS 1. Calculate the mechanical output power developed by the series wound DC motor when the torque is 1.2 NAm [9 lbfain]. Use the equation: where: P out = Mechanical Output Power in watts (W) N = Speed in revolutions per minute (r/min) T = Torque in NewtonAmeter (NAm) where: P out = Mechanical Output Power in horse power (hp) N = Speed in revolutions per minute (r/min) T = Torque in pound-force-inches (lbfain) P out = W [hp] 1. Knowing that 1 hp is equivalent to 746 W, what is the equivalent output power of the motor? Output power = W 2. What is the input power (in watts) of the motor in Question 1? Input power = W 3. Knowing the input and output power in watts, calculate the efficiency of the motor in Question 1. Efficiency = % 5-5

58 The DC Series Motor 4. What are the losses (in watts) of the motor in Question 1? Losses = W 5. How much larger is the starting current than the normal full load current? 6. Compare the shunt wound DC motor and the series wound DC motor on the basis of: a) Starting torque b) Starting current c) Efficiency d) Speed regulation 5-6

59 Experiment 6 The DC Compound Motor OBJECTIVE To study the torque vs speed characteristics of a compound wound DC motor. To calculate the efficiency of the compound wound DC motor. DISCUSSION The high torque capability of the series wound DC motor is somewhat compromised by its tendency to overspeed at light loads. This disadvantage can be overcome by adding a shunt field, connected in such a way as to aid the series field. The motor then becomes a cumulative compound machine. Again, in special applications where DC motors are use in conjunction with flywheels, the constant speed characteristic of the shunt wound motor is not entirely satisfactory, because it does not permit the flywheel to give up its kinetic energy by an appropriate drop in motor speed. This kind of application (which is found in punch-press work), requires a motor with a dropping speed characteristic, that is, the motor speed should drop significantly with an increase in load. The cumulative compound wound DC motor is well adapted for this type of work. The series field can also be connected so that it produces a magnetic field opposing that of the shunt field. This produces a differential compound motor, which has very limited application, principally because it tends to be unstable. Thus, as the load increases, the armature current increases, which increases the strength of the series field. Since it acts in opposition to the shunt winding, the total flux is reduced, with the result that the speed increases. An increase in speed will generally further increase the load which raises the speed still more and could cause the motor runaway. Differential compound motors are sometimes made with weak series fields which compensate somewhat for the normal slowing of a shunt motor under load and, hence, have more constant speed. Differential compound motors are not used very often. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. 6-1

60 The DC Compound Motor PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Using your Power Supply, DC Motor/enerator, DC Voltmeter/Ammeter and Electrodynamometer, connect the circuit shown in Figure 6-1. Figure 6-1. DO NOT APPLY POWER AT THIS TIME! Couple the dynamometer to the DC motor/generator with the timing belt. Notice that the motor is wired for series operation (the shunt field winding and the rheostat not in the circuit as yet), and is connected to the variable DC output of the power supply (terminals 7 and N). The electrodynamometer is connected to the fixed 120 V ac output of the power supply (terminals 1 and N). 2. Set the dynamometer control knob at its full ccw position (to provide a minimum starting load for the motor). 3. a. Turn on the power supply. radually increase the DC voltage until the motor starts to turn. Note the direction of rotation. If it is no cw, turn off the power and interchange the series field connections. 6-2

61 The DC Compound Motor b. Return the voltage to zero and turn off the power supply. 4. Connect the shunt field, in series with the rheostat, to terminals 1 and 4 as shown in Figure 6-2. Figure Turn on the power supply. Adjust the voltage for 120 V dc as indicated by the meter. If the motor is running at an excessively high speed then it is in the differential-compound mode. If this is the case, return the voltage to zero and turn off the power supply. Interchange the shunt field connections to terminals 1 and 4 to obtain the cumulative-compound mode of operation. 6. With the input at exactly 120 V dc adjust the shunt field rheostat for a noload motor speed of 1800 r/min as indicated by your hand tachometer. Note: Do not change the field rheostat adjustment for the remainder of the experiment. 7. a. Apply a load to your DC motor by varying the dynamometer control knob until the scale marked on the stator housing indicates 0.3 NAm [3 lbfain]. (Readjust the power supply, if necessary, to maintain exactly 120 V dc). b. Measure the line current and motor speed. Record these values in Table 6-1. c. Repeat for each of the torque values listed in the Table, while maintaining a constant 120 V dc input. d. Return the voltage to zero and turn off the power supply. 6-3

62 The DC Compound Motor E (volts) I (amps) SPEED (r/min) TORQUE (NAm) Table 6-1. E (volts) I (amps) SPEED (r/min) TORQUE (lbfain) Table 6-1. Note: For an exact torque of 0 NAm [lbfain], uncouple the motor from the dynamometer. 8. a. Plot the recorded motor speed values from Table 6-1 on the graph of Figure 6-3. Figure

63 The DC Compound Motor b. Draw a smooth curve through your plotted points. c. The completed graph represents the speed vs torque characteristics of a typical DC compound wound motor. 9. Calculate the speed vs torque regulation (full load = 1.2 NAm [9 lbfain]) using the equation: Speed regulation = % 10. Set the dynamometer control knob at its full cw position (to provide the maximum starting load for the compound wound motor). 11. a. Turn on the power supply and gradually increase the DC voltage until the motor is drawing 3 A of line current. The motor should turn very slowly or not at all. b. Measure and record the DC voltage and the torque developed. E = V Torque = NAm [lbfain] c. Return the voltage to zero and turn off the power supply. 12. a. The line current in procedure 11 is limited only by the equivalent DC resistance of the compound wound motor. b. Calculate the value of the starting current in the full line voltage 120 V dc) were applied to the compound wound DC motor. Starting current = A REVIEW QUESTIONS 1. Calculate the mechanical output power developed by the compound wound DC motor when the torque is 1.2 NAm [9 lbfain]. Use the equation: where: P out = Mechanical Output Power in watts (W) N = Speed in revolutions per minute (r/min) T = Torque in NewtonAmeter (NAm) 6-5

64 The DC Compound Motor where: P out = Mechanical Output Power in horse power (hp) N = Speed in revolutions per minute (r/min) T = Torque in pound-force-inches (lbfain) P out = W [hp] 1. Knowing that 1 hp is equivalent to 746 W, what is the equivalent output power of the motor? Output power = W 2. What is the input power (in watts) of the motor in Question 1? Input power = W 3. Knowing the input and output power in watts, calculate the efficiency of the motor in Question 1. Efficiency = % 4. What are the losses (in watts) of the motor in Question 1? Losses = W 6-6

65 The DC Compound Motor 5. How much larger is the starting current than the normal full load current? 6. A compound wound DC motor is more stable than a series wound DC motor and its starting characteristics are almost as good. Explain this statement. 7. Compare the compound, series and shunt motors on the basis of: a) Starting torque b) Starting current c) Efficiency d) Speed regulation 6-7

66 Experiment 7 The DC Separately Excited Shunt enerator OBJECTIVE To study the properties of the separately excited DC shunt generator under noload and full-load conditions. To obtain the saturation curve of the generator. To obtain the armature voltage vs armature current load curve of the generator. DISCUSSION A DC machine can run either as a motor or as a generator. A motor converts electrical power into mechanical power while a generator converts mechanical power into electrical power. A generator must, therefore, be mechanically driven in order that it may produce electricity. Since the field winding is an electromagnet, current must flow through it to produce a magnetic field. This current is called the excitation current, and can be supplied to the field winding in one of two ways; it can come from a separate, external DC source, in which case the generator is called a separately excited generator; or it can come from the generator s own output, in which case the generator is called a selfexcited generator. Assume that the shunt field is excited by a DC current, thereby setting up a magnetic flux in the generator. If the rotor (or more correctly, the armature) is rotated by applying mechanical effort to the shaft, the armature coils will cut the magnetic flux, and a voltage will be induced in them. This voltage is AC and in order to get DC out of the generator, a rectifier must be employed. This role is carried out by the commutator and the brushes. The voltage induced in the coils (and, therefore, the DC voltage at the brushes) depends only upon two things - the speed of rotation and the strength of the magnetic field. If the speed is doubled, the voltage doubles. If the field strength is increased by 20%, the voltage also increases by 20%. Although separate excitation requires a separate DC power source, it is useful in cases where a generator must respond quickly and precisely to an external control source, or when the output voltage must be varied over a wide range. With no electrical load connected to the generator, no current flows and only a voltage appears at the output. However, if a resistance load is connected across the output, current will flow and the generator will begin to deliver electric power to the load. 7-1

67 The DC Separately Excited Shunt enerator The machine which drives the generator must then furnish additional mechanical power to the generator. This is often accompanied by increased noise and vibration of the motor and the generator, together with a drop in speed. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! No Load Characteristics 1. Because of its constant running speed, the synchronous motor will be used to mechanically drive the DC generator. Using your Power Supply, AC Ammeter and Three-Phase Synchronous Motor/enerator, connect the circuit shown in Figure 7-1. Figure 7-1. DO NOT APPLY POWER AT THIS TIME! 7-2

68 The DC Separately Excited Shunt enerator 2. Terminals 1, 2 and 3 on the power supply provide fixed three-phase power for the three stator windings. (Three-phase power will be covered in later Experiments). Terminals 8 and N on the power supply provide fixed DC power for the rotor winding. Set the rheostat control knob to its proper position for normal excitation (Experiment 1, procedure 6). 3. a. Using your DC Motor/enerator and DC Voltmeter/Ammeter, connect the circuit shown in Figure 7-2. b. Connect the shunt field of the generator, terminals 5 and 6, to the variable DC output of the power supply, terminals 7 and N, while connecting the 500 ma meter in series with the positive lead. c. Connect the 200 V dc meter across the generator output (armature (terminals 1 and 2). d. Couple the synchronous motor and the DC generator with the timing belt. e. Make sure the brushes are in their neutral position. f. Have your instructor check your circuit. Figure 7-2. CAUTION! The switch in the excitation circuit of the synchronous motor should be closed (I) only when the motor is running. 4. a. Turn on the power supply. The synchronous motor should start running. b. Close the switch S. 7-3

69 The DC Separately Excited Shunt enerator c. Vary the shunt field current I F by rotating the voltage control knob on the power supply. Note the effect on the generator output (armature voltage) E A as indicated by the 200 V dc meter. d. Measure and record in Table 7-1 the armature voltage E A for each of the listed field currents. I F (milliamperes) E A (volts) Table 7-1. e. Return the voltage to zero and turn off the power supply. f. Can you explain why there is an armature voltage even when the field current is zero? 5. a. Reverse the polarity of the shunt field by interchanging the leads to terminals 5 and 6 on the DC generator. b. Turn on the power supply and adjust for a field current I F of 300 ma dc. c. Did the armature voltage reverse its polarity? Yes No d. Return the voltage to zero and turn of the power supply. 6. a. Interchange the leads to the 200 V dc meter. b. Turn on the power supply and adjust for a field current I F of 300 ma dc. 7-4

70 The DC Separately Excited Shunt enerator c. Measure and record the armature voltage. E A = V dc d. Is the armature voltage approximately the same as in procedure 4 (at an I F of 300 ma), except for reversed polarity? Yes No e. Return the voltage to zero and turn off the power supply. 7. a. Reverse the rotation of the driving motor by interchanging any two of the stator lead connections (terminals 1, 2 or 3) to the synchronous motor. b. Turn on the power supply and adjust for a field current I F of 300 ma dc. c. Did the armature voltage reverse its polarity? Yes No d. Return the voltage to zero and turn off the power supply. 8. a. Interchange the leads to the 200 V dc meter. b. Turn on the power supply and adjust for a field current I F of 300 ma dc. c. Measure and record the armature voltage. E A = V dc d. Is the armature voltage approximately the same as in procedure 4 (at an I F of 300 ma), except for reversed polarity? Yes No e. Return the voltage to zero and turn off the power supply. Load Characteristics 9. Set the supply voltage phase sequence of the synchronous motor as shown in Figure 7-1. Using your Resistive Load, connect the circuit shown in Figure 7-3. Place the resistance switches so that the total load resistance is 120 Ω. 7-5

71 The DC Separately Excited Shunt enerator Figure a. Turn on the power supply. The synchronous motor should start running. b. Adjust the shunt field current I F until the generator is delivering an output voltage of 120 V dc. The ammeter I A should indicate 1 A dc. c. Record the shunt field current I F. I F = ma This is the nominal I F at the rated power output (120 V x 1 A = 120 W) of the DC generator. 11. a. Adjust the load resistance to obtain each of the values listed in Table 7-2 while maintaining the nominal I F value found in procedure 10. b. Measure and record E A and I A for each of the resistance values listed in the Table. Note: Although the nominal output current rating of the generator is 1 A dc, it may be loaded up to 1.5 A dc (50% overload) without harm. 7-6

72 The DC Separately Excited Shunt enerator R L (ohms) I A (amps) E A (volts) POWER (watts) Table a. With the load resistance adjusted for an output current I A of 1.5 A, turn the field current I F on and off by removing the connecting lead from terminal 6 to the DC generator. b. Do you notice that the driving motor is obviously working harder when the generator is delivering power to the load? Yes No c. Return the voltage to zero and turn off the power supply. 13. Calculate and record the power for each of the values listed in Table a. Place a dead short across the armature (terminals 1 and 2). b. Make sure that the power supply voltage control knob is turned down for zero field current. c. Turn on the power supply. d. radually increase the field current I F until the motor stalls. CAUTION! Do not leave the motor in the stalled condition for more than a couple of seconds. e. What value of shunt field current I F is needed to stall the motor? I F = ma 7-7

73 The DC Separately Excited Shunt enerator f. Turn off the power supply. Note: With a short-circuit across the armature, its current becomes very large; this produces a strong braking effect sufficient to stall the driving motor. REVIEW QUESTIONS 1. State two ways by which the output polarity of a shunt DC generator can be changed. 2. If a DC generator delivers 180 W to a load, what is the minimum mechanical power (in watts) needed to drive the generator (assume 80% efficiency)? 2. If a DC generator delivers 180 W to a load, what is the minimum hp needed to drive the generator (assume 100% efficiency)? 3. Plot the E A vs I F characteristic curve for your DC shunt generator on the graph of Figure 7-4. Use the data from Table 7-1. Note that the curve bends over as the field current increases. Can you explain why this happens? 7-8

74 The DC Separately Excited Shunt enerator Figure Plot the E A vs I A regulation curve on the graph of Figure 7-5. Use the data from Table

75 The DC Separately Excited Shunt enerator Figure Calculate the regulation from no-load to full-load (1 A dc). Regulation = % 7-10

76 Experiment 8 The DC Self-Excited Shunt enerator OBJECTIVE To study the properties of the self-excited DC shunt generator under no-load and full-load conditions. To learn how to connect the self-excited generator. To obtain the armature voltage vs armature current load curve of the generator. DISCUSSION The separately-excited generator (Experiment 7) has many applications. However, it does have the disadvantage that a separate direct current power source is needed to excite the shunt field. This is costly and sometimes inconvenient; and the selfexcited DC generator is often more suitable. In a self-excited generator, the field winding is connected to the generator output. It may be connected across the output, in series with the output, or a combination of the two. The way in which the field is connected (shunt, series or compound) determines many of the generator s characteristics. All of the above generators can have identical construction. Self-excitation is possible because of the residual magnetism in the stator pole pieces. As the armature rotates a small voltage is induced across its windings. When the field winding is connected in parallel (shunt) with the armature a small field current will flow. If this small field current is flowing in the proper direction, the residual magnetism will be reinforced which further increases the armature voltage and thus, a rapid voltage build-up occurs. If the field current flows in the wrong direction, the residual magnetism will be reduced and voltage build-up cannot occur. In this case, interchanging the shunt field leads will correct the situation. It is the purpose of this Experiment to show these major points. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. 8-1

77 The DC Self-Excited Shunt enerator PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Because of its constant running speed, the synchronous motor will be used to mechanically drive the DC generator. Using your Power Supply, AC Ammeter and Three-Phase Synchronous Motor/enerator, connect the circuit shown in Figure 8-1. Figure 8-1. DO NOT APPLY POWER AT THIS TIME! 2. Terminals 1, 2, and 3 on the power supply provide fixed three-phase power for the three stator windings. (Three-phase power will be covered in later Experiments). Terminals 8 and N on the power supply provide fixed DC power for the rotor winding. Set the rheostat control knob to its proper position for normal excitation (Experiment 1, procedure 6). 3. a. Using your DC Motor/enerator, DC Voltmeter/Ammeter and Resistive Load, connect the circuit shown in Figure 8-2. b. Couple the synchronous motor and the DC generator with the timing belt. 8-2

78 The DC Self-Excited Shunt enerator c. Turn the DC generator field rheostat control knob full cw for minimum resistance. d. Make sure the brushes are in their neutral position. e. Place the resistance switches for no-load (all switches open). Figure 8-2. CAUTION! The switch in the excitation circuit of the synchronous motor should be closed (I) only when the motor is running. 4. a. Turn on the power supply. The synchronous motor should start running. b. Close the switch S. c. Not if voltage E A builds up. Yes No d. If not, turn off the power supply and interchange the shunt field leads at terminals 5 and 6. e. Measure the open circuit armature voltage. E A = V dc 8-3

79 The DC Self-Excited Shunt enerator 5. Vary the field rheostat and notice if the armature voltage E A changes. Explain. Yes No 6. a. Place the resistance switches so that the total load resistance is 120 Ω. Adjust the field rheostat until the generator is delivering an output voltage of 120 V dc. The ammeter I A should indicate 1 A dc. b. This is the correct setting of the field rheostat control for the rated power output 120 V x 1 A = 120 W) of the DC generator. Do not touch the field rheostat control for the remainder of the Experiment! 7. a. Adjust the load resistance to obtain each of the values listed in Table 8-1. b. Measure and record E A and I A for each of the resistance values listed in the Table. R L (ohms) I A (amps) E A (volts) POWER (watts) Table 8-1. Note: Although the nominal output current rating of the generator is 1 A dc, it may be loaded up to 1.5 A dc (50% overload) without harm. c. Turn off the power supply. 8-4

80 The DC Self-Excited Shunt enerator d. Calculate and record the power for each of the resistance shown in Table a. Reverse the rotation of the driving motor by interchanging any two of the stator lead connections (terminals 1, 2, or 3) to the synchronous motor. b. Remove the generator load by opening all of the resistance switches. c. Turn on the power supply. d. Does the generator voltage build up? Explain. Yes No e. Turn off the power supply. REVIEW QUESTIONS 1. If a self-excited generator has lost all of its residual magnetism, can it build up an output voltage? Yes No 2. How would you get a generator to work after it had lost all of its residual magnetism? 3. Does a generator slowly lose its residual magnetism with time? Yes No 4. Plot the E A vs I A regulation curve on the graph of Figure

81 The DC Self-Excited Shunt enerator Figure Calculate the regulation from no-load to full-load (1 A dc). Regulation = % 6. Compare the regulation of the self-excited generator with the regulation of the separately-excited generator (Experiment 7). 7. Explain why one of the generators has better regulation than the other. 8-6

82 Experiment 9 The DC Compound enerator OBJECTIVE To study the properties of compound DC generators under no-load and full-load conditions. To learn how to connect both the compound and the differential compound generators. To obtain the armature voltage vs armature current load curves for both generators. DISCUSSION Self-excited shunt generators have the disadvantage in that changes in their load current from no-load cause their output voltage to change also. Their poor voltage regulation is due to three factors: a) The magnetic field strength drops as the armature voltage drops, which further reduces the magnetic field strength, which in turn reduces the armature voltage, etc. b) The armature voltage drop (I 2 x R losses) from no-load to full-load. c) The running speed of the driving motor may change with load. (This is particularly true of internal combustion engines and induction motors). The two field windings (shunt and series) on the compound generator are connected so that their magnetic fields aid each other. Thus, when the load current increases, the current through the shunt field winding decreases, reducing the strength of the magnetic field. But, if the same increase in load current is made to flow through the series field winding, it will increase the strength of the magnetic field. With the proper number of turns in the series winding, the increase in magnetic strength will compensate for the decrease caused by the shunt winding. The combined magnetic field strength remains almost unchanged and little change in output voltage will take place as the load goes from no-load to full-load. If the series field is connected so that the armature current flows in such a direction as to oppose the shunt field, we obtain a differential compound generator. This type of generator has poor regulation, but is useful in applications such as welding and arc lights where maintaining a constant output current is more important than a constant output voltage. It is the purpose of this Experiment to show these major points. 9-1

83 The DC Compound enerator EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. CAUTION! The switch in the excitation circuit of the synchronous motor should be closed (I) only when the motor is running. PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Because of its constant running speed, the synchronous motor will be used to mechanically drive the DC generator. Using your Power Supply, AC Ammeter and Three-Phase Synchronous Motor/enerator, connect the circuit shown in Figure 9-1. Figure Terminals 1, 2, and 3 on the power supply provide fixed three-phase power for the three stator windings (Three-phase power will be covered in later Experiments). Terminals 8 and N on the power supply provide fixed DC power for the rotor winding. Set the rheostat control knob to its proper position for normal excitation (Experiment 1, procedure 6). 9-2

84 The DC Compound enerator 3. a. Using your DC Motor/enerator, DC Voltmeter/Ammeter and Resistive Load, connect the circuit shown in Figure 9-2. Figure 9-2. b. Couple the synchronous motor and the DC generator with the timing belt. c. Turn the DC generator field rheostat control knob full cw for minimum resistance. d. Make sure the brushes are in their neutral position. e. Place the resistance switches for no-load (all switches open). 4. a. Turn on the power supply. The synchronous motor should start running. b. Close the switch S. c. Note if voltage E A builds up. Yes No d. If not, turn off the power supply and interchange any two of the stator connection leads on the synchronous motor. e. Measure the open circuit armature voltage. E A = V dc 9-3

85 The DC Compound enerator 5. Vary the field rheostat and notice if the armature voltage E A changes. Explain. Yes No 6. Adjust the field rheostat for a no-load current (I A = 0 A) output voltage E A of 120 V dc. Do not touch the field rheostat control for the remainder of the Experiment! 7. a. Adjust the load resistance to obtain each of the values listed in Table 9-1. b. Measure and record E A and I A for each of the resistance values listed in the Table. Note: Although the nominal output current rating of the generator is 1 A dc, it may be loaded up to 1.5 A dc (50% overload) without harm. c. Turn off the power supply. d. Calculate and record the power for each of the resistance shown in Table 9-1. R L (ohms) I A (amps) E A (volts) POWER (watts) Table

86 The DC Compound enerator 8. a. Change the connections to the series field only, so that the armature current flows through it in the opposite direction. b. Complete the drawing shown in Figure 9-3 showing your proposed circuit change. c. Have your instructor check your circuit and your drawing. Figure a. Turn on the power supply. b. Adjust the field rheostat for an E A of 120 V dc. c. Do not touch the rheostat after this. 10. a. Adjust the load resistance to obtain each of the values listed in Table 9-2. b. Measure and record E A and I A for each of the resistance values listed in the Table. c. Turn off the power supply. d. Calculate and record the power for each of the resistances shown in Table

87 The DC Compound enerator R L (ohms) I A (amps) E A (volts) POWER (watts) Table 9-2. REVIEW QUESTIONS 1. State which procedure, (7 or 10) is concerned with: a) the differential compound generator. Procedure b) the compound generator. Procedure 9-6

88 The DC Compound enerator 2. Plot the E A vs I A regulation curve on the graph of Figure 9-4. Use the data from Table 9-2. Figure Over what voltage range is the armature current nearly constant in the differential compound generator? From V dc to V dc 9-7

89 The DC Compound enerator 4. Plot the E A vs I A regulation curve on the graph of Figure 9-5. Use the data from Table 9-1. Figure Calculate the regulation from no-load to full-load (1 A dc) for the compound generator. Regulation = % 6. Compare the regulation of the compound generator with the regulation of the self-excited generator and the separately-excited generator. 9-8

90 The DC Compound enerator 7. Explain briefly why the voltage does not drop with increasing load for the compound generator. 9-9

91 Experiment 10 DC Motor Starter OBJECTIVE To examine the construction of a DC motor starter. To observe the operation of a 3-point DC starter. To observe the operation of a 4-point DC starter. DISCUSSION At start-up, the counter-electromotive force of the armature of a DC motor is zero and the armature current is limited only by the armature and brushes resistance. If a DC shunt motor was to be connected directly across a full-voltage source, the inrush current could be as high as 20 to 30 times its nominal current, and the extremely high current could burn the armature winding, damage the commutator or overload the source. For these reasons, appropriate means shall be used to limit this inrush current to a safe value of approximately two times the nominal current of the motor. In order to reduce the starting current, we normally wire a starting rheostat in series with the armature. This series resistance is then gradually decreased as the motor accelerates and is finally removes from the circuit when the motor reaches its fullload speed. During this operation, the shunt field is generally connected directly across the supply to provide maximum starting torque. The face-plate starter is commonly used for this purpose; it consists of a series of fixed contacts connected to fixed value resistors as shown on the front face of the Manual DC Motor Starter. When the movable contact, driven by the handle, is first moved from its rest position, all the resistors are placed in series with the armature winding. As the motor speed increases, the handle is moved further so that the total series resistance is decreased. When the handle has been moved all the way, it brings a holding plate across an electromagnet which holds the handle in this position as long as the current flows through the electromagnet; the resistance is also taken out of the circuit. The Manual DC Motor Starter can also be operated as a 3-point or 4-point starter. If the front switch is placed in the 3-point position, current from the source passes through the electromagnet coil to the shunt field winding of the DC motor. When the starter handle is moved off its rest position, the electromagnet and the shunt field of the motor are energized. In the event of loss of field, such as an open shunt field circuit or too low a shunt field current, the electromagnet is de-energized and the handle is spring returned to the off position. 10-1

92 DC Motor Starter When the switch is in the 4-point position, the current to energize the electromagnet is set by the 600 Ω internal resistor placed in series with it. The loss of field feature is not provided in this position. Usually, the 3-point starter is used for shunt or compound motors, while the 4-point starter is used for series motors (the shunt winding being unused). EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Examine the construction of the Manual DC Motor Starter, paying particular attention to the high wattage fixed resistor equipped with two taps, the movable and fixed contacts, the return spring, the holding plate and the holding electromagnet. Note that a circuit breaker is placed in series with terminal 3 and another one in series with the electromagnet. These are to prevent the burning of the magnet coil or the high wattage resistor in case of overcurrent in the magnet or prolonged use of the starting resistor. The resistor is designed for intermittent use only and the handle should never be left in any intermediate position for an extended period of time. Move the handle gradually from the off position to the holding position. Note that the movable contact is successively connected to the fixed contacts until all of the resistor has been removed from the circuit when the holding plate makes contact with the holding electromagnet. Release the handle; it returns to the off position because the electromagnet is not energized. 2. a. Using your Power Supply, Manual DC Motor Starter, DC Voltmeter/Ammeter and DC Motor/enerator, connect the circuit shown in Figure b. Place the Manual DC Motor Starter toggle switch to the 3-point position. Turn the rheostat on the DC Motor/enerator fully ccw for maximum resistance; this assures minimum starting torque for the motor, extends the starting period and enhances the demonstration. 10-2

93 DC Motor Starter Figure c. Install the electrodynamometer to the right of the motor and couple them together with the timing belt. The electrodynamometer should not be connected to the supply. It is used only to increase the inertia of the system. 3. Turn on the power supply and adjust for an output voltage of 135 V dc. The motor should not turn. Set the shunt-field rheostat control knob at its full ccw position to provide minimum starting torque and to extend the starting period to an easily measurable value. 4. a. Move the starter handle slowly until the motor starts turning. Measure the peak armature current. Note that the current decreases thereafter and that the voltage across the armature increases. I A = A dc b. Repeat procedure (a) and measure how long the current remains above the value of 3 A. Time = s c. Move the handle slowly the second, third and fourth position and note small jumps in the current at each step. 10-3

94 DC Motor Starter d. Push the handle as far as it will go and release it. Is it held in place by the electromagnet? Yes No Does the motor keep turning? Yes No e. Force the handle back to the off position. Move the handle rapidly until it reaches the electromagnet. Note that the current goes beyond the full scale range of the current meter. How long does the current stay above 3 A? Time = s f. Open momentarily the shunt field circuit by removing the connection lead from one of the shunt field winding terminals (5 or 6). Be extremely careful not to touch any of the terminal connections or any metal part during this procedure. g. Explain what happened to the 3-point face plate starter and to the motor when the motor lost power to its hunt field. 5. a. Move the toggle switch of the Manual DC Motor Starter to the 4-point position and turn the shunt field rheostat to its maximum clockwise position. b. Move the starter handle slowly and by steps, as in procedure 4 and as far as it will go. Release the handle. Is it held in place by the electromagnet? Yes No Does the motor keep turning? Yes No c. Repeat procedure 4 (f). d. Explain what happened to the 4-point face starter and to the motor when the motor lost power to its shunt field. e. Return the voltage to zero and turn off the power supply. 10-4

95 DC Motor Starter REVIEW QUESTIONS 1. Explain why the shunt field rheostat must normally be set for minimum resistance during the starting of a motor with a face plate starter. 2. Why would you prefer a 3-point to a 4-point face plate starter to supply power to a DC motor? 3. What would happen if the motor were heavily loaded and the starter handle were maintained in an intermediate position for too long? 4. Why is the power to the shunt field taken from the first fixed contact of the face plate starter rather than from terminal 3 of the module? 5. What is the purpose of the 600 Ω resistor placed in series with the holding electromagnet of the Manual DC Motor Starter when used as a 4-point starter? 10-5

96 Experiment 11 Thyristor Speed Controllers Note: If you are a user of the Thyristor (SCR) Speed Controller module, Model 9011, you should perform the experiment SCR Speed Control - Part I included in Appendix D. EXERCISE OBJECTIVE C C C Completing this exercise will give you an introduction to thyristor speed controllers. You will learn how to control the speed of a DC motor by varying the armature voltage using a thyristor speed controller; You will also learn that thyristor speed controllers offer poor speed stability with varying loads in the open-loop mode of control. DISCUSSION In Experiment 3, you saw that the speed of a DC motor can be controlled from zero to maximum by varying the armature voltage while keeping the shunt field voltage constant. Thyristor speed controllers are specially designed for this application. The Thyristor Speed Controller of your training system contains a thyristor singlephase bridge rectifier. This type of rectifier operates on the same principle as a diode rectifier except that each thyristor begins to conduct only when a current pulse is injected into its gate. Once a thyristor begins to conduct, it continues to conduct until the current flowing through it becomes zero. Since the conduction can be initiated at any angle in the network sinewave between 0 and 180E, the average output voltage and therefore the average current can be varied between 0 and 100%. The resulting output waveform is a pulsated voltage. By controlling the firing angle of the thyristors, the voltage applied to the armature of the motor can be varied and its speed controlled. As Figure 11-1 shows, the front panel of the Thyristor Speed Controller shows many important characteristics about its operation: C C C C C Terminals L and N are referred to as the POWER INPUT. Q 1, Q 2, D 3 and D 4 make up the thyristor single-phase, full-wave bridge rectifier that converts the fixed AC-voltage into a variable DC-voltage. D 1, D 2, D 3 and D 4 make up the diode single-phase, full-wave bridge rectifier that converts the fixed AC-voltage into a fixed DC-voltage. D 5 is a free-wheeling diode required to ensure that the circuit can turn off an inductive load like the winding of a motor. Without this diode, when the gate pulses are stopped, the current may never drop to zero and a thyristor may continue to conduct. Terminals 1 and 2 are referred to as the variable DC-voltage output. The armature of the DC-motor connects to these terminals. Terminal 1 is positive with respect to terminal

97 Thyristor Speed Controller C C C C C C C C C C C C C C Terminals 3 and 4 are referred to as the fixed DC-voltage output. The DC motor shunt winding connects to these terminals. Terminal 3 is positive with respect to terminal 4. VOLTAE REFERENCE potentiometer allows for adjustment of the armature voltage. VOLTAE MINIMUM potentiometer allows for adjustment of the voltage reference lower limit. RUN selector allows the starting and the stopping of the DC motor. REFERENCE INTERATOR potentiometer allows for adjustment of the motor acceleration and deceleration. CURRENT FEEDBACK potentiometer allows for adjustment of the circuitry that controls the armature voltage to compensate for the voltage loss caused by the armature resistance. CURRENT LIMIT potentiometer allows the setting of the armature current maximum value to provide protection against overloads. COMPARATOR compares the current feedback to a current limit set by the user with the CURRENT LIMIT potentiometer. VOLTAE FEEDBACK potentiometer allows for adjustment of the voltage feedback amplitude. This makes it possible to use motors having different supply voltages by matching the voltage feedback to the voltage reference. The VOLTAE FEEDBACK potentiometer is also used to set an upper limit to the armature voltage. ERROR DETECTOR compares the voltage feedback signal to the sum of the voltage reference and current feedback signals. The ERROR DETECTOR produces an error signal equal to the difference. ERROR INTERATOR adjusts the integral gain of the error signal sent by the ERROR DETECTOR. The output signal of the ERROR INTERATOR is the control signal of the FIRIN CIRCUIT. FIRIN ANLE potentiometer allows manual adjustment of the firing angle of the thyristors. CLOSED LOOP selector allows selection of open-loop or closed-loop mode of control. FIRIN CIRCUIT detects when the thyristor voltage becomes positive, and controls the time delay before a current pulse is sent to the thyristor gate. The Thyristor Speed Controller is designed for both open-loop and closed-loop modes of control. In the open-loop mode of control, the armature voltage is set manually by controlling the firing angle of the thyristors using the FIRIN ANLE potentiometer. This allows the control of the motor speed using the FIRIN ANLE potentiometer. In the PROCEDURE of this Experiment, you will observe that the armature voltage decreases rapidly when the load applied to the motor increases, causing the motor speed to decrease. Therefore, the open-loop mode of control is not adequate to control the speed of a motor whose load varies. The decrease in voltage, when the current increases, is a characteristic of the pulsated voltage produced by thyristor rectifiers. 11-2

98 Thyristor Speed Controller Figure Thyristor Speed Controller. In the closed-loop mode of control, the firing angle of the thyristors is determined by the controller. In this mode of control, the armature voltage is measured and its value is sent back to an error detector to be compared with a voltage reference set by the user. The output signal of the error detector is sent to a firing circuitry to determine the firing angle of the thyristors to produce the correct armature voltage. However, even if the armature voltage is maintained constant when the load varies, the motor speed will still vary slightly because of the armature resistance. To compensate for the voltage drop produced by the armature resistance, the armature current is also measured. Its value is sent back to the error detector as a voltage corresponding to the armature current (I) multiplied by the armature resistance (R). 11-3

99 Thyristor Speed Controller The purpose of the closed-loop mode of control is to maintain a constant speed despite the load variations. Procedure Summary In the first part of the exercise, Setting up the Equipment, you will set up the equipment in the EMS Workstation. In the second part of the exercise, Motor Speed Versus Armature Voltage, you will vary the voltage applied to the armature of the DC Motor/enerator and observe how its speed varies. In the third part of the exercise, Motor Speed versus Load, you will vary the mechanical load applied to the DC Motor/enerator and observe how its speed is affected. In the last part of the exercise, Motor Speed versus Load with Constant Armature Voltage, you will vary the mechanical load applied to the DC Motor/enerator. At each load setting, you will adjust the armature voltage to maintain its value constant and observe how the motor speed is affected. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Install the DC Motor/enerator, DC Voltmeter/Ammeter, Power Supply, Electrodynamometer, and Thyristor Speed Controller modules in the EMS Workstation. 2. On the Power Supply, make sure the main power switch is set to the O (off) position, and the voltage control knob is turned fully counterclockwise. Ensure the Power Supply is connected to a three-phase power source. Do not couple the DC Motor/enerator to the Electrodynamometer with the timing belt now. 3. Connect the circuit shown in Figure

100 Thyristor Speed Controller Figure Motor Speed versus Armature Voltage 4. Set the Thyristor Speed Controller controls as follows: CLOSED LOOP selector... O RUN selector... O FIRIN ANLE... MIN. 5. On the Power Supply, set the main power switch to the I (on) position, and increase the voltage control knob to MAX. 6. On the Thyristor Speed Controller, adjust the FIRIN ANLE potentiometer so that the armature voltage increases as indicated in the following table. AC NETWORK VOLTAE INCREASE THE ARMATURE VOLTAE UP TO 120 V 100 V by increments of 10 V 220 V 200 V by increments of 20 V 240 V 200 V by increments of 20 V For each armature voltage setting, measure and record the armature current I A, field current I F, and motor speed in Table

101 Thyristor Speed Controller E A (V) I A (A) I F (ma) SPEED (r/min) Table Do your results confirm that the speed of the DC Motor/enerator can be controlled by varying the armature voltage using the Thyristor Speed Controller? Yes No 8. Do your results confirm that the DC Motor/enerator offers a wide speed range when controlling the armature voltage using the Thyristor Speed Controller? Yes No 9. Return the FIRIN ANLE potentiometer at MIN., and turn off the Power Supply. Motor Speed versus Load 10. Couple the DC Motor/enerator to the Electrodynamometer with the timing belt. On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading. Turn on the Power Supply. 11-6

102 Thyristor Speed Controller 11. On the Thyristor Speed Controller, set the FIRIN ANLE potentiometer to obtain the motor speed indicated in the following table. AC NETWORK VOLTAE MOTOR SPEED (r/min) 120 V V V Measure and record the armature voltage E A, armature current I A, field current I F, and motor speed in Table On the Electrodynamometer, adjust the dynamometer control knob so that the torque indicated on the module, increases by 0.1 N@m [1.0 lbf@in] increments up to 1.0 N@m [10.0 lbf@in]. For each load setting, record your data in Table LOAD SETTIN E A (V) I A (A) I F (ma) SPEED (r/min) Table On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading. Turn off the Power Supply. 14. Plot the graph of the Motor Speed as a function of Load in Figure

103 Thyristor Speed Controller 15. Do your results confirm that the Thyristor Speed Controller offers poor speed stability with varying loads in the open-loop mode of control? Yes No Motor Speed (r/min) Load Figure Motor Speed versus Load with Constant Armature Voltage 16. Turn on the Power Supply. 17. Make sure that the FIRIN ANLE potentiometer is still set as indicated in step 11. Adjust if necessary. 18. Record the armature voltage E A armature current I A field current I F, and motor speed in Table On the Electrodynamometer, adjust the dynamometer control knob so taht the torque indicated on the module, increases by 0.1 NAm [1.0 lbfain] increments up to 1.0 NAm [10.0 lbfain]. For each load setting, readjust the FIRIN ANLE potentiometer to maintain the armature voltage E A constant, then record your data in Table

104 Thyristor Speed Controller LOAD SETTIN E A (V) I A (A) I F (ma) SPEED (r/min) Table Do your results confirm that by maintaining the armature voltage constant when the load varies, the speed is maintained fairly constant with a small decrease caused by the armature resistance voltage drop? Yes No 20. On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading. 21. Turn off the Power Supply. CONCLUSION In this exercise, you were introduced to thyristor speed controllers. You learned how to control the speed of a DC motor by varying the firing angle of the thyristors. You plotted the speed versus load curve of the Thyristor Speed Controller operating in the open-loop mode of control, and observed that it offers poor speed stability with varying loads in this mode. In the last part of the exercise, you maintained the armature voltage constant when the load varied. You observed that the speed was fairly constant with a small decrease caused by the armature resistance voltage drop. 11-9

105 Thyristor Speed Controller REVIEW QUESTIONS 1. What is the purpose of the diode single-phase, full-wave bridge rectifier of the Thyristor Speed Controller? 2. What is the purpose of the thyristor single-phase, full-wave bridge rectifier of the Thyristor Speed Controller? 3. What is the purpose of the FIRIN ANLE potentiometer on the Thyristor Speed Controller? 4. Explain why the armature voltage decreases when the load applied to the motor increases in the open-loop mode of control

106 Experiment 12 Thyristor Speed Controllers with Regulation Note: If you are a user of the Thyristor (SCR) Speed Controller module, Model 9011, you should perform the experiment SCR Speed Control - Part II included in Appendix E. EXERCISE OBJECTIVE C C C C In this exercise, you will be introduced to thyristor speed controllers operating in the closed-loop mode of control; You will learn how the closed-loop mode of control regulates the motor speed by detecting the armature voltage and current; You will learn how to control the acceleration of the DC Motor/enerator; You will also learn how to limit the current and the torque of the DC Motor/enerator; DISCUSSION You have noticed in the previous Experiment that although the open-loop control of the DC Motor/enerator offered a wide speed range, it had poor speed stability with varying loads. The non-feedback speed controller consisted of a fixed field supply and a manually adjustable armature power supply. Speed regulation was not improved by the controller, when the load changed. However, you saw that by maintaining the armature voltage of the motor constant, it is possible to obtain a motor speed fairly constant even when the load varies. In this Experiment, you will experiment with the effects of feedback upon motor speed stability. You will learn how feedback, or closed-loop, control systems automatically maintain the armature voltage constant when the current increases. All feedback systems, where a quantity such as speed, torque, or temperature, is to be kept at a predetermined value, must have a reference level against which the quantity can be compared. The voltage reference level, on the Thyristor Speed Controller, is set using the VOLTAE REFERENCE potentiometer. It determines the armature voltage which called for a specific motor speed. The lower limit of the voltage reference can be set using the VOLTAE MINIMUM potentiometer. The voltage reference signal is sent to an integrator where the time taken by the control signal to be fully applied to the motor can be set using the REFERENCE INTERATOR potentiometer. This allows smooth acceleration and deceleration of the motor. The acceleration time of the Thyristor Speed Controller can be set between 0.5 and 8 s, and the deceleration time can be set between 0.06 and 0.8 s. The voltage reference is then sent to the ERROR DETECTOR to be compared with the feedback signals. 12-1

107 Thyristor Speed Controllers with Regulation In the closed-loop mode of control, the Thyristor Speed Controller senses the armature voltage (voltage feedback) and armature current (current feedback). The controller uses these feedback signals to regulate the motor speed when the load varies. The voltage feedback signal is sent to the ERROR DETECTOR to be compared with the voltage reference. The difference, or error signal, between the voltage feedback and the voltage reference can then be used to "tell" the FIRIN CIRCUIT whether it should increase the armature voltage, or reduce it. Before it is sent to the ERROR DETECTOR, the voltage feedback amplitude is set using the VOLTAE FEEDBACK potentiometer. The voltage feedback amplifier allows the use of motors having different supply voltages by matching the voltage feedback and the voltage reference. Setting the upper limit of the armature voltage by controlling the gain of this amplifier is also possible. The current feedback signal is sent to the CURRENT FEEDBACK potentiometer and to the COMPARATOR. The CURRENT FEEDBACK potentiometer allows the setting of the IR compensation that corresponds to the voltage loss caused by the armature resistance. The current feedback is then sent to the ERROR DETECTOR as a voltage corresponding to the armature current (I) multiplied by the armature resistance (R). The COMPARATOR compares the current feedback to a current limit set by the user with the CURRENT LIMIT potentiometer. The purpose of the current limiter and COMPARATOR is to set an armature current limit to prevent damage to the motor and the controller circuitry, and to limit the torque developed by the motor to prevent damage to the component driven by the motor. The output signals of the COMPARATOR and ERROR DETECTOR are sent to the ERROR INTERATOR before they are sent to the FIRIN CIRCUIT. Procedure Summary In the first part of the exercise, Setting up the Equipment, you will set up the equipment in the EMS Workstation. In the second part of the exercise, Maximum Speed Setting, you will use the VOLTAE FEEDBACK potentiometer to set the gain of the voltage feedback amplifier to set the upper limit of the motor speed. In the third part of the exercise, Minimum Speed Setting, you will use the VOLTAE MINIMUM potentiometer to set the lower limit of the motor speed. In the fourth part of the exercise, Motor Speed versus Load in Closed Loop, without IR Compensation, you will determine and plot the speed versus load curve from the data obtained in closed loop without IR compensation. You will compare this curve to the one obtained in the open-loop mode of control. In the fifth part of the exercise, Motor Speed versus Load in Closed Loop, with IR Compensation, you will determine and plot the speed versus load curve from the data obtained in closed loop with IR compensation. You will compare this curve with the one obtained without IR compensation. 12-2

108 Thyristor Speed Controllers with Regulation In the sixth part of the exercise, REFERENCE INTERATOR Setting, you will experiment with the acceleration control of the Thyristor Speed Controller. In the last part of the exercise, Current Limit, you will experiment with the current limiter of the Thyristor Speed Controller. You will observe that the torque developed by the DC Motor/enerator can be limited by limiting the armature current. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE CAUTION! High voltages are present in this Experiment! do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Install the DC Motor/enerator, DC Voltmeter/Ammeter, Power Supply, Electrodynamometer, and Thyristor Speed Controller modules in the EMS Workstation. 2. On the Power Supply, make sure the main power switch is set to the O (off) position, and the voltage control knob is turned fully counterclockwise. Ensure the Power Supply is connected to a three-phase power source. Couple the DC Motor/enerator to the Electrodynamometer with the timing belt. 3. Connect the circuit shown in Figure Maximum Speed Setting 4. On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading. 12-3

109 Thyristor Speed Controllers with Regulation Figure Set the Thyristor Speed Controller controls as follows: VOLTAE REFERENCE potentiometer... MAX. VOLTAE MINIMUM potentiometer... MAX. RUN selector... I REFERENCE INTERATOR potentiometer... MIN. CLOSED LOOP selector... I CURRENT FEEDBACK potentiometer... MIN. CURRENT LIMIT potentiometer... MAX. 6. On the Power Supply, set the main power switch to the I (on) position, and increase the voltage control knob to MAX. 7. On the Thyristor Speed Controller, set the VOLTAE FEEDBACK potentiometer to obtain the motor speed indicated in the following table. AC NETWORK VOLTAE MAXIMUM MOTOR SPEED (r/min) 120 V V V 1000 Minimum Speed Setting 8. On the Thyristor Speed Controller, set the VOLTAE REFERENCE potentiometer at MIN. 12-4

110 Thyristor Speed Controllers with Regulation Rotate slowly the VOLTAE MINIMUM potentiometer counterclockwise until the motor stops. Note: The lower speed limit is now set at 0 r/min. 9. Vary the VOLTAE REFERENCE from MIN. to MAX. Measure the motor speed at each of these positions. Minimum motor speed = Maximum motor speed = r/min r/min Note: It may necessary to readjust the maximum speed setting using the VOLTAE FEEDBACK potentiometer once the minimum speed setting is completed. Motor Speed versus Load in Closed Loop, without IR Compensation 10. On the Thyristor Speed Controller, set the VOLTAE REFERENCE potentiometer to obtain the motor speed indicated in the following table. AC NETWORK VOLTAE MOTOR SPEED (r/min) 120 V V V Record the armature voltage E A, armature current I A, field current I F, and motor speed in the Without IR COMP. columns of Table On the Electrodynamometer, adjust the dynamometer control knob so that the torque indicated on the module, increases by 0.1 N@m [1.0 lbf@in] increments up to 1.0 N@m [10.0 lbf@in]. For each load setting, record your data in Table

111 Thyristor Speed Controllers with Regulation LOAD SETTIN Without IR COMP. E A (V) I A (A) I F (ma) SPEED (r/min) With IR COMP. Without IR COMP. With IR COMP. Without IR COMP. With IR COMP. Without IR COMP. With IR COMP. Table Use your data to plot the graph of the Motor Speed as a function of Load without IR compensation in Figure Compare the Motor Speed versus Load curves you plotted in Figure Do the curves confirm that the closed-loop control system helps in maintaining the motor speed constant as the load varies? Yes No 14. Compare the armature voltage and speed values with those obtained in the open-loop mode of control when you maintained the armature voltage constant manually (Table 11-2). What can you conclude from your comparison? 12-6

112 Thyristor Speed Controllers with Regulation Motor Speed versus Load in Closed Loop, with IR Compensation IR Compensation Setting 15. On the Thyristor Speed Controller, set the CURRENT FEEDBACK potentiometer to obtain the motor speed indicated in step 10. On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading, then measure the motor speed. If the motor speed with and without load is different, readjust the current feedback level using the CURRENT FEEDBACK potentiometer to obtain the same speed "with and without" load. Note: The IR compensation is now set to compensate for the voltage loss caused by the armature resistance. Once the current feedback level is correctly set, adjust the VOLTAE REFERENCE potentiometer to obtain the motor speed indicated in step 10. Motor Speed versus Load Characteristics 16. For each load settings listed in Table 12-1, measure and record the armature voltage E A, armature current I A, field current I F and motor speed in the With IR COMP. columns. 17. Plot the graph of the Motor Speed as a function of Load with IR compensation in Figure Compare the speed versus load curves you plotted in Figure 11-3 with and without IR compensation. Do the curves confirm that the IR compensation improves the speed regulation of the motor? Yes No REFERENCE INTERATOR Setting 19. On the Electrodynamometer, set the dynamometer control knob at its full counterclockwise position for minimum loading. 20. On the Thyristor Speed Controller, set the VOLTAE REFERENCE potentiometer at MAX., and position the RUN selector at O. Ensure that the REFERENCE INTERATOR potentiometer is at MIN. Position the RUN selector at I while observing the time the DC Motor/enerator takes to run at maximum speed. 12-7

113 Thyristor Speed Controllers with Regulation Note: The breaker on the Power Supply and/or on the Thyristor Speed Controller may trip when performing this manipulation. Then, turn off the Power Supply, reset the breaker(s). Turn the REFERENCE INTERATOR potentiometer clockwise c of a turn, turn on the Power Supply and continue the manipulation. 21. Does the DC Motor/enerator start to run and attain maximum speed very rapidly? Yes No 22. Repeat your observation when the REFERENCE INTERATOR potentiometer is set at MAX. To do so, position the RUN selector at O and set the REFERENCE INTERATOR potentiometer at MAX. Position the RUN selector at I while observing the time the DC Motor/enerator takes to run at maximum speed. 23. Is the acceleration of the motor smooth? Yes No 24. Do your observations confirm that the REFERENCE INTERATOR potentiometer allows the control of motor acceleration? Yes No 25. Repeat your observations with different REFERENCE INTERATOR potentiometer settings. Current Limit 26. Set the Thyristor Speed Controller controls as follows: RUN selector... I REFERENCE INTERATOR potentiometer... MAX. CURRENT LIMIT potentiometer... MIN. Set the VOLTAE REFERENCE potentiometer to obtain the motor speed indicated in step Record the armature voltage E A, armature current I A, field current I F, and motor speed in Table On the Electrodynamometer, adjust the dynamometer control knob so that the torque indicated on the module, increases by 0.1 N@m [1.0 lbf@in] increments up to 1.0 N@m [10.0 lbf@in]. 12-8

114 Thyristor Speed Controllers with Regulation After each load setting, wait 10 s then record your data in Table12-2. LOAD SETTIN E A (V) I A (A) I F (ma) SPEED (r/min) Table Were you able to increase the load to the value indicated in the following table? AC NETWORK VOLTAE MOTOR TORQUE 120 V 1 N@m [10 lbf@in] 220 V 1 N@m 240 V 1 N@m Yes No 29. Does this confirm that the torque developed by the DC Motor/enerator can be limited using the current limiter function of the Thyristor Speed Controller? Yes No 30. Repeat your observations with different CURRENT LIMIT potentiometer settings. 31. Once your observations are completed, set the dynamometer control knob at its full counterclockwise position for minimum loading. 12-9

115 Thyristor Speed Controllers with Regulation 32. Turn off the Power Supply. CONCLUSION In this exercise you were introduced to thyristor speed controllers operating in the closed-loop mode of control. You saw that the Thyristor Speed Controller detects the armature voltage and current to determine the motor speed and torque, and to regulate the speed as the load varies. You plotted the speed versus load curves from the data measured in closed loop with and without IR compensation, you compared these curves to the one obtained in open-loop mode of control, and observed that the closed-loop mode of control improves speed regulation as the load varies. You also learned how to control the acceleration, and how to limit the torque developed by the DC Motor/enerator. REVIEW QUESTIONS 1. Explain how the closed-loop control improves speed regulation as the load varies. 2. What is the purpose of the VOLTAE REFERENCE potentiometer on the Thyristor Speed Controller. 3. What is the purpose of the REFERENCE INTERATOR of the Thyristor Speed Controller? 12-10

116 Thyristor Speed Controllers with Regulation 4. What is the purpose of the CURRENT LIMIT potentiometer on the Thyristor Speed Controller? 12-11

117 Appendix A Equipment Utilization Chart MODEL EQUIPMENT 1 EXPERIMENT ELECTRICAL POWER TECHNOLOY Mobile Workstation DC Motor/enerator Three-Phase Synchronous Motor/enerator Resistive Load DC Voltmeter/Ammeter AC Ammeter AC Voltmeter Manual DC Motor Starter Power Supply ² Electrodynamometer Digital Tachometer Timing Belt Connection Leads Thyristor Speed Controller The module storage facilities Storage Cabinet have not been included in this chart 2 The Electrodynamometer Module EMS 8911 may be replaced by a Prony Brake Module EMS A-1

118 Appendix B Impedance Table for the Load Modules The following table gives impedance values which can be obtained using either the Resistive Load, Model 8311, the Inductive Load, Model 8321, or the Capacitive Load, Model Figure B-1 shows the load elements and connections. Other parallel combinations can be used to obtain the same impedance values listed. 120 V 60 Hz IMPEDANCE (Ω) 220 V 50 Hz 240 V 50 Hz SWITCH POSITIONS FOR LOAD ELEMENTS I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I Table B-1. Impedance table for the load modules. B-1

119 Impedance Table for the Load Modules (cont'd) Figure B-1. Location of the load elements. B-2

120 Appendix C Performing the Electrical Power Technology Courseware Using the Lab-Volt Data Acquisition and Management System The exercises in the Electric Power / Controls courseware have been designed to be performed using conventional instruments (AC/DC voltmeters and ammeters, power meters, etc). All these exercises can also be carried out using the Lab-Volt Data Acquisition and Management (LVDAM) System. The LVDAM System consists of the Data Acquisition Interface (DAI) module, model 9062, and the corresponding LVDAM software. The system includes a user manual (p.n EX) designed to familiarize users with the operation of the LVDAM System. The Electrodynamometer (model 8911) and Precision Hand Tachometer (model 8920) are replaced in the LVDAM System by the Prime Mover / Dynamometer module (model 8960). In some exercises, the Prime Mover / Dynamometer module can also replace the Synchronous Motor/enerator (model 8241) to drive rotating machines mechanically. Refer to the manual titled AC/DC Motors and enerators (p.n ) to familiarize yourself with the operation of the Prime Mover / Dynamometer module. When performing the Electric Power / Controls courseware with the LVDAM System, the following guidelines should be taken into account: The "AC and DC voltmeters" are implemented using the high-voltage inputs E1, E2, and E3 of the DAI module. The voltage values are displayed on meters E1, E2, and E3 in the Metering application of the LVDAM System. The "AC and DC ammeters" are implemented using the high-current inputs I1, I2, and I3 of the DAI module. The current values are displayed on meters I1, I2, and I3 in the Metering application of the LVDAM System. The "Single-Phase Wattmeter" (model 8431) is implemented using one highvoltage input combined with one high-current input. This can be done using the inputs E1 with I1, E2 with I2, and E3 with I3 of the DAI module. The power values are displayed on meters PQS1, PQS2, and PQS3 in the Metering application of the LVDAM System. Note that the voltage and current values used in the power measurements can be displayed on the voltage and current meters in the Metering application of the LVDAM System. This is a useful feature which, in certain cases, can reduce the number of inputs required to measure the various parameters in a circuit. As an example, in a circuit where the line-to-line voltage is measured using input E1, it would be a wise choice to use inputs E1 and I1 to measure single-phase power in this circuit. This would prevent the line-to-line voltage from being measured twice and using two voltage inputs. The "Three-Phase Wattmeter" (model 8441) is implemented using two "wattmeters" (each of them being implemented using one high-voltage input combined with one high-current input). This can be done using inputs E1 with I1 to produce P1, E2 with I2 to produce P2, and E3 with I3 to produce P3. The C-1

121 Performing the Electrical Power Technology Courseware Using the Lab-Volt Data Acquisition and Management System (cont'd) power values are displayed on meters PQS1, PQS2, or PQS3 in the Metering application of the LVDAM System. As an example, using inputs E1 with I1 and E3 with I3, and selecting functions PQS1 and PQS3, allow one to measure W1 and W2 present on the Three-Phase Wattmeter. EXERCISES WHERE THE NUMBER OF AVAILABLE INPUTS IS EXCEEDED In some exercises of the Electric Power / Controls courseware, four or five highcurrent inputs are required to perform all current measurements. Unfortunately, only three high-current inputs are available in the LVDAM system. In the exercises where it is asked to measure the currents I F, I A, and the line currents I 1, I 2, and I 3, the number of available inputs is exceeded. Since the line currents are usually measured only to make sure that the motor operation is normal, you may consider measuring only one line current (or all but one at the same time). In some exercises, the implementation of the Single-Phase Wattmeter, or Three- Phase Wattmeter, using high-voltage and high-current inputs also causes the number of available inputs to be exceeded. The exercises where the number of available inputs is exceeded are listed below. A solution is also suggested for each case. The DC Separately-Excited Shunt enerator In the circuits of Figures 1, 2 and 3, use input I1 to measure current I F, input I2 to measure current I A, and input I3 to measure line current I 3. The DC Self-Excited Shunt enerator In the circuits of Figures 1 and 2, use input I1 to measure current I A, and inputs I2 and I3 to measure line currents I 2 and I 3. The DC Compound enerator In the circuits of Figures 1 and 2, use input I1 to measure current I A, and inputs I2 and I3 to measure line currents I 2 and I 3. The DC Series enerator In the circuits of Figures 4 and 5, use input I1 to measure current I A, and inputs I2 and I3 to measure line currents I 2 and I 3. Transformers in Parallel In the circuit of Figure 1, the implementation of the Single-Phase Wattmeter using one high-voltage input and one high-current input from the LVDAM System increases the number of required high-current inputs to four. Use a programmable meter programmed to calculate I1 + I2 to measure the load current I L (I 1 + I 2 = I L in this circuit). C-2

122 Performing the Electrical Power Technology Courseware Using the Lab-Volt Data Acquisition and Management System (cont'd) Frequency Conversion In the circuits of Figures 1 and 2, the implementation of the Three-Phase Wattmeter using two high-voltage inputs and two high-current inputs from the LVDAM system increases the number of required high-current inputs to four. Use inputs E1-I1 and E3-I3 to implement the Three-Phase Wattmeter. In step 3d, use temporarily input I3 (currently used in the power measurement circuit) to adjust the DC excitation of the synchronous motor. Once the adjustment is completed, return input I3 in the power measurement circuit. C-3

123 Appendix D SCR Speed Control Part I OBJECTIVE To show the operation of an open-loop electronic variable speed control for a DC motor. DISCUSSION In Experiment 3, it was seen that the speed of a DC motor could be controlled from zero to maximum by varying the armature voltage, while keeping the shunt field voltage constant. Controllers for DC shunt motors often use silicon controlled rectifiers as the controlled element for varying the power applied to the motor. The silicon controlled rectifier (SCR), is a solid state device whose function is analogous to that of the grid controlled thyratron tube. It will pass current in only one direction; hence, it rectifies. In addition, it will turn on and pass current only upon receipt of a trigger signal on a control electrode called the gate. Once turned on, an SCR will continue to conduct until the voltage is removed or the polarity of the voltage across it is reversed. When an SCR is used to rectify alternating current, the point during the positive halfcycle of the input current at which the rectifier is turned on, can be adjusted by the timing of the application of a trigger signal to the gate. At the end of the positive halfcycle, the SCR will turn off as the polarity of the applied voltage reverses. By controlling the phase relationship of the trigger signal to the zero axis crossing of the positive half-cycle of alternating current, the amount of power transmitted through the SCR can be varied. This is called phase control. While there are a number of different types of electronic speed controllers for use with DC shunt motors, all of them have in common the conversion of alternating current to pulsating unidirectional current, using half-wave, or full-wave rectification. Both of these currents depart considerably from steady direct current. The measure of the departure of a pulsating, unfiltered, unidirectional current of this nature from a steady direct current is called a form factor. Pure direct current has a form factor of one. The form factor of unidirectional currents is determined by the rms value of the current divided by the average value of the current. For half-wave, unfiltered current, the form factor is 1.57; for full-wave, unfiltered current, the form factor is Capacitor or inductor-capacitor filters can be used to improve the form factor. In considering speed controllers for DC shunt motors, the form factor of the DC supplied to the motor is of considerable importance. When operating from rectified power, the increase in motor heating is approximately proportional to the square of the form factor. For example, a motor operating from unfiltered half-wave rectified current with a from factor of 1.57 will have approximately 2 ½ times the heat rise of the same motor operating on unity form factor DC. In addition to the I 2 R losses, there D-1

124 SCR Speed Control Part I are losses in the motor frame and pole pieces, due to pulsating flux produced by intermittent high peak currents. A second consideration is brush and commutator life. When operating from high form factor current, high peak currents are required to maintain average current input for a given power output, thus accelerating brush and commutator wear. High form factor adversely affects motor operation at low speeds. The current pulse repetition rate, using half-wave rectification, is 60 pulses per second. Using full-wave rectification, it is 120 pulses per second. At low speeds, these pulsating currents reinforce the motor s tendency to cog (have non-uniform velocity). Therefore, smooth, low-speed operation becomes impractical. The Thyristor (SCR) Speed Controller has the following features: a) It will operate from a 120 V, alternating current source. b) It rectifies the alternating current, changing it to direct current. c) The DC armature current can be varied by advancing or retarding the firing angle of the SCR. (It also employs a free-wheeling diode circuit to establish a constant magnetic field.) d) A phase-shift circuit comprising a capacitor ad a variable resistance, permits a change in the SCR firing angle from zero to approximately 150 E. e) It can be adapted for either open-loop (no feedback) or closed-loop (with feed-back) control. Major System Components It is possible to gain a reasonable understanding of the major parts of the Thyristor (SCR) Speed Controller by referring to the schematic diagram on the face of the module. In moving from left to right, the following reference numbers and components may be identified. 1. Transformer T 1 is an autotransformer which changes the 120 V ac, input (points 2 and 1) to 200 V ac (points 3 and 1). The transformer is center-tapped (point 4) giving 100 V ac between points 4 and 1 or 4 and Capacitor C 1 and Rheostat R 1. As the resistance of R 1 is varied, the phase angle of the voltage between points 4 and 5 changes from zero (R 1 at minimum resistance) to approximately 150 E lag (R 1 at maximum resistance). 3. Transformer T 2. The voltage between points 4 and 5 is applied to the primary winding of transformer T 2. The stepped-down secondary voltage of T 2 appears between points 7 and 9. As rheostat R 1 is varied, the phase angle of the secondary voltage of T 2 (points 7 and 9) changes from zero to approximately 150 E with respect to the 200 V output (points 1 and 3) of autotransformer T Diode D 1 and Potentiometer R 2 are part of a DC reference voltage supply. Potentiometer R 2 allows the reference voltage (between points 6 and 1) to be varied from 0 to 140 V dc. This circuit is only required on closed-loop control studies. D-2

125 SCR Speed Control Part I 5. Reactance X L is a filter choke to provide smoother operation of the DC motor. It also tends to prevent heavy surges of armature current. The choke is located between points 3 and Silicon controlled Rectifier (SCR). The anode, cathode and gate of the SCR correspond respectively to points 10, 11 and 9. The AC voltage across the secondary of transformer T 2 (points 7 and 9) causes the gate (point 9) of the SCR to trigger the SCR on earlier or later in the cycle, depending upon the phase shift which is controlled by the setting of rheostat R 1. For open-loop control, point 7 is connected to the cathode (point 11) of the SCR. 7. The Armature A. The armature of the DC motor is connected between points 1 and 11. Point 11 is positive with respect to point 1 which is at ground potential. 8. Capacitor C 2 (between points 8 and 1) is an electrolytic filter capacitor that can be connected across the armature winding of the DC motor by joining points 8 and 11. This will result in smoother motor operation because the capacitor will discharge through the armature winding during the periods when the SCR is not conducting. The motor periods when the SCR is not conducting. The motor will vibrate less and run cooler because the capacitor, rather than the armature, will absorb the current peaks during each cycle. 9. Diodes D 2, D 3 and the Shunt Field. The shunt field of the DC motor is connected between points 12 and 1. The action of diodes D 2 and D 3 is such that the field current is kept nearly constant. The DC voltage between points 12 and 1 should be approximately 45% of the AC voltage (points 3 and 1) applied to the freewheeling circuit. EQUIPMENT REQUIRED DESCRIPTION PART NUMBER DC Motor/enerator 8211 DC Voltmeter/Ammeter 8412 Power Supply 8821 Electrodynamometer 8911 Digital Tachometer 8920 Timing Belt 8942 Connection Leads 8951 Thyristor (SCR) Speed Controller 9011 PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Using your Thyristor (SCR) Speed Controller, DC Motor/enerator, Electrodynamometer, DC Voltmeter/Ammeter, AC Ammeter and AC Voltmeter, connect the circuit shown in Figure The reference numbers D-3

126 SCR Speed Control Part I appearing on the schematic diagram refer to the numbered points of the SCR speed control unit. Figure D-1. a. Connect the armature of the DC motor, in series with the ammeter, to points 11 and 1. b. Connect the shunt field of the DC motor, in series with the milliammeter, to points 12 and 1. c. Connect the V dc voltmeter across the armature power source, points 11 and 1. d. Connect points 7 and 11 together so that the triggering signal from the secondary of T 2 can be applied to the gate of the SCR. e. Set rheostat R 1 at its full ccw position for maximum resistance. f. Short out reactance X L by connecting a lead between points 3 and 10. g. Connect the input of the Thyristor (SCR) Speed Controller to any convenient source of 120 V ac power. 2. a. Turn on the power; the indicating lamp should light. D-4

127 SCR Speed Control Part I b. Vary the position of rheostat R 1 and note the effect upon motor speed and armature voltage. c. Adjust R 1 until the armature voltage E A = 90 V dc. Measure and record in Table D-1 the armature current, field current and motor speed. E A (volts) I A (amps) I 1 (amps) SPEED (r/min) Table D1. d. Repeat (c) for each of the armature voltages listed in Table D-1. e. At low motor speed, does the SCR conduct early or late in the cycle? f. What is the highest motor speed you can attain? Speed = r/min g. Turn off the power. 3. a. Couple the motor to the electrodynamometer with the timing belt. b. Set the dynamometer control knob at its full ccw position for minimum loading. c. Set rheostat R 1 at its full ccw position for maximum resistance. d. Turn on the power and adjust the speed control rheostat R 1 and the dynamometer control for a motor speed of 1500 r/min at a load of 0.7 NAm [6 lbfain]. e. Measure and record the armature voltage, armature current and field current. E A = V dc, I A = A dc, I F = A dc f. Is there appreciable sparking at the brushes? Yes No D-5

128 SCR Speed Control Part I g. Is there appreciable motor vibration? Yes No h. What is the frequency of the vibration in Hz? 4. a. Without altering the position of rheostat R 1, reduce the dynamometer loading to its minimum value by turning the control knob to its full ccw position. b. Measure and record the armature voltage, armature current, field current and motor speed. E A = V dc, I A = A dc, I F = A dc. Motor speed = r/min c. Turn off the power. d. Is there a large speed change from full-load torque to no-load torque? Yes No e. Might this variation in motor speed be objectionable in some cases? Explain. Yes No 5. You will now repeat procedures 3 and 4, using the LC filter in your SCR speed control unit. a. To place the filter choke in the circuit, remove the shorting lead between points 3 and 10. b. To place the filter capacitor in the circuit, connect a lead between points 8 and 11. (The other side of C 2 is permanently connected to point 1). c. Repeat procedure 3 and record the armature voltage, armature current and field current. E A = V dc, I A = A dc, I F = A dc D-6

129 SCR Speed Control Part I d. Is there as much vibration as before? Yes No e. Is there as much sparking at the brushes? Yes No f. Repeat procedure 4 and record the armature voltage, armature current, field current and motor speed. E A = V dc, I A = A dc, I F = A dc Motor speed = r/min g. Turn off the power. REVIEW QUESTIONS 1. Comment on the differences in motor operation with filtered and unfiltered DC power. 2. Can you comment on one of the faults of open-loop speed control as evidence by performing this Experiment? 3. If C 1 = 2µF, what value of R 1 would be required for a phase shift of 90E? D-7

130 Appendix E SCR Speed Control Part II OBJECTIVE To show the operation of a closed-loop electronic variable speed control for a DC motor. DISCUSSION You may have noted that, although the open-loop electronic control of the DC motor of Appendix D offered a wide speed range, it had poor stability with varying loads. In this Experiment, you will learn the effects of feedback upon motor speed stability. Feedback, or closed-loop, control systems give far better performance than openloop systems. The non-feedback speed controller consisted of a field supply and a manually adjustable armature power supply. Speed regulation as the motor load changes is not improved by the controller. The feedback speed controller can be adjusted to provide the desired motor speed. It is provided with circuitry which senses motor speed and automatically adjusts its output power to maintain motor speed constant as the load varies. All feedback systems where a quantity such as peed, torque, temperature, etc., is to be kept at a predetermined value, must have a reference against which the quantity can be compared. Thus, if we want to keep the speed of a motor constant, the speed must be compared with a reference. However, it is cumbersome to compare a speed with a reference speed, particularly in systems which are basically electrical. For this reason, we prefer to use an electrical quantity (such as a voltage which is directly related to the speed) and compare it with a reference voltage. The difference or error between the measured voltage (proportional to the motor speed) and the reference voltage can then be used to tell the system whether it should increase the speed or reduce it, in order to bring it back as close as possible to the reference value. E-1

131 SCR Speed Control Part II EQUIPMENT REQUIRED DESCRIPTION PART NUMBER DC Motor/enerator 8211 DC Voltmeter/Ammeter 8412 Power Supply 8821 Electrodynamometer 8911 Digital Tachometer 8920 Timing Belt 8942 Connection Leads 8951 Thyristor (SCR) Speed Controller 9011 PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! 1. Using your Thyristor (SCR) Speed Controller, DC Motor/enerator, Electrodynamometer, DC Voltmeter/Ammeter and Power Supply, connect the circuit shown in Figure E-1. (This is the same circuit used in the previous Experiment, where the speed control unit operated without feedback). Figure E-1. E-2