ENSC387: Introduction to Electromechanical Sensors and Actuators LAB 5: DC MOTORS WARNING:

Transcription

1 ENSC387: Introduction to Electromechanical Sensors and Actuators LAB 5: DC MOTORS WARNING: Please be extremely cautious to precisely follow the procedures described in this manual. It is very easy to break this setup by applying too much current. Please read the entire manual first before attempting anything.

2 1 Objective Required Materials Introduction Lab procedure Important Notes Regarding the Lab Setup & Equipment Operation Setup Using the tachometer to measure rpm Polarity reversal of armature and field motor windings Speed control Speed-torque characteristics Armature Control Field Control Armature Resistance Control Shunt DC Motor Control of 9

3 1 Objective In this experiment, we will examine some key characteristics of a DC motor. We will use an SCR (silicon controlled rectifier) DC drive controller to operate a DC motor, a hand-held tachometer to measure the motor speed, AC and DC ammeters to measure the AC power-line current and motor current, and finally, a Prony brake (a simple device invented by Gaspard de Prony) to mechanically load and measure motor torque. 2 Required Materials Description Photo Description Photo Hioki 3262 Clamp-on Ammeter SAF SCR (Silicon Controlled Rectifier) Motor Speed Controller DC Motor with Prony brake and connector box Tachometer Monarch Instruments The items depicted above are additional to the standard compliment of test equipment provided at the selected Lab-1 workstation. This station shall have a King Instruments DPS-1306AF highpower 6A, 2x0-30VDC laboratory power supply with both a red and black test lead. Other related lab support documents can be found in the 387 course support folder on the Lab1 Computers: Lab1 Computers > Windows Desktop > Reference Materials > Course Specific ENSC387 3 of 9

4 3 Introduction In engineering applications involving DC motors, it is necessary to know the torque/speed relations so that the right motor can be selected to match the operational requirements. In the following discussions, we are assuming the use of a separately excited motor. In class, we derived the following important equations for such a DC motor: The above equation states that the motor torque is directly proportional to the armature current for a fixed field flux (field current). The back emf (a.k.a. counter emf or cemf) generated by the armature (V b ) is directly proportional to the rotational speed of the motor (ω m ) and field current. On rearranging the above equation and on substitution, we get: (1.1) (1.2) Where: (1.3) T m - motor mechanical torque k, k - motor constants i a V b V t R a i f ω m - armature current - induced armature voltage - terminal voltage (across armature) - armature resistance - field current - angular speed of the motor (rad/s) The above equation implies that as the load torque increases, the motor speed decreases causing the back emf (E) to decrease as well. Provided that the power supply can handle the load and maintain a constant voltage to the armature, the armature current should increase, causing the motor torque to also increase until it is equal to the load torque. Thus, an applied load torque results in an increased armature current, and in a reduction in the motor speed for a constant applied voltage. Based on this equation, we see that there are 3 variables which we can manipulate to control the speed of a DC motor: 1. Terminal voltage 2. Armature resistance 3. Field flux (field current) Sometimes it is more convenient to drive a DC motor with a single power supply (by connecting the field winding directly across the armature input terminals) a shunt motor is created. In order to control the speed of such a motor, we can vary the armature voltage, but we no longer have the clear linear relationship between V t and motor speed. The new relationship is as shown in equation (2.1) ENSC387 4 of 9

5 Where k is the motor constant and linking current and torque and k is the motor constant linking speedof-rotation and back-emf created across the armature. We notice that for a fixed terminal voltage V t, the motor speed is a linear function of torque, and for non-zero T m and V t the motor speed increases with increasing V t. For detailed derivation of this formula, please refer to the textbook. (2.1) 4 Lab procedure 4.1 Important Notes Regarding the Lab Setup & Equipment Operation SAF Drive Systems SCR Motor Speed Controller Power the SAF controller from a 115VAC circuit that is not shared with other workstations. Should an accidental AC circuit overload occur in the course of your experiments, neighbouring workstations will not suddenly lose their power. Workstation and AC circuit assignments will be determined by a Lab Technician. Do not move experiment equipment to another workstation; a technician must do this! To ensure your safety and prevent damage to equipment; a) Be certain that SAF SCR controller is switched OFF and the SPEED control set to 0 before making or adjusting any electrical connections. b) Always monitor the AC current while operating the speed controller. Do this with the clamp-on ammeter as described below. NEVER EXCEED 6A!!! Use the bench DMM to monitor the voltage applied to the motor at the D.C. output leads of the SCR motor speed controller. Blown fuses and Circuit Breakers will Quickly End Your Lab Session For safety reasons, most lab test equipment signal-grounds (esp. oscilloscope, signal generator) are connected directly to power line earth ground. This severely restricts your measurement methods and can cause extensive electrical damage or injury. Do not connect any test equipment unless specified in this document. The SAF SCR controller output is not isolated from the AC powerline. To Start the SAF SCR Motor Speed controller a) Set the Speed control to 0 (fully CCW). b) Set FORWARD/REVERSE switch to FORWARD. c) Power switch to ON. ENSC387 5 of 9

6 d) Depress the START switch (the motor should not rotate). e) Slowly turn the SPEED control clockwise. The motor shaft should start rotating in the desired direction. f) Press STOP. The motor shaft should stop quickly under dynamic braking conditions. Clamp-On Ammeter The Hioki 3262 clamp-on ammeter 1 is needed to detect the current drawn from the 115VAC power line by the SAF Drive Systems control box. Locate the black wire loop protruding from the box. Open the jaws of the ammeter and slip one jaw through the wire loop, then close the jaws again. Switch the meter ON at its side slide switch. Please switch the Ammeter off again upon completion of your lab session. Using the King Instruments DPS-1306AF Power Supply 1 for Shunt Mode Motor Operation Adhere to the following procedure steps; Agilent 34405A DMM a) Loosen the Prony brake on the motor. b) Before connecting the power supply leads to the motor, set the supply for SERIES- TRACKING operation. c) Configure the 2 power supply leads; Master = red terminal = red lead, Slave = black terminal = black lead. Power ON. Set both Master and Slave current controls to 1/8 turn CW. Adjust the Master voltage to indicate around 5V/channel = 10V total. Short the leads and adjust the current limit to 1/2A (this is more than adequate for Field winding excitation). Rotate the Master Voltage to minimum (fully CCW). d) Connect the power supply leads to the motor (Field only) and slowly adjust the voltage to the specified value using the supply meters and/or DMM. This DMM s auto-ranging feature is ineffective for this motor lab. Manually set range to 100V scale. 4.2 Setup Set up the DC motor in the separately excited mode by connecting a 60V source to the field winding. Use ONLY the King Instruments DPS-1306AF lab power supply for this purpose. Ensure that the power supply is configured for SERIES TRACKING mode in order to get 60V from the two 30V supplies. Using the SAF SCR motor controller, connect 40V to the armature winding with the Prony brake set to approximately 0 load torque. 1 Equipment manuals are available on all Lab1 computers. Login > Windows Desktop > Reference Material folder > Equip Manuals. ENSC387 6 of 9

7 CRITICAL WARNINGS!!! Serious damage to the King Instrument DPS-1306AF will happen instantly due to the harsh inductive properties of the motor and its back-emf. Always soft-control the power supplies to energize or de-energize the load by gradually increasing or decreasing the power supply Voltage/Current controls. DO NOT use the DPS-1306AF power switch to perform this function. DO NOT OPERATE THIS DC MOTOR WITHOUT A FIELD CURRENT. Don t allow the motor to overheat beyond the slightly-warm-to-the-touch state Using the tachometer to measure rpm 1 The tachometer is a point-and-shoot, non-contact type of measuring device. Press the middle button to activate. Although a variety of modes are available, you will use the default setting of revolutions per minute (rpm), and this should display briefly during power-up before the unit zeroes. Notice that the Prony brake rotor has a small piece of reflective tape attached to it in one location. Notice also that the backside of the tachometer features both a light source (illuminated when unit is on) and a light detector. To measure rpm, with the motor running, hold the tachometer (backside facing rotor) about 2-3 cm away from the rotor and look along the gun-sight groove on the top of the tachometer, aiming at the tape location so that the light beam sees the reflective tape passing by. Press and hold the middle (measurement) button while doing this, and wait 5 to 10 seconds to get a reliable reading. Release the measurement button while still aiming at the rotor to preserve your reading. Make a note of the reading immediately since it will automatically reset after a while. Your readings will vary slightly, especially when the motor is under load, so decide on a method of getting a good average. You will have to convert your results to radians. 4.3 Polarity reversal of armature and field motor windings Investigate what happens when reversing the polarities for both the armature and the field connections. Softly turn off the ARMATURE first & then the FIELD supplies before changing connections. When powering up, softly energize the FIELD first, followed by the ARMATURE. Question: Why are these power-up and shut-down sequences important? Explain: Investigate reversing only the armature and only the field connections. What happens and why? ENSC387 7 of 9

8 4.4 Speed control LAB 5: DC Motors In order to control the speed of the DC motor, we will softly apply a user-controlled DC voltage to the armature while keeping field flux constant. The armature current is proportional to the AC current measured at the input to the SAF SCR DC drive controller. DO NOT ALLOW ARMATURE CURRENT TO EXCEED 6A RMS! Question: What is the relationship between the input AC current and the DC armature current, I a? Make measurements of both the AC current and the resulting DC armature current to determine this relationship and report your results. Measure the field resistance and the armature resistance (using the DMM) so that you can calculate the field current from the applied field voltage. The armature resistance is likely to be 5Ω or less, so the accuracy of the measurement may be low with the available equipment. On the Fluke 45 DMMs, you can use the RELative feature to null out the test lead resistance. Similarly, Agilent uses the term Null for their 34405A DMM. The DMM user manuals are available on all Lab1 computers. Windows Desktop > Reference Material folder. 4.5 Speed-torque characteristics Armature Control 1. Softly apply 60V to the field using the King 1306AF DC power supply. 2. Softly apply 20V to the armature and measure the no-load speed. Engage the Prony brake and record the speed and input AC current for 4 different torque loads of your choice. Before taking any measurements, determine appropriate loads by trial and error. Don t run the motor at very low speeds (i.e. stay above 100 rpm at all times in these experiments). 3. Repeat (2) for armature voltages of 30, 40, 50 and 60V. Analyze results by plotting speed vs. torque for all five cases on the same graph and label appropriately. From the motor speed equation (1.3), the torque/speed characteristic should be a linear function of V t Field Control Motor speed can also be controlled by varying the field voltage and therefore the field flux φ. It is much easier to control the field of a DC motor since the magnitude of the field current is much smaller than the armature current. Examine the torque-speed and V-I characteristics of a fieldcontrolled arrangement as follows: 1. Softly apply 40V to the field using the King DPS-1306AF lab DC power supply. 2. Softly apply 30V to the armature using the SAF SCR drive controller. Measure the noload speed. Engage the Prony brake and record the speed and input AC current for 4 different torque loads. 3. Repeat (2) for field voltages of 20, 30, 50 and 60V. ENSC387 8 of 9

9 Analyze your experimental results and confirm for yourself whether they agree with the theory according to the motor speed equation (1.3). 4.6 Armature Resistance Control For an armature voltage of 50V and a field voltage of 60V, and with an external armature resistance of 0Ω, 1Ω, 2Ω, and 4Ω, take speed/torque measurements (about 4 to 5 readings for each value of external armature resistance). Plot these results and compare them with what you would expect from the DC motor model. Are there any major discrepancies between the two? 4.7 Shunt DC Motor Control Connect the motor as a shunt motor (connect the field terminals in parallel with the armature terminals and drive the motor from SAF SCR controller). Try to verify the torque/speed equation derived earlier in this write-up for 3 or 4 values of T m and V t. Without numeric values for k and k, you can only characterize the shape of the plot versus what you would expect from the model. ENSC387 9 of 9