Page 1 of 1 Faculty of Engineering, Architecture and Science Department of Electrical and Computer Engineering Course Number EES 612 Course Title Electrical Machines and Actuators Semester/Year Instructor Lab Report # Lab Title Lab Date Lab Section TA s Name Student Name Student ID Signature* *By signing above you attest that you have contributed to this written lab report and confirm that all work you have contributed to this lab report is your own work. Any suspicion of copying or plagiarism in this work will result in an investigation of Academic Misconduct and may result in a 0 on the work, an F in the course, or possibly more severe penalties, as well as a Disciplinary Notice on your academic record under the Student Code of Academic Conduct, which can be found online at: www.ryerson.ca/senate/current/pol60.pdf.
Faculty of Engineering, Architecture and Science Department of Electrical and Computer Engineering LAB INSTRUCTIONS EES 612 Electrical Machines and Actuators Experiment # 3: Three-Phase Induction Motor Introduction The three-phase Induction Machine is the most widely used of all rotating machines. When operated off a constant-voltage, constant-frequency, electric power source, an induction machine offers an almost constant speed within its normal operating range. Induction machines are inexpensive, very rugged, and of little maintenance requirements. However, the control of their speed (and position) is difficult and relatively expensive. Depending on its rotor construction, an induction machine is either a squirrel-cage machine or a wound-rotor machine. In a squirrel-cage machine, the rotor windings are realized by short-circuited conductive bars and, as such, cannot be accessed. In the wound-rotor type, however, the rotor windings are made of insulated copper wires whose terminals are brought out by means of slip rings. Hence, external resistors may be connected in series with the rotor windings, to alter the machine s torque-speed characteristic for speed control. Alternatively, the rotor windings may be energized by a three-phase, variable-voltage, variable-frequency, power source (of power-electronic type), for speed control in a remarkably more efficient manner. An induction machine controlled in the latter way is commonly referred to as a doubly-fed induction machine and used in regenerative drive applications and modern wind turbines. In this experiment, we examine the characteristics of a three-phase wound-rotor induction machine which we will operate as a motor.
Pre-Lab Assignment P1. The motor to be studied in this lab is a 4-pole, three-phase, 60-Hz, 208-V machine whose parameters are R s = 12 Ω, R r = 18 Ω, X m = 160 Ω, and X l = 22 Ω. Using the equations listed in the Appendix of this Lab Instructions document (also presented in the course notes), complete Table P1. Note that we have labeled the calculated developed torque here as theoretical. Also, for this motor calculate the maximum torque, shaft speed at which the maximum torque develops (in rpm), standstill (start-up) torque, the stand-still rotor and stator currents, and stand-still power factor. Table P1: developed torque values. Shaft Speed n (rpm) 1750 Slip s Developed Torque T (Nm) 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500 2
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P2. In many practical situations where a machine is integrated with a larger system and may also be in operation, all we might be able to learn about the machine would be through simple voltage, current, and resistance measurements. Thus, consider an induction motor for which you can measure the per-phase stator rms voltage V s, the stator rms current I s, and the shaft speed (hence the number of poles and slip s). Also, assume that you know the motor s per-phase rotor resistance R r, per-phase stator resistance R s, and per-phase magnetizing inductance X m (all referred to the stator side). Moreover, assume a Further Simplified Gamma Model for the machine; this is the Gamma Model in which the stator resistance is assumed to be located on the rotor side of the magnetizing reactance (see Page 10 of the lecture notes, as well as problems 5.9 and 5.12). Further, assume that the slip is small such that the effect of the leakage reactance is negligible (i.e., X l = 0). Furthermore, assume that the core losses are negligible (i. e., R c = ). a) Provide an expression for the rotor rms current I r, in terms of V s, I s, and X m ; b) Provide an expression for the power factor of the machine, in terms of I r and I s ; c) Provide an expression for the machine s mechanical power P mech in terms of V s, I s, I r, R s, and R r ; and d) Provide an expression for the machine s developed torque T, in terms of P mech and shaft speed n. 4
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Lab Work General safety note To prevent injuries or damage to equipment, the power source must be turned OFF prior to wiring up the circuit. Ask your TA to check. Equipment Three-phase wound-rotor induction motor module EMS 8231 AC power supply module EMS 8821 (for energizing the induction motor) Dynamometer module EMS 8911 (for applying a load torque) Hand-held tachometer (for measuring the shaft speed) Bench-top digital multimeter (for voltage measurements) Hand-held clamp ammeters (for current measurements) Circuit 1.1. Ensure that the three-phase wound-rotor induction motor and the dynamometer are side by side and their shafts are coupled by a belt, as shown in Figure 1. 1.2. Observing the terminal numbers, connect the AC power supply to the stator terminals of the induction motor, as shown in Figure 2. For now, leave open the rotor terminals 7, 8, and 9. 1.3. Connect the AC power supply to the dynamometer, as shown in Figure 3. Coupling Belt Wound-rotor Machine EMS 8231 Dynamometer EMS 8911 Figure 1: Belt coupling the machine and dynamometer
Figure 2: Power supply connections for induction motor. Figure 3: Power supply connections for dynamometer. 8
Experiments E1. Energization with open rotor terminals E1.1. Turn on the AC power supply. This will energize both the induction motor and dynamometer. However, the motor will not spin, implying that a substantial voltage of the same frequency as the supply frequency (i.e., 60 Hz) will appear on the rotor (open) terminals. Using your multimeter, measure the voltage between two of the rotor terminals (for example, between terminals 8 and 9), as the line-to-line rotor voltage V LLr. Then, the rotor per-phase voltage (or line-to-neutral voltage) is V r = V LLr / 3. Record the aforementioned voltages in the boxes below: V LLr = V and V r = V E1.2. Knowing that the stator per-phase voltage V s is 120 V in this experiment, and using the rotor per-phase voltage V r you calculated in E1.1, calculate and record the stator-to-rotor winding turns ratio: N s N r = V s V r = E1.3. Turn off the AC power supply and park the dynamometer s knob at the fully counter-clockwise position.
E2. Energization with short rotor terminals E2.1. With the AC power supply off and dynamometer s knob turned fully counterclockwise, short the rotor terminals by means of two short wires, as shown in Figure E2.1. Place the ammeter clamp around the wire that connects node 1 of the power supply to node 1 of the stator. Also, connect the voltmeter across nodes 1 and 4 of the stator. E2.2. Turn on the power supply to make the motor run clockwise. If the motor runs counter clockwise, turn off the power supply and interchange two of the stator terminal connections (for example, swap the supply ends of the wires that connect to the black and red terminals of the power supply). Figure E2.1: Wound-rotor induction motor with short rotor terminals. E2.3. Gradually turn the dynamometer s knob clockwise to load the shaft. Measure the shaft speed by your tachometer. For each shaft speed specified in Table E2.3, measure the stator current I s, power factor, and per-phase power P s. Note that we have labeled these as measured. Disregard the dynamometer s scale reading, since we are using the dynamometer to only load the motor and not to measure the shaft torque. Make sure that the stator current does not exceed 1.5 A. 10
Table E2.3: Different measured variables versus shaft speed. Shaft Speed n (rpm) 1750 Stator Current I s (A) Power Factor cosφ Per-Phase Stator Power P s (W) Three-Phase Input Power P in = 3P s (W) 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500
Conclusions and Remarks C1. Using the measurements you reported in Table E2.3, the machine s parameters introduced in P1, and the expressions you derived in P2, complete Table C1. Note that we have labeled these calculated values as calculated. Table C1: Developed torque and mechanical power calculated from measured quantities. n (rpm) Stator Current I s (A) Rotor Current I r (A) Power Factor cosφ Mechanical Power P mech (W) Developed Torque T (Nm) 1750 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500 12
C2. Compare the theoretical torque values which you found in P1 with the calculated torque values of Table C1, and complete Table C2. Define the percent error as follows: error percent = calculated value theoretical value 100 calculated value Based on Table C2, comment on the closeness of the theoretical values to their calculated counterparts, and provide reasons for discrepancies. Table C2: Comparison between theoretical and calculated torque values. n (rpm) Developed Torque T (Nm) Percent Error 1750 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500
C3. Compare the measured power factor values which you recorded in Table E2.3 with their calculated counterparts recorded in Table C1. Complete Table C3. Define the percent error as follows: error percent = calculated value measured value 100 measured value Based on Table C3, comment on the closeness of the calculated values to their measured counterparts, and provide reasons for discrepancies. Table C3: Comparison between calculated and measured power factor values. n (rpm) Power Factor cosφ Percent Error 1750 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500 l 14
C4. Using Table C2, plot on one graph, Graph C4, the shaft speed n versus the calculated and theoretical values of the developed torque T. Clearly identify the curves with the labels calculated and theoretical. Also, for both curves, assume that n = 1800 rpm at T = 0. Use proper data range and grid resolution for the two axes. In view of the curves of Graph C4, comment on the torque-speed characteristic of the induction machine and its linearity. Also, based on your comment, explain what type of dc motor does the induction motor resemble as far as the torquespeed characteristic is concerned. n (rpm) ( ) N r (rpm) T (N.m) Graph C4: and theoretical torque-speed characteristic of the induction motor.
C5. Using the data recorded in Table E2.3 and Table C1, calculate the percent efficiency of the motor, for each of the shaft speeds of Table C5. Then, plot on Graph C5 the efficiency (in percent) versus the shaft speed. Use proper data range and grid resolution for the two axes. Comment on the behavior and provide your reason. Table C5: Efficiency versus the shaft speed. n (rpm) P mech (W) from Table C1 P in (W) from Table E2.3 η = P mech P in 100 1750 1725 1700 1675 1650 1625 1600 1575 1550 1525 1500 16
η ( (%) ) n (rpm) N r (rpm) Graph C5: Efficiency of the induction motor versus its shaft speed.
Appendix: General Induction Machine Torque-Speed Characteristic In general, the torque of an induction machine can be formulated as 1 + 1/ 1 + Q 2 T = 2T max s s + s (A1) max max s + 2/ 1 + Q2 where the maximum torque T max is given by T max = 3p 4 V s 2 1 1 (A2) ω s R s 1 + 1 + Q 2 and takes place at a slip of where the ratio R r R s max = s 1 + Q 2 Q = X l R s (A3) (A4) is analogous to the quality factor in circuit theory; thus, it is a measure of the magnitude of X l relative to R s. In large machines, X l is remarkably larger than R s and thus Q is very large. In other words, the baseline for deciding whether or not R s may be ignored in an induction machine analysis problem is X l, and not R r. Convince yourself that, if Q is large, the above equations take the simple forms presented in the course. Last updated Sep. 4, 2013 AY 18