EEE 360 lab 13

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Apr 3, 2024

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EEE 360 Lab 13 The Synchronous Motor - Part I October 23, 2023 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-1

Experiment 13. The Synchronous Motor – Part I OBJECTIVE  To examine the construction of the three-phase (3Φ) synchronous motor.  To obtain the starting characteristics of the 3Φ synchronous motor. DISCUSSION The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the rotating magnetic field of the stator. As you have learned, this natural speed of rotation is controlled strictly by the number of pole pairs and the frequency of the applied power. Like the induction motor, the synchronous motor makes use of the rotating magnetic field. Unlike the induction motor, however, the torque developed does not depend on the induction currents in the rotor. Briefly, the principle of operation of the synchronous motor is as follows. A multiphase source of AC is applied to the stator windings, and a rotating magnetic field is produced. A direct current is applied to the rotor windings, and a fixed magnetic field is produced. The motor is so constructed that these two magnetic fields react upon each other causing the rotor to rotate at the same speed as the rotating magnetic field. If a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will continue to rotate at the same synchronous speed. The falling behind is analogous to the rotor being tied to the rotating field with a rubber band. Heavier loads will cause stretching of the band, so the rotor position lags the stator field, but the rotor continues at the same speed. If the load is made too large, the rotor will pull out of synchronism with the rotating field and, thus, will no longer rotate at the same speed. The motor is then said to be overloaded. The synchronous motor is not a self-starting motor. The rotor is heavy, and from a dead stop, it is not possible to bring the rotor into the magnetic lock with the rotating magnetic field. For this reason, all synchronous motors have a starting device. A simple starter is another motor which brings the rotor up to approximately 90% of its synchronous speed. The starting motor is then 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-2

disconnected, and the rotor locks in step with the rotating field. The more commonly used starting method is to have the rotor include a squirrel cage induction winding. This induction winding brings the rotor almost to synchronous speed as an induction motor. The squirrel cage is also useful even after the motor has attained synchronous speed because it tends to dampen rotor oscillations caused by sudden changes in loading. Your Three-Phase Synchronous Motor/Generator contains a squirrel-cage-type rotor. PROCEDURE The grading in this section is 1 point for each correct answer. 1. Procedure 1 Open the Virtual Laboratory, and from the Virtual Lab, Welcome Window click on the Experiment 13 Procedure 1 button. Your screen should look like Figure 13-1. Examine the construction of the Three-phase Synchronous Motor/Generator, paying attention to the motor, slip rings, connection terminals and the wiring. The rheostat is a variable resistor to adjust the current and voltage in the circuit. Figure 13-1. Screen capture of Procedure 1. View the motor from the rear of the module: a. Identify the two slip rings and brushes. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-3

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b. Note that the two rotor windings are brought out to the two slip rings via a slot in the rotor shaft. c. Identify the DC damper windings on the rotor. Although there are only two windings, they are connected so that their magnetomotive forces act in opposition, thus, creating four poles. d. Identify the four salient poles just beneath the damper windings. e. Identify the stator winding and note that it is identical to that of the three-phase squirrel cage and wound rotor motors. f. Close the Experiment 13 Procedure 1 window. 2. Procedure 2 Circuit : Click on the Experiment 13 Procedure 2 button. Your screen should look similar to Figure 13- 2. Using your Three-Phase Synchronous Motor/Generator, Power Supply, and Ammeter, connect the circuit shown in Figure 13-3. Note that the three stator windings are wye- connected to the fixed 208 V 3Φ output of the power supply. Also, note that the rotor is left unconnected. If you are unsure how to make/remove connections, please refer to the Experiment 1 manual. Figure 13-2. Screen capture for Procedure 2. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-4

Figure 13-3. Connection circuit of Procedure 2. Motor terminals are 1 = A, 2 = B, 3 = C, Neutral point is terminals 4,5,6. The rotor DC excitation terminals are 7 and 8. S is a switch and potentiometer to provide fine regulation of DC excitation current. Measurements: a. Click on the Run button. If you receive an error message, check your wiring and the voltage magnitude. Once you have fixed your circuit, click the Run button again. Note that the motor starts smoothly and continues to run as an ordinary induction motor. b. Note the direction of rotation (clockwise/counterclockwise) and the line current. Rotation = counterclockwise I A = 1.218 A c. Interchange any two of the leads from the power supply. If you are unsure how to make/remove connections, please refer to the Experiment 1 manual. d. Click the Run button and note the direction of rotation (clockwise/counterclockwise) and the line current. Rotation =clockwise I A =1.218 A e. Close the Experiment 13 Procedure 2 window. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-5

3. Procedure 3 Circuit : Click on the Experiment 13 Procedure 3 button. Your screen should look like Figure 13-4. Connect the circuit shown in Figure 13-5. The motor is now coupled to the electrodynamometer with a belt. Figure 13-4. Screen capture of Procedure 3. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-6

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Figure 13-5. Circuit diagram for Procedure 3. Generator terminals 1 = A, 2 = B, 3 = C. Neutral point N is terminals 4,5,6. The rotor DC excitation terminals are 7 and 8. S is a switch and potentiometer (rheostat) which provides fine regulation of DC excitation current. a. The dynamometer is used to load the synchronous motor. Set the dynamometer control slider to 40% excitation (torque reference). The motor is loaded. b. The rotor of the synchronous motor is connected to the fixed 120 V DC output of the power supply, terminals 7 and 8. Set the field rheostat for zero resistance (and close the switch {ON position}). Note that if this switch is OFF, no voltage will be applied to the rotor windings. Measurements (round one) : i. With the rotor switch in the ON position, click on the Run button. If you receive an error message, check your wiring and the voltage magnitude. Once you have fixed your circuit, click the Run button again. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-7

ii. Describe what happened. The motor shook erratically and created a lot of noise when the current climbed to 4.028 Amperes. The rotor scarcely rotates. iii. What was the maximum reading on the ammeter? 4.028 A iv. Should a synchronous motor, under load, be started with DC excitation on its field?  Yes  No Measurements (round two) : v. Connect the rotor of the synchronous motor to the variable 0-120 V DC output of the power supply. Do not disturb any of the other connections or change any control settings. vi. The generator is unloaded when the Dynamometer voltage is zero. With the variable output voltage control at zero, turn on the power supply. Apply 3Φ power by closing the synchronizing switch and observe what happens. vii. Describe what happened. Current stayed at 1.24 viii. Is your motor operating as an induction motor?  Yes  No ix. Is your motor operating as a synchronous motor?  Yes  No x. Close the Experiment 13 Procedure 3 window. REVIEW QUESTIONS 1. What precautions should be taken during the start-up period of a synchronous motor? (4) The stator magnetic field's speed should be initially reduced. The synchronous motor should then be accelerated to synchronous speed using an external prime mover. Use of damper winds is also required. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-8

2. If the squirrel-cage winding were removed from a synchronous motor, could it start by itself? (4)  Yes  No 3. What function does the damper winding serve? (4) The main winding is assisted in correcting the field without discontinuity by damper windings connected in series with the field windings, placing the in synchronous operation. 7933adfb48e80c98034e562f8d4929d7dba026a4.docx 13-9

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