EXPERIMENT 19 Starting and Synchronizing Synchronous Machines PURPOSE: To discover the method of starting synchronous motors. BRIEFING: When three-phase is applied to the stator of a three-phase motor, a revolving stator magnetic field is created. This field revolves at synchronous speed, which is a speed determined by the number of poles per phase of the motor and the frequency of the incoming power. The rotor of a synchronous motor becomes locked in on the revolving stator field. It then rotates at synchronous speed. To accomplish this, the rotor contains a DC field winding. The problem is in starting. If you have DC applied to the field coil, while the rotor is standing still, the revolving field passes the stationary field much too fast to be locked onto. First the DC field coils on the rotor must be made to rotate almost as fast as the revolving stator field. Then, when you apply DC to it, the rotor is pulled into synchronism. That means that the rotor turns at synchronous speed. To get the rotor turning in the first place, a squirrel-cage winding is used. The bars are imbedded in the rotor core. When power is applied to the stator, the revolving field induces voltage into these windings. In other words, a synchronous motor starts as an induction motor. When the rotor reaches 95% of synchronous speed, DC is switched into the rotor field winding. Now, during the start process, there will also be voltage induced into the DC field winding as the rotor turns. Rather than have a charged-up field coil, its terminals are shorted through a resistor while the squirrel-cage winding is getting the rotor started. Because synchronous motors must achieve 95% synchronous speed before being synchronized, they are rarely started under load. Load is applied after it is running as a synchronous motor. The rotor, however, continues to turn at synchronous speed. 19-1
It is possible to load a synchronous motor beyond its ability to stay in synchronism. The counter-torque of a load overcomes the torque (pull) on the rotor from the revolving stator field. When that happens, the motor pulls out of step with the stator field. It will not simply fall back to running smoothly as an induction motor, however. Induced currents, added to the excitation current in the DC field winding, make the rotor pulsate. Therefore, field excitation should be removed as soon as possible after the rotor pulls out of synchronism. Then, if you want to re-synchronize the motor, you must first remove the overload. PERFORMANCE OBJECTIVES: Upon successful completion of this experiment, the student will be able to: 1. Explain the principle of synchronous motors. 2. Start and synchronize a synchronous motor. MACHINES REQUIRED SM-100-3A Synchronous Machine DYN-100A-DM Dynamometer POWER REQUIRED Fixed 3 AC Supply 0-150 volt Variable DC, 1 amp 0-125 volt Variable DC, 5 amps METERS REQUIRED (2) 0-150 volt DC Voltmeters 0-1 amp DC Ammeter 0-2.5 amp DC Ammeter 0-300 volt AC Voltmeter 0-2 amp AC Ammeter ADDITIONAL MATERIAL REQUIRED MGB-100-DG Bedplate RL-100A Resistance Load SLA-100 Series Strobe-Tachometer 19-2
PROGRAM PLAN: Step 1. Place the two machines on the bedplate. Motor on the left, dynamometer on the right. Couple and clamp the machines securely. Step 2. Connect the synchronous motor as shown in Figure 19-1. Note that the stator is wye-connected. Step 3. Connect the dynamometer as shown in Figure 19-1. Note that this is a separately-excited shunt generator connection. Be sure that all of the load switches on the RL-100A are in the downward (OFF) position. Figure 19-1 Step 4. Have someone check your connections to be sure they are correct. Adjust the field rheostat on the dynamometer to its maximum resistance position, fully clockwise. 19-3
Step 5. With the motor circuit breaker OFF, turn ON the main AC and the 0-125V DC excitation supply. Step 6. Put the motor switch in the SYNC RUN position and adjust the excitation supply until 0.6 amps flows in the field coil. Then return the switch to the IND START position. Step 7. Turn ON the motor circuit breaker. Step 8. The synchronous motor is now running as an induction motor. Measure the speed, and stator current. Record these values in TABLE 19-1. SPEED STATOR CURRENT BEFORE SYNCHRONIZING AFTER SYNCHRONIZING TABLE 19-1 Step 9. Switch the toggle on the motor to SYNC RUN. Step 10. Turn ON the 0-150 volt DC supply and adjust its output to 115 volts. Adjust the dynamometer s field rheostat until the terminal voltage is 120 volts. Step 11. Adjust the 0-125V DC excitation current until the stator current is at its lowest point. Step 12. Measure speed and stator current. Record these values in TABLE 19-1. Step 13. Zero the dynamometer scale by positioning the weight at the rear. Step 14. Turn ON the resistance load switches one at a time. As each one is turned ON, measure speed, stator current and torque. Record these values in TABLE 19-2. Step 15. As you turn ON the ninth or tenth load switch, the motor will pull out of synchronism. Immediately switch back to IND START and start removing load steps one at a time. After each, attempt to re-synchronize. Make a note of the number of switches that were still on when you were able to re-synchronize. Step 16. Turn OFF all circuit breaker switches. Disconnect all leads. 19-4
TEST RESULTS: LOAD STEPS 1 2 3 4 5 6 7 8 9 10 SPEED TORQUE I STATOR TABLE 19-2 DE-BRIEFING: 1. The damper winding (squirrel-cage) starts the rotor and gets it up to 95% synchronous speed. After DC is applied to the rotor field coil and the rotor pulls up to synchronous speed, what is the job of the damper winding? Explain. 2. Why isn t DC applied to the rotor field coil right away instead of waiting until the rotor is up to speed? 3. What happened to rotor speed as you synchronized the rotor? What happened to stator current? Explain why rotor speed changed the way it did. 4. What was the pull-out torque for this motor? (Pull-out torque is the maximum torque the motor will produce before dropping out of synchronism). 19-5
5. As you attempted to re-synchronize, how many switches were ON when your attempt was successful? QUICK QUIZ: 1. When a synchronous motor is loaded, its speed: (a) Increases. (b) Decreases. (c) Remains the same. 2. Normally, the stator current of a synchronous motor is: (a) Higher than that of an induction motor. (b) Lower than that of an induction motor. (c) The same as that of an induction motor. 3. A synchronous motor will not synchronize if: (a) The stator has a revolving field. (b) The load on the rotor is too great. (c) DC is applied to the field coil on the rotor. 4. As the motor was loaded, stator current: (a) Increased. (b) Decreased. (c) Remains the same. 5. You can easily tell when a synchronous motor drops out of synchronism, because the rotor begins to: (a) Speed up. (b) Pulsate. (c) Reverse direction. 19-6
EXPERIMENT 20 Synchronous Motor V-Curves PURPOSE: To discover the effect that changing the excitation current of a synchronous motor has on its stator current. BRIEFING: Synchronous motors have the unique ability to run at different power factors. Motors actually require electric power for two reasons. The first is to supply power current that gets converted to mechanical power for the load and rotational losses. The second kind of electrical input to a motor is the excitation current. Excitation current stores energy in the magnetic field and releases it back to the source. Excitation current, which does no actual work, is ninety degrees out of phase with power current. Induction motors must draw both the power current and the excitation current from the AC lines. That s why typical induction motors operate with 0.8 lagging power factor. Synchronous motors, on the other hand, have a separate source of excitation current. If you wanted to supply less than normal excitation current to the DC field coil on the rotor, a synchronous motor would run at 0.8 lagging power factor, the same as induction motors. This is seldom done, however. Instead the excitation current is increased to point where it magnetizes the rotor, stator, and air gap so that no excitation current is taken from the AC lines at all. The entire stator current, therefore, is converted to mechanical power. The synchronous motor has a unity power factor. Excitation current can, however, be increased above normal. Now, not only does the motor not take any excitation current from the AC lines, it actually supplied excitation current to the AC lines. Typical synchronous motors can run at 0.8 P.F., leading. 20-1
PERFORMANCE OBJECTIVES: Upon successful completion of this experiment, the student will be able to: 1. Demonstrate change in synchronous motor power factor. 2. Explain the effect of excitation current in terms of V-curves. MACHINES REQUIRED SM-100-3A Synchronous Machine DYN-100A-DM Dynamometer POWER REQUIRED Fixed 3 AC Supply 0-150 volt Variable DC, 1 amp 0-125 volt Variable DC, 5 amps METERS REQUIRED (2) 0-150 volt DC Voltmeter 0-1 amp DC Ammeter 0-2.5 amp DC Ammeter 0-300 volt AC Voltmeter 0-2 amp AC Ammeter ADDITIONAL MATERIAL REQUIRED MGB-100-DG Bedplate RL-100A Resistance Load PROGRAM PLAN: Step 1. Place the two machines on the bedplate: motor on the left, dynamometer on the right. Couple and clamp the machines securely. Install guards. Step 2. Connect the Synchronous motor as shown in Figure 20-1. Note that the stator is wye-connected. Step 3. Connect the dynamometer as shown in Figure 20-1. Note that this is a separately-excited shunt generator connection. Be sure that all of the load switches on the RL-100A are in the downward (OFF) position. 21-2
Step 4. Have someone check your connections to be sure they are correct. Adjust the field rheostat on the dynamometer to its maximum resistance position, fully clockwise. Figure 20-1 Step 5. With the motor circuit breaker OFF, turn ON the main AC and the 0-125V DC excitation supply. Step 6. Put the motor switch in the SYNC RUN position and adjust the excitation supply until 1.0 amps flow in the field coil. Then return the switch to the IND START position. Step 7. Turn ON the motor circuit breaker. The motor should now be running as an induction motor. Step 8. Move the motor s toggle switch to the SYNC RUN position. The motor should now be synchronized with the stator s revolving field. Step 9. With 1.0 amperes flowing in the DC field, record stator current in TABLE 20-1. Step 10. Reduce the value of rotor current by 0.1 amps. Read and record stator current in TA- BLE 20-1. 20-3
Step 11. Continue to repeat Step 10 until the motor pulls out of synchronism. Step 12. When the motor pulls out of synchronism, first switch to IND START, then increase rotor current and re-synchronize. Step 13. Turn ON the 0-150 volt DC supply and adjust its output to 115 volts. Step 14. Adjust the dynamometer field rheostat until its terminal voltage is 120 volts. Step 15. Turn ON load steps 1 through 4 on the RL-100A. Step 16. Repeat Step 14. Step 17. Repeat Steps 9, 10, 11, 12, and 14 for TABLE 20-2. Step 18. Turn ON load steps 1 through 8 on the RL-100A. Step 19. Repeat Steps 9, 10, 11, 12, and 14 for TABLE 20-3. Step 20. Turn OFF all circuit breakers. Disconnect all leads. TEST RESULTS: FIELD AMPS 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 STATOR AMPS TABLE 20-1 - NO LOAD FIELD AMPS 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 STATOR AMPS TABLE 20-2 - HALF LOAD FIELD AMPS 1.0 0.9 0.8 0.7 0.6 0.5 0.4 STATOR AMPS TABLE 20-3 - FULL LOAD 20-4
DE-BRIEFING: 1. From the data you recorded in TABLE 20-1, plot a curve on the graph provided showing how stator current changes as the field excitation current changes with no load on the motor. Label this curve NO LOAD. 2. From the data you recorded in TABLE 20-2, plot a curve on the same graph showing how stator current changes with field excitation at half load. Label this curve HALF LOAD. 3. From the data you recorded in TABLE 20-3, plot a curve on the same graph showing how stator current changes with field excitation at full load. Label this curve FULL LOAD. 4. Connect with a dotted line the lowest point of the three curves. Label this line UNITY POWER FACTOR. 5. On the left side of the unity power factor line, label the area LAGGING POWER FACTOR. On the right side of the line, label the area LEADING POWER FACTOR. QUICK QUIZ: 1. As excitation current was decreased from 1.0 amps to pull-out, the stator current: (a) Went up then down. (b) Went down then up. (c) Remained the same. 2. At the lowest point on the stator current curve: (a) Current is leading voltage. (b) Current is lagging voltage. (c) Current is in-phase with voltage. 3. Normal excitation is when the synchronous motor has: (a) Unity power factor. (b) Leading power factor. (c) Lagging power factor. 20-5
4. Under-excitation produces a: (a) Unity power factor. (b) Leading power factor. (c) Lagging power factor. 5. Over-excitation produces a: (a) Unity power factor. (b) Leading power factor. (c) Lagging power factor. 20-6