of University Alaska Fairbanks Electrical and Computer Engineering Department EE 303 - Electric Machinery Laboratory #5 OPERATING FEATURES OF A SYNCHRONOUS MOTOR (V-CURVES)
OBJECTIVE The purpose of this laboratory is to study the operating characteristics of a synchronous motor, specifically synchronous motor V-curves. REFERENCES "Electric Machinery" by Fitzgerald, Kingsley, and Umans "Electric Machinery Fundamentals" by Chapman "The Faraday's Law Machines Laboratory" by Summers BACKGROUND The three-phase synchronous motor is first started as an induction motor by shorting its three rotor coils. The armature current IA(m) in the synchronous motor field is varied by adjusting the field current IF(m) supplied to the motor or by changing the mechanical load. The speed of the synchronous motor remains constant as the load current (armature current) varies while keeping the field current of the motor constant. A set of curves for a synchronous motor known as V-curves are generated by varying the field current of the motor and recording the corresponding armature current at various load conditions. The example plot of synchronous motor V-curves in Figure 5.1 shows the left side of the V representing the under-excited condition (lagging power factor) and the right side of the V representing the over-excited condition (leading power factor). As the load on the motor increases, the V-curve shifts upward and to the right. The minimum point of the V represents normal excitation or unity power factor.
P=P2
IA(m)
P=P1 Leading PF
Lagging PF
PF = 1.0
IF(m) Figure 5.1: Synchronous Motor V-Curves
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University of Alaska Fairbanks Electrical and Computer Engineering Department EE 303 - Electric Machinery Laboratory #5 At the far left of the V-curve (under-excited case) with higher armature currents (higher load), the effect of "hunting" can be observed. In this case, the field current is too small to provide the appropriate amount of excitation necessary in the rotor to maintain the steady state rotor angle . The effect is a damped mechanical oscillation known as hunting where the machine oscillates about the rotor angle. This effect is also present for short periods of time after sudden changes in the load occur.
PROCEDURE 1. Connect the machines as shown in Figure 5.2. Supply three-phase 120V line-line from the variable three-phase autotransformer sitting on the bench (external to the bench) to the stator of the synchronous motor (Item #2). The DC supply for the synchronous motor rotor field is the one located in the bottom left corner of the lab bench panel. The DC supply for the separately excited DC generator field is the one located in the bottom right corner of the lab bench panel. The switch SW is one pole of the three-pole double-throw switch available on the lab bench. With the switch in position SW1 so the that rotor coils are short-circuited, start the synchronous motor as an induction motor and bring it up to rated line voltage (120VAC). After the motor is started, switch SW1 to SW2 and then turn on the DC supply to the field winding of the motor. Be careful here! If DC voltage is supplied with the rotor short-circuited the fuse in the back of the rectifier section of the bench will blow since this is a short circuit. 2. With no load applied by the separately-excited DC generator, vary the field current IF(m) of the synchronous motor and record the armature current IA(m) and the field current IF(m) of the motor to get the no load V-curve for the motor. Make sure to record the minimum armature current point. For a very short period of time take the field current IF(m) up to 2.0A. Bring the armature current back to the minimum value by decreasing the field current IF(m) to the motor. 3. Load the separately-excited DC generator (Item #4) by means of the DC load panel on the lab bench. Adjust the mechanical load provided to the synchronous motor by adjusting the field current supplied to the DC generator to obtain a minimum armature current of 0.4A in the motor. Repeat Procedure #2 with this load to generate a second V-curve. Repeat this again for a minimum armature current of 0.5A. Observe the effect of "hunting" at low field currents for the 0.5A minimum armature current. 4. Keep the field current IF(m) of the motor at 1.5A. Vary the armature current IA(m) of the motor by varying the mechanical load using the DC generator field current. Record the armature current and speed of the synchronous motor. The speed of the motor is measured using the voltage transducer connected between the motor and generator. Note the volts/RPM rating of the voltage transducer.
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University of Alaska Fairbanks Electrical and Computer Engineering Department EE 303 - Electric Machinery Laboratory #5
Figure 5.2: Synchronous Motor with Separately Excited DC Generator as the Load
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University of Alaska Fairbanks Electrical and Computer Engineering Department EE 303 - Electric Machinery Laboratory #5 RESULTS 1. Plot the three V-curves from Procedure #2 and #3 with the motor field current IF(m) on the x-axis and the motor armature current IA(m) on the y-axis. 2. Draw phasor diagrams for the over-excited, under-excited, and unity power factor cases and use these to explain the nature of the V-curves you plotted. 3. Plot the speed versus the armature current IA(m) from Procedure #4 with speed nm on the x-axis and armature current IA(m) on the y-axis.
QUESTIONS 1. What information do the V-curves give us about the operation of the synchronous motor? 2. What can you say about the speed of a synchronous motor under the condition of constant field excitation and varying load? 3. What does the term "hunting" mean in synchronous motor operation?
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