Electrical Machines Multiple Choice Questions on “Synchronous Motor Phasor Diagram”.
1. Below is the phasor diagram of synchronous generator.
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If the machine is made to run as synchronous motor, what will be the phasor diagram changes then?
A. Reverse Ia
B. Reverse Ef
C. Reverse Vt and Ia
D. Reverse Vt
Answer: A
Clarification: Only the direction of the armature current changes in the motor as it absorbs the electrical energy.
2. Below is the phasor diagram of synchronous generator.
If the machine is made to run as synchronous motor, what will be the new phasor diagram?
A.electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2a”>
B.
C.electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2c”>
D.electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2d”>
Answer: A
Clarification: Only the direction of the armature current changes in the motor as it absorbs the electrical energy. It has a component in phase opposite to Ef.
3. The voltage equation of synchronous motor is?
A. Ef = Vt + Ia*(ra+jXA.
B. Ef = Vt – Ia*(ra+jXA.
C. Ef = Vt + Ia*(ra-jXA.
D. Ef = Vt – Ia*(ra-jXA.
Answer: A
Clarification: The voltage equation of synchronous motor is Ef = Vt + Ia*(ra+jXA..
4. Synchronous motor delivers lagging power at __________
A. leading pf
B. lagging pf
C. zero pf
D. unity pf
Answer: A
Clarification: As Q < 0 for synchronous motor at leading pf.
5. A 420 V synchronous motor is working at 30° lagging load. What can be the expected phasor?
A.electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2c”>
B.electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q5b”>
C.
D. None of the mentioned
Answer: A
Clarification: The Ia should lag Ef by 30°.
6. The synchronous motor is operating at upf load. The most suitable phasor will be?
A.
B. electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q5b”>
C. electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2b”>
D. electrical-machines-questions-answers-synchronous-motor-phasor-diagram-q2c”>
Answer: A
Clarification: Ia must be in phase with the Ef.
7. The zero power factor of an alternator can be obtained by _________ at rated Ia.
A. over exciting
B. conducting short circuit of secondary terminals
C. under excitation
D. running as reluctance motor
Answer: A
Clarification: By over exciting an alternator gives the additional flux required to make it operate at zero power factor.
8. From the figure shown below for the OCC and SCC of a three phase alternator, the point ‘L’ corresponds to _______ and the point ‘A’ can be obtained by _____ the alternator.
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A. terminal voltage, over exciting
B. excitation voltage, over exciting
C. terminal voltage, under exciting
D. excitation voltage, under exciting
Answer: A
Clarification: The point ‘L’ corresponds to Vt and the point ‘A’ can be obtained by over exciting the alternator.
9. In the figure below, the point ‘F’ corresponds to _______ which can be obtained by ________
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A. field current required to circulate short circuit current, SCC
B. field current required to circulate full load current, SCC
C. mmf required to cancel the hysteresis losses, SCC
D. mmf required to compensate the leakage reactances, SCC
Answer: A
Clarification: Point ‘F’ corresponds to the short circuit current and it is obtained by SCC plot.
10. The operating point for an over excited alternator working near zpf lagging power factor gives ‘A’ as the operating point. Then if the same machine is working as an over excited synchronous motor will give same operating point at __________
A. zpf leading
B. zpf lagging
C. upf
D. there will never be the same operating point for the machine as motor as well as generator
