Electrical Machines Questions and Answers – Synchronous Motor Phasor Diagram

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Synchronous Motor Phasor Diagram”.

1. Below is the phasor diagram of synchronous generator.

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
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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)
b)
c)
d)
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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)
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4. Synchronous motor delivers lagging power at __________
a) leading pf
b) lagging pf
c) zero pf
d) unity pf
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5. A 420 V synchronous motor is working at 30° lagging load. What can be the expected phasor?
a)
b)
c)
d) None of the mentioned
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6. The synchronous motor is operating at upf load. The most suitable phasor will be?
a)
b)
c)
d)
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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
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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.

a) terminal voltage, over exciting
b) excitation voltage, over exciting
c) terminal voltage, under exciting
d) excitation voltage, under exciting
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9. In the figure below, the point ‘F’ corresponds to _______ which can be obtained by ________

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
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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
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11. Identify the over excited alternator operating at zpf.
a)
b)
c)
d)
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12. Identify the over excited synchronous motor operating at zpf.
a)
b)
c)
d)None of the mentioned
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13. For the below graph depicting the performance of a synchronous motor, coordinates ‘x’ and ‘y’ are respectively?
In the ‘V’ curve shown in the above figure for a synchronous motor, the parameter of x and y coordinates are, respectively.

a) Armature current and field current
b) Power factor and field current
c) Armature current and torque
d) Torque and field current
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14. We can increase the torque of a reluctance synchronous motor if we __________
a) Increase reluctance of the magnetic circuit along the direct axis
b) Decrease the reluctance of the magnetic circuit along the quadrature axis
c) Increase the ratio of the quadrature axis reluctance to direct axis reluctance
d) Decrease the ratio of quadrature axis reluctance to direct axis reluctance
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15. Switched reluctance motors are basically __________
a) salient pole synchronous motor but without excitation winding
b) stepper motor with salient poles
c) synchronous motor with salient poles on stator and rotor
d) stepper motor with closed loop control and with rotor position sensor
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Sanfoundry Global Education & Learning Series – Electrical Machines.

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