Electrical Machines Questions and Answers – Synchronizing Power and Synchronizing Torque

This set of Electrical Machines Multiple Choice Questions & Answers focuses on “Synchronizing Power and Synchronizing Torque”.

1. The coupling angle or load angle of a synchronous motor is defined as the space angle between __________
a) rotor and stator poles of opposite polarity
b) rotor and stator poles of same polarity
c) rotor and stator teeth
d) rotor and resultant magnetic field
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2. What is Synchronizing power?

(i) Stiffness coefficient of electromagnetic coupling between stator and rotor
(ii) Stability measure of synchronous machine
(iii) Power consumption factor

a) i,ii
b) ii, iii
c) iii, i
d) i, ii, iii
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3. Large stiffness coefficient represents ____________
a) motor speed remains practically constant
b) large fluctuations due to mechanical load
c) motor speed is variable to changes
d) no such significance with speed
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4. An over excited synchronous machine is more stable than under excited.
a) True
b) False
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5. Consider the below remarks for an alternator.

(i) An over excited machine is more stiff
(ii) Machine with longer air-gap is less stable than one with smaller air-gap

a) (i) is True, (ii) is False
b) (i) is False, (ii) is True
c) (i) is True, (ii) is True
d) (i) is False, (ii) is False
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6. As load angle is increased, degree of stability __________
a) reduces
b) increases
c) remains same
d) no change in stability
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7. Synchronizing power is __________
a) transient in nature
b) steady state in nature
c) pulsating at small frequencies
d) constant for a machine
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8. Which of the following makes synchronizing power to come to play in the machine?
a) Disturbance in power mover
b) Disturbance in field current
c) Disturbance in load
d) All of the mentioned
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9. Which of the following is/are used in synchronous machines to maintain mechanical stability?
a) Damper winding
b) Interpole winding
c) Compensating winding
d) Equalizer rings
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10. For a 4-pole 3 phase, 400V alternator has synchronizing power of 300 units. Then the synchronizing power per mechanical degree is?
a) 300
b) 1200
c) 900
d) 600
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11. Electromagnetic torque is present in rotating machines when ____________
a) both stator and rotor windings carry current
b) rotor windings carry current
c) stator windings carry current
d) any of the mentioned
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12. The operation of a synchronous motor operating on constant excitation across infinite bus will not be stable if power angle δ?
a) exceeds internal angle θ
b) is less than θ
c) is more than θ/2
d) is less than θ/2
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13. In a synchronous motor the back emf peak set up in the stator depends on ___________
a) rotor excitation
b) supply voltage
c) rotor speed
d) load on the motor
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14. Variation in the dc excitation of a synchronous motor causes variation in ___________
a) armature current and power factor
b) power factor
c) armature current
d) speed
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15. Which motor can conveniently operate at lagging as well as leading power factor?
a) Synchronous motor
b) Slip ring induction motor
c) Squirrel cage induction motor
d) Stepper motor
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