# Electrical Machines Questions and Answers – Power Factor Correction by Synchronous Motors

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Power Factor Correction by Synchronous Motors”.

1. The magnetization current drawn from an AC supply a synchronous motor is used to _________
a) set up flux in magnetic circuit of device
b) compensate core losses
c) set up magnetizing armature reaction
d) all of the mentioned

Explanation: Magnetizing current sets up the flux needed in the machine.

2. A 3 phase synchronous motor is working at normal excitation, then the flux deficient in circuit is ________
a) given by armature winding mmf
b) given by field winding mmf
c) supplied to armature winding mmf
d) supplied to field winding mmf

Explanation: it is given by armature winding mmf.

3. The excess flux in synchronous motor is neutralized by ________
a) armature winding by drawing demagnetizing component of the current from ac supply
b) armature winding by drawing magnetizing component of the current from ac supply
c) field winding by drawing demagnetizing component of the current from dc supply
d) field winding by drawing magnetizing component of the current from dc supply

Explanation: Excess flux is neutralized y the demagnetizing component.
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4. The V-curves of synchronous motor is plotted between ________
a) Ia Vs If with constant shaft load
b) If Vs Ia with constant shaft load
c) power factor vs If
d) power factor vs Ia

Explanation: The V-curves of synchronous motor is plotted between Ia Vs If with constant shaft load.

5. When a constant power output is desired to maintain ________
a) Ef*sinδ and Ia*cosθ are to be maintained constant
b) Ef*sinδ is be maintained constant
c) Ia*cosθ is to be maintained constant
d) Ef*sinδ and Ia*sinθ are to be maintained constant

Explanation: For constant real power output the component of the excitation voltage lagging to Vt should remain constant as per the phasor diagram.

6. When the excitation voltage is increased from 1 to 1.3 pu of a 3-phase synchronous motor. Then load angle for the constant power operation ________
a) must be decreased
b) can also be increased
c) increased
d) decreased

Explanation: ‘Ef*sinδ’ is to be maintained constant. so if excitation voltage increases then the load angle must decrease.

7. The armature current is ________ at UPF when compared to leading power factor.
a) minimum
b) maximum
c) equal
d) none of the mentioned

8. The armature current is _______ at UPF than motor at lagging power factor.
a) minimum
b) maximum
c) zero
d) none of the mentioned

9. The inverted V plots is plotted between ________
a) power factor vs field current for constant shaft load
b) field current vs power factor for variable shaft load
c) armature current vs field current
d) terminal voltage vs power factor

Explanation: Inverted v-curves are plotted between power factor and field current for constant shaft load.

10. For a power system having induction motor loads, an overexcited synchronous motor is also attached. Then the over all power factor?
a) improves
c) becomes upf
d) remains same

Explanation: An over excited synchronous motor acts as a source of lagging reactive power and so the overall power factor improves.

11. For a power system having induction motor loads, an overexcited synchronous motor is also attached. The induction motor will now operate at ________
a) lagging
c) reduced power factor
d) increased power factor

Explanation: Adding synchronous motor will improve the pf of the system but then the induction motor will still work at lagging pf only.

12. Synchronous compensators are ________
a) over excited synchronous motor with no mechanical load
b) over excited synchronous motor with mechanical load
c) under excited synchronous motor with no mechanical load
d) normally excited synchronous motor with no mechanical load

Explanation: Synchronous compensators are synchronous motor with no mechanical load which are overexcited.

13. For a synchronous motor, mark the appropriate. a) A-Inductor, B-Capacitor
b) A-Capacitor, B-Resistor
c) A-Inductor, B-Inductor
d) A-Capacitor, b-Inductor

Explanation: A will act as the inductor curve consuming the reactive power while the B curve is like capacitor.

14. (I) Speed of an isolated alternator can be altered.
(II) Speed of an alternator connected to IBB can be also altered.
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

Explanation: Speed of the alternator connected to the infinite bus bas can not be changes as it has to be of fixed frequency.

15. (I)For constant power of an isolated alternator, V-curves can be obtained.
(II)For constant power of an alternator connected to IBB, V-curves can be obtained.
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

Explanation: For an isolated alternator working at constant power, we can not alter the armature current to have a v-curve while the is possible for the machine connected to infinite bus bar.

16. The operating frequency and voltage of an isolated alternator ________ if the prime mover input to motor is increased.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and frequency

Explanation: By varying the prime mover torque, the load angle as well as the operating frequency also get affected.

17. The active power delivered by an alternator will _______ when the prime mover input is increased for the alternator connected to infinite bus bar.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and prime mover droop characteristic

Explanation: The load angle will increase and so the real power will increase in case the prime mover input is increased.

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