# Electrical Machines Questions and Answers – Power Flow Through an Inductive Impedance – II

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This set of Electrical Machines Interview Questions and Answers for Experienced people focuses on “Power Flow Through an Inductive Impedance – II”.

1. When excitation given to a 3-phase alternator is equal to terminal voltage then ____________
a) the absorbed reactive power is zero
b) the absorbed real power is zero
c) the delivered real power is zero
d) any of the mentioned

Explanation: When excitation is equal to the terminal voltage there will not be any net flow of energy across the system.

2. The generator(3 phase alternator) absorbs reactive power from the infinite bus at leading power factor.
a) True
b) False

Explanation: Infinite bus bar provides the flux needed to set up the reactive power to produce the real power in the generator.

3. When terminal voltage exceeds the excitation, then operating power factor is?
b) lagging
c) uPF
d) zpf

Explanation: As the reactive power is negative the power factor based on the standard convention should be lagging.
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4. Under the conditions of maximum reactive power output, the reactive power flowing out of an alternator is __________
a) positive
b) negative
c) zero
d) equal to power

Explanation: The alternator will always absorb the reactive power to meet its flux requirements to generate the real power.

5. Under the condition of maximum power delivered, reactive power is __________
a) negative
b) positive
c) zero
d) same as previous condition

Explanation: The alternator will always absorb the reactive power to meet its flux requirements to generate the real power.

6. Under the condition for 3 phase alternator, maximum active power output, the delivered reactive power output is ________ while under maximum reactive power output, the delivered reactive power is ________
a) negative, negative
b) positive, negative
c) negative, positive
d) positive, positive

Explanation: Under both the situation, there is need of reactive power which will set up the flux in the machine to make it run.

7. For an overexcited 3 phase synchronous motor, connected to infinite bus is __________
a) motor is delivering reactive power to IBB
b) motor is absorbing reactive power to IBB
c) motor is delivering zero reactive power to IBB
d) none of the mentioned

Explanation: An over excited synchronous motor will act as a source of the reactive power.

8. If the synchronous motor connected to the infinite bus bar delivers positive reactive power to it, then it must be operating at __________
b) lagging pf
c) zero pf
d) unity pf

Explanation: An over excited synchronous motor will act as a source of the reactive power and it will operate at leading pf.

9. For an underexcited 3 phase synchronous motor, connected to IBB is __________
a) absorbing reactive power from IBB
b) delivering reactive power to IBB
c) absorbing zero reactive power from IBB
d) delivering zero reactive power to IBB

Explanation: An under excited synchronous motor will act as a source of the reactive power.

10. If the synchronous motor connected to the infinite bus bar absorbs reactive power to it, then it must be operating at __________
b) lagging pf
c) zero pf
d) unity pf

Explanation: An under excited synchronous motor will act as a sink of the reactive power and it will operate at leading pf.

11. What is Synchronous motor?
a) absorbs reactive power for maximum input real power
b) absorbs reactive power for maximum input reactive power
c) delivers reactive power for maximum input real power
d) delivers reactive power for maximum input reactive power

Explanation: Synchronous motor always absorbs reactive power for maximum real as well as reactive power.

12. Under the conditions of maximum values of active or reactive power, a synchronous machine always absorbs reactive power.
a) True
b) False

Explanation: Synchronous motor always absorbs reactive power for maximum real as well as reactive power.

13. An ________ synchronous machine delivers reactive power whereas an ________ one absorbs reactive power.
a) over excited, under excited
b) under excited, over excited
c) over excited, over excited
d) under excited, normal excited

Explanation: An over excited synchronous machine delivers reactive power whereas an under excited one absorbs reactive power.

14. An alternator, with synchronous reactance of 0.8 pv is connected to an IBB at rated voltage. With its excitation emf adjusted to 1.3 pv, the alternator delivers an output of 0.5 pv. The load angle is __________
a) 17.92
b) 72.08
c) 34.92
d) 90

Explanation: δ = atan(0.5*0.8/1.3) = 17.92°.

15. An 3 phase alternator star connected 400 V synchronous motor takes a power input of 5472 watts at rated voltage. Its synchronous reactance is 10** per phase and negligible resistance. If its excitation voltage is adjusted equal to the rated voltage of 400 V, its power factor is _______
a) cos(10°)
b) cos(20°)
c) cos(40°)
d) cos(30°)

Explanation: Vt = Ef = 400/1.73 = 231 V
V*Esinδ/X = P
δ = 20°
θ = δ/2.

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