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

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Power Flow Through an Inductive Impedance – I”.

1. Which is the correct relation below?
a) Pim = -Pog
b) Pim = (Pog)*
c) Pim = Pog
d) Pim = -(Pog)*
View Answer

Answer: a
Explanation: Electrical input to the motor is equal output power of the generator but in opposite direction.
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2. The condition for maximum power output for an alternator having its load impedance as (2+j) ohms is?
a) 26.56°
b) 63.43°
c) 90°
d) 116.56°
View Answer

Answer: a
Explanation: Maximum power output is arctan(1/2)= 26.56°.

3. For an alternator, the specified load angle should be _________ for an impedance angle of ‘θ’.
a) δ=θ
b) δ=90-θ
c) δ=90+θ
d) δ=-θ
View Answer

Answer: a
Explanation: dP/dδ = EVcos(δ+α) = 0
δ = 90-α=θ.
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4. For a 3 phase synchronous motor, the load angle should be equal to impedance angle.
a) True
b) False
View Answer

Answer: a
Explanation: dP/dδ = EVcos(δ+α)/Z = 0
δ = 90-α=θ.

5. With constant E,V,Z, the maximum input power can be obtained at load angle of value _______
a) 180-θ
b) 90+θ
c) θ
d) 180
View Answer

Answer: a
Explanation: dPi/dδ = EVcos(δ-α) = 0
δ = 90+α = 2α+θ=180-θ.
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6. for a 3 phase 315V alternator having impedance angle of 60 degree, the required load angle for the maximum power input to the generator is _______
a) 120°
b) 60°
c) 150°
d) 30°
View Answer

Answer: a
Explanation: δ = 90+α = 2α+θ=180-θ
So δ = 180-60=120°.

7. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1.2 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) upf
b) zpf
c) lagging pf
d) leading pf
View Answer

Answer: a
Explanation: The Value for Ef-Ia*ra = Vt; Here it is Vt = 0. So it is upf.
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8. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) Leading
b) Lagging
c) Upf
d) Zpf
View Answer

Answer: a
Explanation: Here Vt > Ef-Ia*ra.

9. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1.5 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) Leading
b) Lagging
c) Upf
d) Zpf
View Answer

Answer: b
Explanation: Here Vt < Ef-Ia*ra.
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10. For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at ____________

Vt < Ef-Ia*ra

a) Leading
b) Lagging
c) Upf
d) Zpf
View Answer

Answer: a
Explanation: The reactive power will be positive for this condition.

11.For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at ____________

Vt = Ef-Ia*ra

a) Leading
b) Lagging
c) Upf
d) Zpf
View Answer

Answer: c
Explanation: The net reactive power flow will be zero.

12. For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at ____________

Vt > Ef-Ia*ra

a) Leading
b) Lagging
c) Upf
d) Zpf
View Answer

Answer: b
Explanation: The motor will absorb the reactive power which will be lagging in nature.

13. Reactive power is a matter of interest at ____________
a) output terminals of generator
b) input terminal of motor
c) output terminals of generator and input terminal of motor
d) none of the mentioned
View Answer

Answer: a
Explanation: Reactive power is a matter of interest at output terminals of generator and input terminal of motor.

14. The reactive power flow is outward from an alternator, then it is ____________
a) over excited
b) under excited
c) critically excited
d) none of the mentioned
View Answer

Answer: a
Explanation: Because Q > 0; For an alternator, Q = V(Ecosδ -V)/Z.

15. The reactive power flow is outward from an alternator, then the alternator works at ____________
a) lagging power factor
b) leading power factor
c) zero power factor
d) normal power factor
View Answer

Answer: a
Explanation: As the reactive power is positive the power factor based on the standard convention should be lagging.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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