This set of Basic Electrical Engineering Question Bank focuses on “Relationship between Frequency, Speed and Number of Pole Pairs”.

1. The waveform of the emf generated undergoes one complete cycle when?

a) Conductors move past north pole

b) Conductors move past south pole

c) Conductors move past north and south poles

d) Conductors are stationary

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Explanation: The waveform of the e.m.f. generated in an a.c. generator undergoes one complete cycle of variation when the conductors move past an N and an S pole.

2. When is the shape of the negative half of the emf waveform equal to the positive half?

a) When the conductors move past north pole

b) When conductors move past south pole

c) When conductors move past both north and south pole

d) When conductors are stationary

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Explanation: The waveform of the e.m.f. generated in an a.c. generator undergoes one complete cycle of variation when the conductors move past an N and an S pole and the shape of the wave over the negative half is exactly the same as that over the positive half.

3. Which is the correct formula for frequency in an ac generator?

a) f=p*n

b) f=p/n

c) f=n/p

d) f=n^{2}p

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Explanation: The frequency in an ac generator is p*n, where p is pairs of poles and speed is n revolutions per second.

4. What will happen to the frequency if the number of revolutions increases?

a) Increases

b) Decreases

c) Remains the same

d) Becomes zero

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Explanation: We know that:

f=p*n, therefore, as n increases, f also increases.

Hence frequency increases if number of revolutions increases.

5. What happens to the frequency if the number of pairs of poles increases?

a) Increases

b) Decreases

c) Remains the same

d) Becomes zero

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Explanation: We know that:

f=p*n, therefore, as p increases, f also increases.

Hence frequency increases if number of pair of poles increases.

6. Calculate the frequency if the number of revolutions is 300 and the paired poles are 50.

a) 15kHz

b) 150kHz

c) 1500kHz

d) 150Hz

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Explanation: We know that f=p*n

f=50*300=15000 Hz = 15kHz.

7. Calculate the number of revolutions if the frequency is 15kHz and the paired poles are 50.

a) 100

b) 200

c) 300

d) 400

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Explanation: We know that f=p*n

f=15kHz=15000Hz, p=50

15000=50*n => n=15000/50=300.

8. Calculate the number of paired poles if the frequency is 15kHz and the number of revolutions is 300.

a) 10

b) 30

c) 50

d) 70

View Answer

Explanation: We know that f=p*n

f=15kHz=15000Hz, n=300

15000=p*300 => p=15000/300=50.

9. What is the frequency of a two pole machine having n=50?

a) 100Hz

b) 200Hz

c) 50Hz

d) 25Hz

View Answer

Explanation: The frequency in an ac generator is p*n, where p is pairs of poles and speed is n revolutions per second. For a two pole machine, p=1.

f=pn = 1*50 = 50Hz.

10. What is the minimum number of poles that a machine must have __________

a) 1

b) 2

c) 4

d) 10

View Answer

Explanation: The minimum number of poles that a machine must have is 2 because a machine must have at least one pair of poles = 2 poles.

**Sanfoundry Global Education & Learning Series – Basic Electrical Engineering.**

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