# Design of Electrical Machines Questions and Answers – Selection of Number of Poles

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Selection of Number of Poles”.

1. How is the selection of number of poles made in AC and DC machines?
a) any number of poles can be used for both AC and DC machines
b) fixed number of poles in both AC and DC machines
c) fixed number of poles in DC machines and any number of poles in AC machines
d) fixed number of poles in AC machines and any number of poles in DC machines

Explanation: When it comes to DC machines, any number of poles can be made use of, but it should be within a range. Whereas in AC machines the number of poles is fixed by supply frequency and speed.

2. How many considerations are present in the selection of number of poles?
a) 4
b) 5
c) 6
d) 7

Explanation: There are 7 consideration in selection of number of poles. They are frequency, weight of iron parts, weight of copper, length of commutator, labor charges, flash over, distortion of field form.

3. What is the formula for frequency of flux reversals?
a) f = p*n
b) f = p/n
c) f = n/p
d) f = (p*n)/2

Explanation: Frequency is directly proportional to the number of poles (p). It is also proportional to the speed of the machine as well.

4. What is the range of frequency during the selection of number of poles?
a) 20-50 Hz
b) 25-40 Hz
c) 25-50 Hz
d) >50 Hz

Explanation: While selecting the number of poles, the lowest value of frequency should be minimum 25 Hz. The highest value of frequency should be limited to 50 Hz.

5. What is the relation of hysteresis loss and weight of iron parts with respect to increase of number of poles?
a) decrease in hysteresis loss, increase in weight
b) decrease in hysteresis loss, decrease in weight
c) increase in hysteresis loss, increase in weight
d) increase in hysteresis loss, decrease in weight

Explanation: With larger number of poles, the area of cross section can be reduced, henceforth decreasing the hysteresis loss. Also by increasing pole number, weight of iron parts is reduced.

6. What happens to the weight of copper in both armature and field windings when the number of poles increase?
a) weight of copper in armature winding decreases and weight of copper in field winding increases
b) weight of copper in armature winding increases and weight of copper in field winding decreases
c) weight of copper in armature winding and field winding decreases
d) weight of copper in armature winding and field winding increases

Explanation: The weight of copper is indirectly proportional to the number of poles. As the number of poles increases, the weight of the copper decreases.

7. What happens to the length of the commutators with the increase in number of poles?
a) The length of commutators are increased
b) The length of commutators are decreased
c) The length of commutators are stable
d) The length of commutators are higher

Explanation: The area of the brushes decreases if the number of poles are being increased. As the area of the brushes are decreased, the length of the commutators also decrease.

8. What happens to the labor charges when there is an increase in number of poles?
a) labor charges are reduced
b) labor charges are increased
c) labor charges are fixed always
d) labor charges vary

Explanation: With increase in the number of poles, the armature windings increase, and more work increases to insulate. The commutator segments also increase, and the work increases.

9. What is the effect of the distortion of field form with respect to the small number of poles?
a) small number of poles cause no distortions
b) small number of poles clears all distortions
c) small number of poles reduces distortions
d) small number of poles increases distortions

Explanation: When there is small number of poles, that time the armature mmf per pole increases. As the armature mmf increases, it results in increase of distortion.

10. Large number of poles lead to large flashover between brushes.
a) true
b) false

Explanation: The number of brushes is equal to number of poles. For the same diameter of the commutator, the distance between the adjacent brush arms decreases and this increases the possibility of flashover.

11. What is the dependency of the cost of the armature and field windings with respect to large number of poles?
a) high cost for armature windings, low cost for field windings
b) high cost for armature windings, high cost for field windings
c) low cost for armature windings, high cost for field windings
d) low cost for armature windings, low cost for field windings

Explanation: With large number of poles the armature and the field windings reduce in number. Thus the cost of the field and armature windings also decrease.

12. Lower values of frequency are used for small machines.
a) true
b) false

Explanation: Lower values of frequency are actually used for the large machines. Also, higher values of frequency are actually used for small machines.

13. What is the range of the current per parallel path for the choice of number of poles?
a) limited to 100 A
b) limited to 150 A
c) limited to 200 A
d) limited to 250 A

Explanation: The current per parallel path should be limited to maximum of 200 A. If the limit gets exceeded then there occurs damage to the machine.

14. What should be the range of the current per brush arm?
a) limited to 400 A
b) limited to 200 A
c) limited to 100 A
d) limited to 300 A

Explanation: The current per brush arm should be limited to maximum of 400 A. If the limit gets exceeded then there occurs damage to the machine.

15. What should be the armature mmf per pole for output over 1500 kW?
a) 5000 A
b) 5000-7500 A
c) 7500-10000 A
d) upto 12500

Explanation: 5000 A is for output of about 100 kW. 5000 to 7500 A for output voltage of 100 to 500 kW. 7500 to 10000 A is for the output of 500 to 1500 kW.

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