# Design of Electrical Machines Questions and Answers – Design of Rotor

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

1. How many design steps are available for the design of rotor?
a) 5
b) 6
c) 7
d) 8

Explanation: There are 6 design steps involved in the design of the rotor. They are number of rotor slots, area of rotor bars, area of end rings, rotor resistance, rotor teeth, rotor core.

2. What is the main motive while choosing the number of rotor slots?
a) increasing the efficiency
b) decreasing the losses
c) no noise is produced
d) high output is produced

Explanation: There are basically 6 steps involved in the rotor design. The number of slots is chosen such that no noise is produced.

3. What is the formula for the harmonic poles due to slots?
a) harmonic poles due to slots = 2 * (number of slots ± number of poles / 2)
b) harmonic poles due to slots = 2 / (number of slots ± number of poles / 2)
c) harmonic poles due to slots = 2 * (number of slots ± number of poles * 2)
d) harmonic poles due to slots = 1/ 2 * (number of slots ± number of poles / 2)

Explanation: First the number of slots and number of poles are first calculated. On substitution we get the harmonic poles due to the slots.

4. What factors are used fixing the number of stator slots?
a) winding arrangement
b) number of poles
c) winding arrangement or number of poles
d) winding arrangement and number of poles

Explanation: The number of poles are fixed according to the winding arrangement. The number of poles are also fixed according to the number of poles.

5. Which condition satisfies the quiet operation in machines?
a) number of stator slots is divisible by number of pairs of poles
b) number of rotor slots differs from the number of stator slots by more than the number of poles
c) number of rotor slots is not divisible by number of pairs of poles
d) number of stator slots differs from the number of rotor slots by more than the number of poles

Explanation: The number of rotor slots are decided for quieter operation of the machine. The number of rotor slots differs from the number of stator slots by more than the number of poles.

6. What among the following are considered for the selection of number of rotor slots?
a) magnetic locking
b) cusps
c) magnetic locking or cusps
d) magnetic locking and cusps

Explanation: The selection of number of rotor slots depends on the magnetic locking. The selection of number of rotor slots depends on the cusps also.

7. What is the formula for the total stator copper section for main winding?
a) total stator copper section for main winding = number of turns in the running winding * area of the running winding conductor
b) total stator copper section for main winding = 2 * number of turns in the running winding * area of the running winding conductor
c) total stator copper section for main winding = number of turns in the running winding / area of the running winding conductor
d) total stator copper section for main winding = 2* number of turns in the running winding / area of the running winding conductor

Explanation: First the number of turns in the running winding is calculated along with the area of the running winding conductor. On substitution it gives the total stator copper section for main winding.

8. What is the formula for the total cross section of rotor bars?
a) total cross section of rotor bars = number of rotor slots * area of each bar
b) total cross section of rotor bars = number of rotor slots / area of each bar
c) total cross section of rotor bars = number of rotor slots + area of each bar
d) total cross section of rotor bars = number of rotor slots – area of each bar

Explanation: The number of rotor slots and area of each bar is first calculated. On substitution it gives the total cross section of rotor bars.

9. What is the range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding for copper?
a) 0.4-0.8
b) 0.3-0.7
c) 0.5-0.8
d) 0.8-0.9

Explanation: The minimum value of range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 0.5. The maximum value range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 0.8.

10. What is the formula of the end ring current?
a) end ring current = number of rotor slots * bar current * 3.14 * number of poles
b) end ring current = number of rotor slots * bar current * 3.14 / number of poles
c) end ring current = number of rotor slots / bar current * 3.14 * number of poles
d) end ring current = number of rotor slots * bar current / 3.14 * number of poles

Explanation: The number of rotor slots and the bar current along with the number of poles is calculated. On substitution it gives the end ring current value.

11. What is the range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding for aluminium?
a) 1-1.3
b) 1-1.4
c) 1-1.6
d) 1.2-1.5

Explanation: The minimum value of range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 1. The maximum value range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 1.6.

12. What is the formula for the area of each bar?
a) area of each bar = current through each bar / current density through each bar
b) area of each bar = current through each bar * current density through each bar
c) area of each bar = current density through each bar / current through each bar
d) area of each bar = current density through each bar * current through each bar

Explanation: The current through each bar and the current density through each bar is calculated. On substitution the area of each bar is obtained.

13. What is the formula of the area of each end ring?
a) area of each end ring = 0.32 * total cross section of rotor bars * number of poles
b) area of each end ring = 0.32 / total cross section of rotor bars * number of poles
c) area of each end ring = 0.32 * total cross section of rotor bars / number of poles
d) area of each end ring = 1/0.32 * total cross section of rotor bars * number of poles

Explanation: First the total cross section of rotor bars along with the number of poles are calculated. On substitution the area of each end ring is obtained.

14. What is the formula of the rotor teeth flux density?
a) flux density of rotor teeth = maximum flux / (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
b) flux density of rotor teeth = maximum flux * (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
c) flux density of rotor teeth = 1/maximum flux * (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
d) flux density of rotor teeth = maximum flux / (number of rotor slots * number of poles) * length of the teeth * depth of rotor core

Explanation: The maximum flux, the number of rotor slots per pole and the length of teeth along with the depth of rotor core is calculated. On substitution the flux density of the rotor teeth is obtained.

15. What is the range for the ratio of the resistance to reactance in the split phase motors?
a) 0.40-0.55
b) 0.45-0.55
c) 0.45-0.8
d) 0.45-0.6

Explanation: The range for the ratio of the resistance to reactance in the split phase motors is 0.45-0.55. The range for the ratio of the resistance to reactance in the capacitor start motors is 0.45-0.8.

Sanfoundry Global Education & Learning Series – Design of Electrical Machines.

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