# Design of Electrical Machines Questions and Answers – Armature Design – 1

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Armature Design – 1”.

1. How many factors does the design of armature of synchronous machines depend upon?
a) 2
b) 4
c) 3
d) 5

Explanation: There are 5 factors that decide the design of the armature of synchronous machines. They are i) single or double layer winding, ii) number of armature slots, iii) coil span, iv) turns per phase, v) conductor section.

2. When are the double layer bar windings made use of during the armature design?
a) large values of flux per pole and small number of turns per phase
b) large values of flux per pole and large number of turns per phase
c) small values of flux per pole and small number of turns per phase
d) small values of flux per pole and large number of turns per phase

Explanation: The double layer bar windings are made use of when there is large values of flux per poles. The double layer bar windings are made use of when there are small number of turns per phase.

3. Which type of machines have a large number of poles per phase?
a) high voltage machines and machines with high value of flux per pole
b) high voltage machines and machines with small value of flux per pole
c) small voltage machines and machines with high value of flux per pole
d) small voltage machines and machines with low value of flux per pole

Explanation: The high voltage machines have a large number of poles per phase. The small value of flux per pole per phase also results in large number of poles per phase.

4. Which among the following makes double layer windings advantageous than the single layer windings?
a) ease in the manufacture of coils and lower cost of winding
b) less number of coils are required as spare in the case of winding repairs
c) fractional slot windings can be employed
d) ease in the manufacture of coils and lower cost of winding, fractional slot windings can be employed, less number of coils are required as spare in the case of winding repairs

Explanation: The double layer windings have an advantage over single layer windings because of ease in the manufacture of coils and lower cost of winding, less number of coils are required as spare in the case of winding repairs, fractional slot windings can be employed.

5. The single layer windings have higher efficiency and quieter operation because of narrow slot openings.
a) true
b) false

Explanation: The single layer winding has high efficiency and quiet operation because of narrow slot openings. They also have higher space factor owing to the absence of inter layer separator.
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6. When is the double layer bar or wave windings made use of?
a) when single turns coils are necessary as with turbo alternators and unipolar low voltage machines
b) when single turns coils are necessary as with turbo alternators and bipolar low voltage machines
c) when single turns coils are necessary as with turbo alternators and multipolar low voltage machines
d) when double turns coils are necessary as with turbo alternators and unipolar low voltage machines

Explanation: The double layer bar or wave windings are used when the single turns coils are necessary. They are also made use of with the multipolar low voltage machines.

7. How many factors are related in the selection of the armature slots?
a) 5
b) 6
c) 7
d) 4

Explanation: There are 6 factors associated with the selection of the armature slots. They are i) Balanced windings ii) Cost iii) Hot spot temperature iv) Leakage reactance v) Tooth ripples vi) flux density in iron.

8. How is the number of armature slots associated with the armature windings?
a) number of slots should be such that unbalanced winding is obtained
b) number of slots should be such that balanced winding is obtained
c) number of slots should be so low as possible
d) number of slots should be high as possible

Explanation: The number of slots should be such that balanced winding is obtained. Balanced windings should be obtained because it may lead to losses and heating effects.

9. How is the number of armature slots associated with the cost factor?
a) small number of slots leads to less cost
b) small number of slots leads to high cost
c) large number of slots leads to high cost
d) large number of slots leads to low cost

Explanation: The smaller the number of slots, the less will be the cost. This is because there are fewer coils to wind, form insulate, place into slots, and connect.

10. How is the number of armature slots associated with the hotspot temperature?
a) small number of slots leads to less hotspot temperature
b) small number of slots leads to high hotspot temperature
c) large number of slots leads to high hotspot temperature
d) large number of slots leads to low hotspot temperature

Explanation: The small number of slots leads to an increase in hotspot temperature. The small number of slots results in bunching of conductors leaving small space for circulation of air.

11. How is the number of armature slots associated with the leakage reactance?
a) small number of slots leads to less leakage reactance
b) small number of slots leads to high leakage reactance
c) large number of slots leads to high leakage reactance
d) large number of slots leads to low leakage reactance

Explanation: When the number of slots is small the leakage flux is increased. As the leakage flux is increased, the leakage reactance is increased owing to conductors lying near each other.

12. How is the number of armature slots associated with the tooth ripples?
a) tooth ripples are increased, if the number of slots are increased
b) tooth ripples are decreased, if the number of slots are increased
c) tooth ripples are increased, if the number of slots are decreased
d) tooth ripples are decreased, if the number of slots are decreased

Explanation: The tooth ripples in the field form and the consequent pulsation losses in pole face decrease if a large number of slots are used. Also, the waveform of generated voltage is free from ripples.

13. How is the number of armature slots associated with the flux densities in iron?
a) tooth ripples are increased, if the number of slots are increased
b) tooth ripples are decreased, if the number of slots are increased
c) tooth ripples are increased, if the number of slots are decreased
d) tooth ripples are decreased, if the number of slots are decreased

Explanation: The large number of slots a greater space is taken up by the insulation. This results in the narrower teeth giving flux densities which may go beyond acceptable limits.

14. The value of slot pitch depends upon the voltage of the machine.
a) true
b) false

Explanation: The value of the slot pitch serves as a guide when choosing the number of armature slots. The value of the slot pitch depends upon the voltage of the machine.

15. What is the value of the slot pitch for the low voltage machines?
a) slot pitch < 25 mm
b) slot pitch = 25 mm
c) slot pitch less than equal to 25 mm
d) slot pitch greater than equal to 25 mm

Explanation: The value of the slot pitch depends upon the voltage of the machine. The slot pitch is less than equal to 25 mm for low voltage machines.

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