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

1. How many type of armature windings are present in the DC machine armature?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are basically two types of armature windings present. They are known as lap winding and wave winding.

2. Which factor determines the difference between the types of armature windings?

a) brush connection

b) slip ring connection

c) commutator connection

d) pole connection

View Answer

Explanation: The armature windings are all connected to the commutator in the case of DC machines. Commutator is nothing but device which converts dc voltage to ac voltage.

3. What is the meaning of pole pitch?

a) type of armature slot

b) number of armature slots/pole

c) number of poles/armature slot

d) number of poles/number of armature slots

View Answer

Explanation: Pole pitch is the concept which is used in the armature slot design. It is nothing but the ratio between the number of armature slots present to the number of poles used in the machine design.

4. A 4 pole DC machine has 36 number of armature conductors. What is the pole pitch?

a) 9

b) 1/9

c) 9^{2}

d) 1/9^{2}

View Answer

Explanation: Pole pitch = Number of armature slots/pole

Pole Pitch = 36/4 = 9.

5. The choosing of the slot values for armature depends on the type of windings.

a) true

b) false

View Answer

Explanation: The slot values are chosen based on the type of windings. If wave winding is used, then multiples of pole pair is chosen as slot values and if lap windings are used the slot values are continuous and not multiples of pole pair.

6. What is the condition for choosing the armature current/parallel path in armature design?

a) > 200 ampere for both lap and wave windings

b) >200 ampere for lap winding, <200 ampere for wave winding

c) < 200 ampere for both lap and wave windings

d) <200 ampere for lap winding, >200 ampere for wave winding

View Answer

Explanation: While designing the armature slots, the armature current should be less than 200 ampere. The armature current should also never cross 200 ampere for the lap winding.

7. What is the range of armature slot pitch in the armature slot design?

a) 20mm – 25mm

b) 25mm – 35mm

c) 30mm – 35mm

d) > 35mm

View Answer

Explanation: For the armature design, the lowest value of slot pitch is generally chosen as 25mm. The highest value of slot pitch is generally chosen as 35mm.

8. For a DC machine, the armature slot pitch is 35mm and the diameter is 0.2m. What is the number of armature slots for the machine?

a) 17

b) 18

c) 19

d) 20

View Answer

Explanation: Number of armature slots = (3.14*D)/Number of armature slot pitch

Number of armature slots = (3.14*0.2)/35*10

^{-3}

Number of armature slots = 17.94 = 18(approx).

9. What is the formula to reduce flux pulsations?

a) armature slot/number of poles = integer

b) armature slot/number of poles = integer + 0.5

c) armature slot/number of poles = integer – 0.5

d) armature slot/number of poles = integer +-0.5

View Answer

Explanation: First the number of armature slots is calculated. Then the ratio of the number of armature slots to the number of poles is approximately equal to integer+-0.5.

10. The number of coils chosen should be minimum in number.

a) true

b) false

View Answer

Explanation: The number of coils chosen for the armature slot design should always be minimum in number. This is because the machine cost reduces and it becomes cost efficient.

11. What should be the range of the integer value while calculating the formula to reduce the flux pulsations?

a) 8-14

b) 9-15

c) 9-16

d) 9-17

View Answer

Explanation: While calculating the formula to reduce the flux pulsations, the lowest value is generally chosen as 9. The highest value of integer chosen is generally 16.

12. What is ‘Z/Sa’ calculation during design of armatures?

a) armature poles/slots

b) armature conductors/slots

c) armature slots/poles

d) armature slots/conductors

View Answer

Explanation: This is the second step in the armature design. Firstly, the number of armature conductors is found out. Then the number of slots used is taken and the ratio of both gives the value of ‘Z/Sa’.

13. What is the formula for calculating the minimum number of coils?

a) Cmin = Voltage

b) Cmin = Voltage/5

c) Cmin = Voltage/10

d) Cmin = Voltage/15

View Answer

Explanation: The voltage through the armature is first calculated from the data given. Then the value of voltage is divided by 15, to get the value of minimum number of coils.

14. Given Z=228 conductors, number of coils C=38 for a DC machine. What is the turns/coils ratio?

a) 4

b) 3

c) 7

d) 3

View Answer

Explanation: Turns/coils = Number of armature conductors/(2*Number of coils)

Turns/coils = 228/38*2 = 3.

15. For a DC generator, given the load current is 510 ampere, field current is 1.4 ampere. What is the value of armature current?

a) 508.6 ampere

b) 714 ampere

c) 511.4 ampere

d) 364.28 ampere

View Answer

Explanation: For DC generator, Armature Current = Load Current + Field Current

Armature Current = 510 + 1.4 = 511.4 ampere.

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