# Design of Electrical Machines Questions and Answers – Slot Dimensions

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

1. What is the range of the flux density in the teeth at no load?
a) 1.7-1.8 Wb per mm2
b) 1.3-1.5 Wb per mm2
c) 1.3-1.6 Wb per mm2
d) 1.4-1.6 Wb per mm2

Explanation: The minimum value of the flux density in the teeth is given to be 1.7 Wb per mm2. The maximum value of the flux density in the teeth is given to be 1.8 A per mm2.

2. What is the formula for the minimum width of the tooth?
a) minimum width of tooth = flux * pole proportion * (number of stator slots / number of poles) * length * 1.8
b) minimum width of tooth = flux / pole proportion * (number of stator slots / number of poles) * length * 1.8
c) minimum width of tooth = flux * pole proportion / (number of stator slots / number of poles) * length * 1.8
d) minimum width of tooth = flux * pole proportion * (number of stator slots / number of poles) * length / 1.8

Explanation: The flux value, pole proportion and the length values are first obtained. Then the ratio of the number of stator slots to number of poles is obtained and on substitution gives the minimum width of tooth.

3. Name the slots that are commonly used.
a) parallel sided
b) square sided
c) rectangular
d) circular

Explanation: The most commonly used type of slots are parallel sided. The other type of slots may be used for the purposes required.

4. How is the teeth and the minimum width designed in the machines?
a) teeth is tapered and minimum width is across the medium
b) teeth is sharpened and minimum width occurs across the air gap
c) teeth is widened and minimum width occurs across the air gap
d) teeth is reduced and minimum width occurs across the medium

Explanation: Parallel sided slots are made use of. Hence, the teeth is tapered and their minimum width occurs at the air gap surface.

5. What is the formula for the maximum permissible width of slot?
a) maximum permissible width = slot pitch * minimum width of the teeth
b) maximum permissible width = slot pitch + minimum width of the teeth
c) maximum permissible width = slot pitch / minimum width of the teeth
d) maximum permissible width = slot pitch – minimum width of the teeth

Explanation: The slot pitch is first calculated with its respective formula. Next, the minimum width of the teeth is calculated and the difference between both gives the maximum permissible width.

6. By how much should the depth of slot not exceed the width?
a) two times
b) three times
c) four times
d) six times

Explanation: The depth of the slot depends upon the width of the slot. The depth should not exceed three times the width of the slot.

7. Why are slot made deeper in the machine?
a) to increase the short circuit current
b) to reduce the short circuit current
c) to increase the open circuit current
d) to reduce the open circuit current

Explanation: The slots used in the machine are basically deeper slots. The slots are made deeper even more to increase the leakage reactance and to limit the short circuit current.

8. What is the formula for the height of length of mean turn of armature?
a) length of mean turn = 2*length + 2.5*pole pitch + 0.06 kV + 0.2
b) length of mean turn = 2*length + 2*pole pitch + 0.06 kV + 0.2
c) length of mean turn = 2*length + 2.5*pole pitch – 0.06 kV – 0.2
d) length of mean turn = 2*length – 2.5*pole pitch – 0.06 kV – 0.2

Explanation: The length of the slots is obtained along with the pole pitch. The output kV is calculated and on substituting we get the length of mean turn.

9. The flux density in the armature core of salient pole machines lies between 1-1.2 Wb per m2.
a) true
b) false

Explanation: The value of depth of core can be calculated by assuming a suitable value of flux density. The value of the flux density varies from 1-1.2 Wb per m2.

10. What is the formula for the depth of armature core?
a) depth of armature core = flux / length of the iron core * flux density
b) depth of armature core = flux * length of the iron core * flux density
c) depth of armature core = flux / 2 * length of the iron core * flux density
d) depth of armature core = flux * 2 * length of the iron core * flux density

Explanation: The flux value is calculated along with the length of the iron core. Next, the suitable flux density is chosen and the depth of armature core is calculated.

11. What is the formula for the outer diameter of the stator?
a) outer diameter = inner diameter + depth of the slots + depth of armature core
b) outer diameter = inner diameter + 2*depth of the slots + depth of armature core
c) outer diameter = inner diameter + 2*(depth of the slots + depth of armature core)
d) outer diameter = inner diameter + depth of the slots + 2*depth of armature core 