# Design of Electrical Machines Questions and Answers – Dispersion Coefficient

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

1. How many factors influence the power factor of an induction motor?
a) 3
b) 2
c) 1
d) 4

Explanation: There are 2 factors which influence the power factor of an induction motor. They are magnetizing current and ideal short circuit current.

2. What is the relation between the magnetizing current and power factor?
a) magnetizing current is directly proportional to the power factor
b) magnetizing current is indirectly proportional to the power factor
c) magnetizing current is directly proportional to the square of the power factor
d) magnetizing current is indirectly proportional to the square of the power factor

Explanation: Magnetizing current is indirectly proportional to the power factor. As the magnetizing current is large, the power factor is poor.

3. What is the relation between the leakage current and power factor?
a) leakage current is directly proportional to the power factor
b) leakage current is indirectly proportional to the power factor
c) leakage current is directly proportional to the square of the power factor
d) leakage current is indirectly proportional to the square of the power factor

Explanation: Leakage current is indirectly proportional to the power factor. A small leakage current means a very good power factor.

4. What is the formula for dispersion coefficient?
a) dispersion coefficient = magnetizing current / ideal short circuit current
b) dispersion coefficient = magnetizing current * ideal short circuit current
c) dispersion coefficient = magnetizing current + ideal short circuit current
d) dispersion coefficient = magnetizing current – ideal short circuit current

Explanation: First the magnetizing current is calculated. Next the ideal short circuit current is calculated. The ratio of both gives the value of dispersion coefficient.

5. What is the formula for dispersion coefficient?
a) dispersion coefficient = 0.838 * 106 * 3.14 / air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
b) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length / effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
c) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance * pole pitch * (window space factor)2 * number of slots per pole per phase
d) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase

Explanation: For the calculation of dispersion coefficient, first the air gap length, effective specific permeance is calculated. Next the pole pitch, window space factor and the number of slots per pole per phase.
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6. The increase in number of poles, the dispersion coefficient increases and this gives a low power factor.
a) true
b) false

Explanation: The increase in number of poles increases the dispersion coefficient. The increases in dispersion coefficient gives a low power factor.

7. What is the relation between the number of poles and pole pitch with power factor?
a) number of poles increases, pole pitch increases, bad power factor
b) number of poles increases, pole pitch decreases, good power factor
c) number of poles increases, pole pitch decreases, good power factor
d) number of poles increases, pole pitch increases, bad power factor

Explanation: As the number of poles increases, the pole pitch decreases and the number of slots per pole per phase also decreases. This increases the dispersion coefficient and it leads to poor power factor.

8. What is the relation between the power factor and the air gap length?
a) small air gap length, dispersion coefficient increases, good power factor
b) small air gap length, dispersion coefficient decreases, bad power factor
c) small air gap length, dispersion coefficient increases, bad power factor
d) small air gap length, dispersion coefficient decreases, good power factor

Explanation: If the air gap length is small, the dispersion coefficient decreases. As the dispersion coefficient decreases, the power factor increases.

9. What is the relation between the dispersion coefficient and maximum power factor?
a) dispersion coefficient is directly proportional to the power factor
b) dispersion coefficient is indirectly proportional to the power factor
c) dispersion coefficient is directly proportional to the square of the power factor
d) dispersion coefficient is indirectly proportional to the square of the power factor

Explanation: The dispersion coefficient is indirectly proportional to the maximum power factor. As the dispersion coefficient increases, the power factor reduces drastically.

10. What is the value of the no. of poles for obtaining a dispersion coefficient = 0.5?
a) 5
b) 7
c) 10
d) 6

Explanation: The machines with 6 poles can result in a dispersion coefficient of 0.5. The dispersion coefficient of 0.5 can be obtained for 2 pole and 4 pole machines also.

11. What is the relation between the overload capacity and dispersion coefficient?
a) overload capacity is directly proportional to the dispersion coefficient
b) overload capacity is indirectly proportional to the dispersion coefficient
c) overload capacity is directly proportional to the square of the dispersion coefficient
d) overload capacity is indirectly proportional to the square of the dispersion coefficient

Explanation: Overload capacity is indirectly proportional to the dispersion coefficient. The overload capacity of induction motors decreases with an increase in the dispersion coefficient.

12. What is the relation between the overload capacity and magnetizing current?
a) overload capacity is directly proportional to the magnetizing current
b) overload capacity is indirectly proportional to the magnetizing current
c) overload capacity is directly proportional to the square of the magnetizing current
d) overload capacity is indirectly proportional to the square of the magnetizing current

Explanation: Overload capacity is directly proportional to the magnetizing current. Overload capacity increases the magnetizing current and this increases the dispersion coefficient and this gives a poor power factor.

13. What is the relation between the ideal short circuit current and the number of poles?
a) short circuit current is directly proportional to the number of poles
b) short circuit current is directly proportional to the square of the number of poles
c) short circuit current is indirectly proportional to the number of poles
d) short circuit current is indirectly proportional to the square of the number of poles

Explanation: The ideal short circuit current is indirectly proportional to the number of poles. As the number of poles increases, the ideal short circuit current decreases.

14. What is the relation between maximum power and the number of poles?
a) maximum power factor is directly proportional to the number of poles
b) maximum power factor is directly proportional to the square of the number of poles
c) maximum power factor is indirectly proportional to the number of poles
d) maximum power factor is indirectly proportional to the square of the number of poles

Explanation: Short circuit current is indirectly proportional to the number of poles. The short circuit current increases the dispersion coefficient. As the dispersion coefficient increases, the maximum power factor decreases.

15. The magnetizing current decreases as the number of poles is decreased.
a) true
b) false

Explanation: As the number of poles is reduced the magnetizing current is reduced. As the magnetizing current is reduced, the dispersion coefficient decreases and the power factor increases.

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