This set of Design of Electrical Machines Multiple Choice Questions & Answers focuses on “Specific Permeance & Calculation of Magnetizing Current”.

1. What is specific permeance?

a) specific permeance is product of permeance of unit length and depth of field

b) specific permeance is ratio of permeance of unit length and depth of field

c) specific permeance is the permeance per unit length

d) specific permeance is the permeance per unit pole

View Answer

Explanation: Specific permeance is defined as permeance per unit length. It is also known as the depth of field.

2. What is the formula of the specific permeance?

a) specific permeance = permeability in air * ∫small change in width + length

b) specific permeance = permeability in air * ∫small change in width/length

c) specific permeance = permeability in air * ∫small change in width * length

d) specific permeance = 1/permeability in air * ∫small change in width * length

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Explanation: The permeability in air, small change in width and length is calculated first. On substitution the specific permeance is calculated.

3. What is the assumption made in the calculation of the specific permeance?

a) voltage is kept constant

b) current is kept constant

c) mmf is kept constant

d) speed is kept constant

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Explanation: The mmf is kept constant over all the flux tubes. The mmf should be kept constant when the integration is carried out during the calculation of specific permeance.

4.What is the formula of the effective permeance?

a) effective permeance = effective flux/total mmf

b) effective permeance = effective flux/mmf of air gap

c) effective permeance = effective flux * total mmf

d) effective permeance = effective flux * mmf of air gap

View Answer

Explanation: The effective flux and the total mmf is first calculated. On substitution of the values the effective permeance is calculated.

5. What is the formula of the flux dividing into infinitesimal parts?

a) flux dividing into infinitesimal parts = mmf producing the flux / permeance of infinitesimal part

b) flux dividing into infinitesimal parts = mmf producing the flux * permeance of infinitesimal part

c) flux dividing into infinitesimal parts = mmf producing the flux + permeance of infinitesimal part

d) flux dividing into infinitesimal parts = mmf producing the flux – permeance of infinitesimal part

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Explanation: The mmf producing the flux and the permeance of infinitesimal part is calculated. On substitution the flux dividing into infinitesimal parts is calculated.

6.What is the relation between the specific permeance of a differential path and the length?

a) specific permeance of a differential path is directly proportional to the length

b) specific permeance of a differential path is indirectly proportional to the length

c) specific permeance of a differential path is directly proportional to the square of the length

d) specific permeance of a differential path is indirectly proportional to the square of the length

View Answer

Explanation: The specific permeance is defined as the permeance per unit length. It is indirectly proportional to the length.

7.How many factors does the value of the magnetizing current depends upon?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are 3 factors upon which the magnetizing current depends upon. They are total mmf required, number of turns in the exciting winding and upon the way in which the winding is distributed.

8. What is the formula for the magnetizing current?

a) magnetizing current = total mmf * number of turns

b) magnetizing current = total mmf / number of turns

c) magnetizing current = total mmf + number of turns

d) magnetizing current = total mmf – number of turns

View Answer

Explanation: First the total mmf is calculated along with the number of turns of the magnetizing winding. Then on substitution the magnetizing current is obtained.

9. What is the formula for the rms value of the magnetizing current?

a) rms value of the magnetizing current = maximum magnetizing current / peak factor

b) rms value of the magnetizing current = maximum magnetizing current * peak factor

c) rms value of the magnetizing current = maximum magnetizing current + peak factor

d) rms value of the magnetizing current = maximum magnetizing current – peak factor

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Explanation: The maximum value of the magnetizing current and the peak factor are calculated first. On substitution the rms value of the magnetizing current is obtained.

10. What is the relation of the type of winding with the flux linkage?

a) in distributed windings the flux does not link with all the turns

b) in distributed windings the flux links with all the turns

c) in concentrated windings the flux links with all the turns

d) in concentrated windings the flux does not link with all the turns

View Answer

Explanation: The magnetizing current is actually calculated for the concentrated windings and the distributed windings. In the distributed windings, the flux does not link with all the turns.

11. What is the relation of the magnetizing current with the turns per phase?

a) magnetizing current is directly proportional to the turns per phase

b) magnetizing current is directly proportional to the square of the turns per phase

c) magnetizing current is indirectly proportional to the turns per phase

d) magnetizing current is indirectly proportional to the turns per phase

View Answer

Explanation: The magnetizing current is actually calculated for the concentrated windings and the distributed windings. In the distributed windings, magnetizing current is indirectly proportional to the turns per phase.

12. The plot of the flux density distribution curve is between the interpolar axis consisting of the flux density and the angle difference between phases?

a) true

b) false

View Answer

Explanation: The flux density distribution curve is used to calculate the magnetizing current in the non sinusoidal flux distribution of the distributed windings. The curve is between the flux density and the phase angle.

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