This set of Design of Electrical Machines Questions and Answers for Campus interviews focuses on “Determination of Iron Losses & Slot Leakage”.

1. What is the other name for the iron loss?

a) field loss

b) armature loss

c) winding loss

d) core loss

View Answer

Explanation: When ferromagnetic materials are subjected to a flux in a fixed direction in space and having a magnitude varying in time, losses are produced in the machine. This is called iron loss or core loss.

2. How many types of iron losses are present?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are basically 2 losses present under the core loss or iron loss. They are hysteresis loss and the eddy current loss.

3. What is the formula to obtain the hysteresis loss devised by Steinmetz?

a) hysteresis loss = hysteresis coefficient / frequency of magnetization * (maximum flux density)^{steinmetz coefficient}

b) hysteresis loss = hysteresis coefficient * frequency of magnetization / (maximum flux density)^{steinmetz coefficient}

c) hysteresis loss = hysteresis coefficient * frequency of magnetization * (maximum flux density)^{steinmetz coefficient}

d) hysteresis loss = 1/hysteresis coefficient * frequency of magnetization * (maximum flux density)^{steinmetz coefficient}

View Answer

Explanation: The hysteresis coefficient along with the frequency of magnetization and maximum flux density is calculated first. Next the Steinmetz coefficient is calculated and on substitution it gives the hysteresis loss.

4. How are the eddy current losses in the machine reduced?

a) by using conducting materials

b) by using magnetic materials

c) by using insulating materials

d) by laminating the core

View Answer

Explanation: Eddy current loss is one type of loss present under the iron or core loss. It can be reduced by laminating the core.

5. What is the relation between resistivity, magnetizing mmf and magnetizing current?

a) high resistivity, low magnetizing mmf, high magnetizing current

b) high resistivity, high magnetizing mmf, high magnetizing current

c) low resistivity, high magnetizing mmf, high magnetizing current

d) low resistivity, high magnetizing mmf, low magnetizing current

View Answer

Explanation: The high resistivity materials are made use of to reduce the eddy current loss. The high resistivity materials have high magnetizing mmf which in turn leads to high magnetizing current.

6. What must be done in order to take additional iron losses into account in dc machines?

a) iron loss obtained from iron loss curve is multiplied by 1.2-1.4

b) iron loss obtained from iron loss curve is multiplied by 1.4-1.6

c) iron loss obtained from iron loss curve is multiplied by 1.3-1.5

d) iron loss obtained from iron loss curve is multiplied by 1.15-1.25

View Answer

Explanation: In order to take additional losses into account the iron loss obtained from the iron loss curves is multiplied into 1.4-1.6 in dc machines and synchronous machines. The iron loss obtained from the iron loss curves is multiplied into 1.2-1.4 in induction motors.

7. What is the value of constant ‘a’ in the core part of the ac machines?

a) 4.7

b) 6.5

c) 6.7

d) 2.3

View Answer

Explanation: The value of constant ‘a’ in the core part of the ac machines is 4.7 and that in the teeth part is 6.5. The value of constant ‘a’ in the core part of the dc machines is 6.5 and that in the teeth part is 2.3.

8. What are the equations that are to be satisfied by flux patterns?

a) Poisson equation

b) Laplace equation

c) Laplace and Poisson equation

d) Gauss equation

View Answer

Explanation: The flux patterns must satisfy the Poisson equation within the section of the conductors. The flux patterns must satisfy the Laplace equation in the other parts.

9. How many assumptions can be used for the slot leakage calculation?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are 3 assumption made in the slot leakage calculation. They are the current in the slot conductors is uniformly distributed, the leakage path is straight across the slot and around the iron at the bottom and the permeance of air paths is only considered.

10. What are the factors the slot leakage permeance will depend upon?

a) shape of the slot

b) arrangement of winding in the slot

c) number of windings

d) shape of the slot and arrangement of winding in the slot

View Answer

Explanation: The slot leakage permeance depends upon the shape of the slots. The slot leakafe permeance also depends upon the arrangement of winding in the slot.

11. What is the formula of the permeance of the strip in the conductor portion?

a) permeance of the strip = permeability in air * area of the flux path * length of flux path

b) permeance of the strip = permeability in air / area of the flux path * length of flux path

c) permeance of the strip = permeability in air * area of the flux path / length of flux path

d) permeance of the strip =1 / permeability in air * area of the flux path * length of flux path

View Answer

Explanation: The permeability in air, area of the flux path and the length of flux path is first calculated. On substitution, the permeance of the strip is obtained.

12. What is the formula of the effective permeance of conductor portion?

a) effective permeance of conductor portion = 1/total flux linkages * total turns * total mmf

b) effective permeance of conductor portion = total flux linkages / total turns * total mmf

c) effective permeance of conductor portion = total flux linkages * total turns * total mmf

d) effective permeance of conductor portion = total flux linkages * total turns / total mmf

View Answer

Explanation: The total flux linkages, total turns and total mmf is first calculated. On substitution the effective permeance of conductor portion is obtained.

13. Which machine incorporates the usage of the closed slots?

a) dc motors

b) synchronous motors

c) induction motors

d) special motors

View Answer

Explanation: The closed slots are being made use often in the small induction motors. The closed slots are used to increase the leakage reactance which in turn decreases the starting current.

14. The leakage flux on the top of the slot is through iron, which is called ‘bridge’?

a) true

b) false

View Answer

Explanation: The leakage flux on the top of the slot is through iron, which is called ‘bridge’. Therefore the specific permeance of this path depends upon the permeance of iron which in turn depends upon the degree of saturation and hence upon the relative permeability of iron.

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