This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of Core and Winding – 1”.

1. What is stacking factor?

a) the allowance made for the power loss

b) the allowance made for the space loss between laminations

c) the allowance made for the heat loss between laminations

d) the allowance made for the power loss between laminations

View Answer

Explanation: The net cross sectional area is obtained from the dimensions of various packets and an allowance is made for the space lost between laminations. This allowance is called stacking factor.

2. What is utilization factor?

a) utilization factor= cross sectional area + gross area of the core

b) utilization factor= cross sectional area – gross area of the core

c) utilization factor= cross sectional area * gross area of the core

d) utilization factor= cross sectional area / gross area of the core

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Explanation: The utilization factor is equal to the ratio of the cross sectional area to the gross area of the core. The cross sectional area and the gross area of the core are first found out, and the ratio gives utilization factor.

3. What is the relationship between utilization factor and the number of core steps?

a) utilization factor is directly proportional to the number of core steps

b) utilization factor is indirectly proportional to the number of core steps

c) utilization factor is indirectly proportional to the square of number of core steps

d) utilization factor is directly proportional to the square of number of core steps

View Answer

Explanation: The utilization factor increases with the increase in the number of core steps used. This eventually increases the manufacturing cost.

4. What is the optimum number of steps for small and large transformers?

a) 5,10

b) 10,15

c) 6,15

d) 1,10

View Answer

Explanation: The optimum number of steps used for the large transformers is maximum of 15. The optimum number of steps for the small transformers is maximum of 6.

5. What happens if the utilization factor gets improved?

a) core area increases and the volt/turns decreases

b) core area increases and the volt/turns increases

c) core area decreases and the volt/turn decreases

d) core area decreases and the volt/turn increases

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Explanation: When the utilization factor increases the core area gets increased. This leads to the increase in the volt/turn for any particular core diameter and specified flux density.

6. How many types of cores are available for core type of transformer?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are basically 3 types of core section available for core type of transformer. They are rectangular, square or stepped type of core sections.

7. What type of core section is used for shell type transformer?

a) rectangular

b) square

c) stepped

d) cruciform

View Answer

Explanation: Shell type transformers prefer only rectangular core section. Shell type transformer are moderate and low voltage transformer which use only rectangular core section.

8. What is the range of the ratio of depth to width of core in rectangular core?

a) 1-2

b) 1.5-2.5

c) 1.4-2

d) 1.5-2

View Answer

Explanation: In rectangular core, the ratio of the depth to core should be minimum 1.4. The maximum value of ratio of depth to core is 2.

9. When is square and stepped cores used?

a) when circular coils are required for low voltage distribution

b) when rectangular coils are required for low voltage distribution

c) when circular coils are required for high voltage distribution

d) when rectangular coils are required for high voltage distribution

View Answer

Explanation: Circular coils are required for high voltage distribution and power transformer. When circular coils are required square and stepped cores are used.

10. Circular coils are preferred because of their electrical characteristics?

a) true

b) false

View Answer

Explanation: Circular coils are preferred because of their high mechanical strength. Their high mechanical strength allows them to be used in high voltage distribution and power transformer.

11. What is the ratio of the net core area to the area of the circumscribing circle in square cores?

a) 0.58

b) 0.64

c) 0.70

d) 0.80

View Answer

Explanation: 0.64 is the ratio of the gross core area to the area of the circumscribing circle. Net core area is the product of stacking factor and gross iron area.

12. The laminations are manufactured in standard size to minimize the wastage of steel during punching of laminations?

a) true

b) false

View Answer

Explanation: The laminations are manufactured in the standard size of width, 0.75m to 1 m. This is used to avoid excessively wide assortment of laminations and to minimize wastage of steel during punching of laminations.

13. What is the value of ratio of gross core area to the area of circumscribing circle in stepped cores?

a) 0.71

b) 0.79

c) 0.89

d) 0.91

View Answer

Explanation: 0.71 is the ratio of net core area to the area of circumscribing circle in stepped cores. The gross core area for stepped cores is 0.618 * d2.

14. What is the net core area for three stepped transformers?

a) 0.45

b) 0.56

c) 0.6

d) 0.62

View Answer

Explanation: 0.45 is the net core area for the square core transformers. 0.56 is the core area for cruciform or stepped core transformers.

15. What is the relationship between the number of steps to the area of circumscribing circle?

a) number of steps is directly proportional to the area of the circumscribing circle

b) number of steps is indirectly proportional to the area of the circumscribing circle

c) number of steps is directly proportional to square of the area of the circumscribing circle

d) number of steps is indirectly proportional to square of the area of the circumscribing circle

View Answer

Explanation: As the number of steps increase, the area of the circumscribing circle also increases. But as the area of the circumscribing circle increases, the ratio of the net core area and gross core area to the area of circumscribing circle decreases.

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