# Design of Electrical Machines Questions and Answers – Design of Core and Winding – 2

This set of Design of Electrical Machines Questions and Answers for Freshers focuses on “Design of Core and Winding – 2”.

1. What is the formula for the number of turns in primary winding?
a) number of turns of primary winding = Voltage of primary windings * voltage per turn
b) number of turns of primary winding = Voltage of primary windings/voltage per turn
c) number of turns of primary winding = Voltage of secondary windings * voltage per turn
d) number of turns of primary winding = Voltage of secondary windings/voltage per turn

Explanation: For calculating the number of turns of primary windings first we calculate the voltage across the primary windings. Then the voltage per turn is calculated and the ratio gives the number of turns.

2. What is the formula for obtaining the current in the primary winding?
a) current in primary winding = kVA per turn * 103 * primary voltage
b) current in primary winding = kVA per phase * 103 * primary voltage
c) current in primary winding = kVA per turn * 103 / primary voltage
d) current in primary winding = kVA per phase * 103 / primary voltage

Explanation: For obtaining the current in primary winding, the kVA output per phase is obtained. Then the primary voltage is calculated, and the ratio of both gives the current in primary windings.

3. What does the area of conductors in primary and secondary windings depend on?
a) current
b) voltage
c) power
d) current density

Explanation: The area of the conductors is directly dependent on the current density. The area of the conductors are determined after choosing a suitable current density.

4. What does the permissible current density depend upon?
a) local heating
b) efficiency
c) output power
d) local heating and efficiency

Explanation: The permissible current density depends upon the local heating as the heating should not affect the output. It also depends on the efficiency of the transformer.

5. What is the relationship between temperature and the current density?
a) current density is directly proportional to the temperature
b) current density is directly proportional to the square of the temperature
c) current density is indirectly proportional to the square of the temperature
d) current density is indirectly proportional to the temperature

Explanation: As the current density increases, the temperature also increases. As the temperature increases, it can cause damage to the insulation.
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6. What is the relationship between the losses and the maximum efficiency on the current density?
a) current density increases, losses decrease, efficiency increases
b) current density increases, losses increase, efficiency increases
c) current density decreases, losses decrease, efficiency increases
d) current density decreases, losses increase, efficiency increases

Explanation: As the current density decreases, the losses decrease. As the losses decrease the maximum efficiency increases.

7. What is the range of current density for small and medium power transformers?
a) 1-2 A per mm2
b) 1-2.5 A per mm2
c) 1.1-2.2 A per mm2
d) 1.1-2.3 A per mm2

Explanation: In small and medium power transformers, the lowest value of current density is 1.1. The highest permissible value is 2.3 for small and medium power transformers.

8. What is the range of current density for large power transformer with self oil cooled type?
a) 1-2 A per mm2
b) 1.5-2.5 A per mm2
c) 2.2-3.2 A per mm2
d) 2-3 A per mm2

Explanation: For large transformers with self oil cooled type, the highest permissible value of current density is 3.2. The minimum current density value required is 2.2.

9. What is the condition for minimum loss condition?
a) current density in primary < current density in secondary
b) current density in primary > current density in secondary
c) current density in primary = current density in secondary
d) current density in primary >= current density in secondary

Explanation: The condition for the minimum loss should be the current density in primary should be equal to the current density in secondary. Any different condition, could lead to high amount of loss.

10. The current density in relatively better cooled outer winding is made 10 percent greater than the inner winding.
a) true
b) false

Explanation: In practical case, the current density in relatively better cooled outer winding is made greater than that in the inner winding. It is usually made 5 percent greater in practical.

11. How many total high voltage windings are present?
a) 1
b) 2
c) 3
d) 4

Explanation: There are 3 high voltage windings present. They are i) Cylindrical winding, ii) Cross-over winding iii) Continuous disc type winding.

12. The low voltage windings are generally divided into 2 types.
a) true
b) false

Explanation: The low voltage windings are basically divided into 2 types. They are i) cylindrical winding ii) helical winding.

13. What is the rating for cylindrical type of winding with circular conductors?
a) 5000-10000 kVA
b) 5000-8000 kVA
c) 160-10000 kVA
d) 200-10000 kVA

Explanation: 5000-8000 kVA is used for rectangular conductors with cylindrical winding. 160-10000 kVA is used for helical winding. 200-10000 kVA is used for continuous disc type of winding.

14. What is the voltage for cross over type of winding?
a) upto 15 kV
b) upto 33 kV
c) upto 66 kV
d) upto 6 kV

Explanation: Helical windings have a voltage of upto 15 kV. Whereas, the cylindrical winding with rectangular conductors have a voltage of upto 6 kV.

15. What is the maximum current per conductor for helical winding?
a) from 12 A and above 12 A
b) from 300 A and above 300 A
c) upto 40 A
d) upto 80 A

Explanation: The maximum current per conductor for continuous disc winding is from 12 A and above 12 A. The maximum current per conductor for cross over winding is upto 40 A and the maximum current per conductor for cylindrical winding with circular conductors is upto 80 A.

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