# Design of Electrical Machines Questions and Answers – Methods of Cooling of Transformers

This set of Basic Design of Electrical Machines Questions and Answers focuses on “Methods of Cooling of Transformers”.

1. How many types of cooling methods are available for the transformer?
a) 3
b) 2
c) 1
d) 4

Explanation: There are 3 types of cooling methods available for transformers. They are natural cooling, air blast cooling, forced oil circulation.

2. How are the radiators cooled in the present time?
a) by natural cooling
b) by forced cooling using small fans
c) by forced cooling using large fans
d) by using external air

Explanation: At present time the radiators are cooled using forced cooling. The forced cooling takes place with the help of the small fans mounted on each radiator.

3. What type of cooling is being made use of in transformers having a capacity of less than 11MVA?
a) natural cooling
b) forced cooling
c) air blast cooling
d) forced cooling and air blast cooling

Explanation: For transformers having capacity less than 11MVA, natural cooling is made use of. For transformers having capacity more than 11MVA, air blast cooling is used.

4. Compared to the natural cooling, how much of heat dissipation is increased by air blast cooling?
a) 50-70%
b) 60-70%
c) 50-60%
d) 40-60%

Explanation: Air blast cooling helps in increased heat dissipation. The minimum value of increased heat dissipation is 50% and maximum value is 60%.

5. Increase in the velocity of oil circulation increases the transformer output.
a) true
b) false

Explanation: The increases in velocity of the air circulation increases the temperature. The temperature rise increases the transformer output.
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6. What is the relation of the increase of the oil circulation rate with energy losses?
a) increase of the oil circulation rate is not depending with energy losses
b) increase of the oil circulation rate is directly proportional to the energy losses
c) increase of the oil circulation rate is directly proportional to the square of energy losses
d) increase of the oil circulation rate is indirectly proportional to energy losses

Explanation: The increase in the oil circulation rate is unsuitable because this increases the large energy losses In the pumping unit. To cool the oil, it is circulate through a special oil cooler.

7. What is the flow rate of the circulating oil in an air cooler with natural air cooling?
a) 12.5 litre per minute per KW of losses
b) 12 litre per minute per KW of losses
c) 14 litre per minute per KW of losses
d) 13 litre per minute per KW of losses

Explanation: When natural air cooling is used, the flow rate is 12 litres per minute per KW of losses. Even when the air blast cooling is used, the transformer output increases roughly to the same extent.

8. What is the range of the cooler surfaces per 1 KW of losses?
a) 0.1-0.25 m2
b) 0.18-0.2 m2
c) 0.1-0.2 m2
d) 0.18-0.25 m2

Explanation: The minimum value of the cooler surfaces per 1 KW of losses is 0.18 m2. The maximum value of the cooler surfaces per 1 KW of losses is 0.25 m2.

9. What is the range of the flow rate of circulating oil per KW of losses?
a) 6-7 liters per minute
b) 5-6 liters per minute
c) 6-8 liters per minute
d) 6-7 liters per minute

Explanation: The minimum value of the flow rate of circulating oil per KW of losses is derived to be 6 liters per minute. The maximum value of the flow rate of circulating oil per KW of losses is derived to be 8 liters per minute.

10. The temperature difference between the incoming and outgoing water is greater than 10°C.
a) true
b) false

Explanation: The water flow rate is about 1.5 litres per minute. The difference in temperature between the incoming water and outgoing water is 10°C.

11. What is the formula for width of the tank for single phase transformers used?
a) width of tank = 2*distance between adjacent limbs + external diameter of h.v windings + 2*clearance between h.v windings and tank
b) width of tank = distance between adjacent limbs + external diameter of h.v windings + 2*clearance between h.v windings and tank
c) width of tank = 2*distance between adjacent limbs * external diameter of h.v windings + 2*clearance between h.v windings and tank
d) width of tank = distance between adjacent limbs * external diameter of h.v windings + 2*clearance between h.v windings and tank

Explanation: Width of tank = 2*distance between adjacent limbs + external diameter of h.v windings + 2*clearance between h.v windings and tank is the formula for three phase transformer. For single phase transformers, the distance between adjacent limbs is not multiplied.

12. What is the formula for the length of the tank?
a) length of the tank = external diameter of h.v winding + clearance on each side between the winding and tank along the width
b) length of the tank = external diameter of h.v winding * clearance on each side between the winding and tank along the width
c) length of the tank = external diameter of h.v winding + 2*clearance on each side between the winding and tank along the width
d) length of the tank = external diameter of h.v winding / 2*clearance on each side between the winding and tank along the width

Explanation: The external diameter of h.v winding is obtained. Next, the clearance on each side between the winding and tank along the width is calculated and is substituted in the above formula.

13. What is the formula for the height of transformer tank?
a) height of transformer tank = Height of transformer frame + clearance height between the assembled transformer and tank
b) height of transformer tank = Height of transformer frame * clearance height between the assembled transformer and tank
c) height of transformer tank = Height of transformer frame/clearance height between the assembled transformer and tank
d) height of transformer tank = Height of transformer frame – clearance height between the assembled transformer and tank

Explanation: Firstly, the height of the transformer frame is calculated. Next, the clearance height between the assembled transformer and tank is also calculated. Substitute the values to obtain the height of transformer tank.

14. What is the rating of the transformer for the voltage of about 11 kV?
a) 1000-2000 kVA
b) 100-3000 kVA
c) 1000-5000 kVA
d) 100-500 kVA

Explanation: The minimum value of the rating of the transformer for a voltage of about 11 kV should be 1000 kVA. The maximum value of the rating of the transformer for a voltage of about 11 kV should be about 5000 kVA.

15. What is the rating of the transformer for the voltage of above 11 kV upto 33 kV?
a) 1000-5000 kVA
b) less than 1000 kVA
c) above 1000 kVA
d) 100-500 kVA

Explanation: 1000-5000 kVA is the rating of the transformer for the voltage of about 11 kV. When the voltage rating is about 11-33 kV, then the rating of the transformer is less than 1000 kVA.

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