This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Temperature Rise in Transformers”.
1. The problem of temperature rise and cooling of transformers is essentially the same as that of rotating machinery.
a) true
b) false
View Answer
Explanation: There are problems of temperature rise and cooling of transformers which decreases the efficiency of the transformers. The same problems are also seen in the rotating machinery.
2. How are the losses in the transformer and rotating machines converted to?
a) the losses are converted to electrical energy
b) the losses are converted to electrical and mechanical energy
c) the losses are converted to mechanical energy
d) the losses are converted to thermal energy
View Answer
Explanation: In both the transformer and the rotating machines the losses are converted to thermal energy. This thermal energy causes heating of the transformer parts.
3. In how many ways does heat dissipation occur in transformers?
a) 2
b) 3
c) 4
d) 5
View Answer
Explanation: The heat dissipation takes place in 3 ways in transformers. They are radiation, convection and conduction.
4. What type of heat dissipation takes place when the heat flows from the outer surface of the transformer part to the oil that cools it?
a) conduction
b) convection
c) conduction and convection
d) radiation
View Answer
Explanation: When heat flows from the outer surface of transformer part to the oil which cools it, it is convection. In transformers all 3 types of heat dissipation occurs.
5. What type of heat dissipation takes place when heat flows from oil to walls of a cooler?
a) conduction
b) convection
c) radiation
d) conduction and convection
View Answer
Explanation: When heat flows from oil to walls of the cooler, the heat dissipation type is convection. In transformers all 3 types of heat dissipation takes place.
6. What type of heat dissipation takes place when heat flows from the walls of the cooler to the cooling medium?
a) convection
b) radiation
c) convection and radiation
d) conduction and radiation
View Answer
Explanation: When the heat flows from the walls of the cooler to the cooling medium, it is both convection and radiation. In transformer all 3 types of heat dissipation occurs.
7. What is the range of the working temperature of oil determined by the tests?
a) 40-60° C
b) 30-60° C
c) 45-60° C
d) 50-60° C
View Answer
Explanation: The minimum value of the working temperature of oil as cooling medium is determined to be 50°C. The maximum value of the working temperature of oil as cooling medium is determined to be 60°C.
8. What is the formula for specific heat dissipation due to convection of oil?
a) specific heat dissipation = 40.3*(temperature difference of the surface relative to oil/height of the dissipating surface)1/4 W per m2 – °C
b) specific heat dissipation = 40.3 / (temperature difference of the surface relative to oil/height of the dissipating surface)1/4 W per m2 – °C
c) specific heat dissipation = 40.3*(temperature difference of the surface relative to oil * height of the dissipating surface)1/4 W per m2 – °C
d) specific heat dissipation = 40.3*(temperature difference of the surface relative to oil + height of the dissipating surface)1/4 W per m2 – °C
View Answer
Explanation: First the temperature difference of the surface relative to oil is calculated, then the height of the dissipating surface is also calculated. Substituting in the above formula provides the specific heat dissipation due to convection of oil.
9. What is the value of specific heat dissipation for convection due to air?
a) 8 W per m2 – °C
b) 6 W per m2 – °C
c) 9 W per m2 – °C
d) 10 W per m2 – °C
View Answer
Explanation: The value of the specific heat dissipation for convection due to air is 8 W per m2 – °C. The value of specific heat dissipation will vary for different medium.
10. The convection due to air is 10 times the convection due to oil.
a) true
b) false
View Answer
Explanation: The convection due to oil is 10 times the convection due to air. This constitutes a major valuable property of oil as a cooling medium.
11. How do the walls of the transformer tank dissipate heat?
a) by radiation
b) by convection
c) by conduction
d) by convection and radiation
View Answer
Explanation: The plain walled tanks of the transformer also dissipate heat through convection and radiation. The property is similar to that of the rotating machinery.
12. What is the specific heat dissipated by the plain walled tanks of the transformer by radiation and convection?
a) 6.5, 6 W per m2 – °C
b) 6, 6.5 W per m2 – °C
c) 6.5, 6.5 W per m2 – °C
d) 6, 6 W per m2 – °C
View Answer
Explanation: The heat dissipated by the plain walled tanks of the transformer by radiation is 6 W per m2 – °C. The heat dissipated by the plain walled tanks of the transformer by convection is 6.5 W per m2 – °C.
13. What is the formula for the temperature rise of the transformers?
a) temperature rise = total loss * specific heat dissipation * surface temperature
b) temperature rise = total loss / (specific heat dissipation * surface temperature)
c) temperature rise = total loss / specific heat dissipation / surface temperature
d) temperature rise = total loss * specific heat dissipation / surface temperature
View Answer
Explanation: Firstly the surface temperature is calculated along with the specific heat dissipation which is nothing but 12.5. Then the losses are calculated and substituted in the above formula.
14. Can the plain walled tanks accommodate the transformer for both large and small outputs?
a) the plain walled tanks can accommodate for large outputs but cannot accommodate for small outputs
b) the plain walled tanks can accommodate for large outputs and small outputs
c) the plain walled tanks cannot accommodate for large outputs and small outputs
d) the plain walled tanks can accommodate for small outputs but cannot accommodate for large outputs
View Answer
Explanation: The plain walled tanks are large enough to accommodate the transformer and oil has sufficient surface to keep the temperature rise within limits for small outputs. But the plain walled tanks cannot accommodate the transformers for large outputs.
15. How are the ratings of the transformer, losses and temperature rise related?
a) increase, decrease, increase
b) decrease, increase, increase
c) increase, increase, increase
d) decrease, increase, decrease
View Answer
Explanation: As the rating of the transformer increases, the losses also increase. As the losses increase the heat dissipated increases and gives high temperature rise.
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