Design of Electrical Machines Questions and Answers – Design of Tank

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of Tank”.

1. What is the usage of the tanks with tubes?
a) if the temperature rise with plain tank is very low
b) if the temperature rise with plain tank is very high
c) if the temperature rise is zero
d) if the temperature rise with plain tank exceeds the specific limits

Explanation: Temperature rise in transformers is calculated with plain walled tanks. If the limits is exceeded then the plain walled tank is replaced by tank with tubes.

2. What is the relation of the provision of tubes with respect to dissipation of heat?
a) the provision of tubes is directly proportional to the dissipation of heat
b) the provision of tubes is indirectly proportional to the dissipation of heat
c) the provision of tubes is directly proportional to square of the dissipation of heat
d) the provision of tubes is indirectly proportional to square of the dissipation of heat

Explanation: The provision of tubes increases the dissipating area. The increase in dissipation of heat is not proportional to area because tube screen some of the tank surface preventing radiation from there.

3. What is the relation of the transformer surface with respect to dissipation of heat?
a) transformer surface has no relation with respect to dissipation of heat
b) transformer surface has minor changes with respect to dissipation of heat
c) transformer surface has major changes with respect to dissipation of heat
d) transformer surface has no change with respect to dissipation of heat

Explanation: When the tanks with tubes are provided, the dissipation of heat increases. The dissipation of heat has no effect on the transformer surface.

4. How is the circulation of oil improved in tanks with tubes?
a) it can be improved by using dissipating heat
b) it can be improved by using more effective air circulation
c) it can be improved by using more effect power flow
d) it can be improved by using more effective heads of pressure

Explanation: The circulation of oil is improved in tanks with tubes. It takes place with the help of using more effective heads of pressure.

5. An addition of 35 percent should be made to tube area of the transformers.
a) true
b) false

Explanation: An addition of 35 percent should be made to tube area of the transformer. This should be done in order to take into account this improvement in dissipation of loss by convection.
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6. What is the loss dissipated by tubes by convection, given area of the tubes = 3.5?
a) 12.3 W per °c
b) 2.51 W per °c
c) 5.3 W per °c
d) 30.8 W per °c

Explanation: Loss dissipated by tubes by convection = 8.8 * Area of tubes
Loss = 8.8 * 3.5 = 30.8 W per °c.

7. What is the formula for temperature rise with tubes?
a) temperature rise with tubes = total loss / dissipating surface*(12.5 + 8.8x)
b) temperature rise with tubes = total loss * dissipating surface*(12.5 + 8.8x)
c) temperature rise with tubes = total loss / dissipating surface / (12.5 + 8.8x)
d) temperature rise with tubes = total loss + dissipating surface*(12.5 + 8.8x)

Explanation: The total losses in the transformers are obtained firstly the iron loss and copper loss. Next the dissipating surface temperature is obtained and substituting in the above formula gives the temperature rise.

8. What is the formula for number of tubes?
a) number of tubes = (1/ 8 * area of each tube) * (total loss / temperature rise with tubes – 12.5 * dissipating surface)
b) number of tubes = (1* 8 * area of each tube) * (total loss / temperature rise with tubes – 12.5 * dissipating surface)
c) number of tubes = (1/ 8 * area of each tube) / (total loss / temperature rise with tubes – 12.5 * dissipating surface)
d) number of tubes = (1/ 8 * area of each tube) + (total loss / temperature rise with tubes – 12.5 * dissipating surface)

Explanation: First the temperature rise with tubes is obtained. Then the iron loss and copper loss are obtained and added. Area of each tube is also obtained. Substituting all the values in the above formula provides the number of tubes.

9. What is the range of the diameter of the tubes used?
a) 50-60 mm
b) 60-70 mm
c) 70-80 mm
d) 50-70 mm

Explanation: The minimum value of the diameter of tubes is derived to be around 50 mm. The maximum value of the diameter of tubes should be less than 70 mm.

10. Elliptical tubes with pressed radiators are increasingly been used.
a) true
b) false

Explanation: Elliptical tubes with pressed radiators are on high demand now a days. This is because they give a greater dissipating surface for the small volume of oil.

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 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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