# Multiple Choice Questions on Transformer | Page – 4

1. If iron loss and full load copper loss of any transformer are denoted by Pi and Pc, then the load corresponding to maximum efficiency is given by

At maximum efficiency of transformer, iron-loss = copper-loss e.i. Pi = I22R2'. Where I2 is load current at maximum efficiency and R2' resistance of transformer referred to secondary. Therefore, I2 = ( Pi / R2' )0.5 = ( Pi / IL2R2' )0.5 × IL = (Pi / Pc)0.5 × IL.
Where IL is full load current of transformer and R2' is the equivalent resistance of the transformer referred to load side.

2. Secondary winding of an auto transformer is also called

The auto transformer has a single continuous winding which is used for both input and output voltages. Secondary winding of auto transformer carries both the supply current and load current. But these two currents are practically in phase opposition. Hence secondary winding of auto transformer is also called common winding.

3. Saving of copper in auto transformer is increased as compared to two winding transformer, when transformation ratio K approaches to

Saving of copper weight in auto transformer = Transformation ratio (K) × weight of copper in equivalent two winding transformer. Thus it can be visualized that saving will increase as K approaches unity.

4. Tertiary winding in a transformer is mostly

When faults occur on the primary or secondary sides, considerable unbalanced of phase voltage may be produced which is compensated by large circulating current through this closed delta connected tertiary winding.

5. The main purpose of providing tertiary winding in a three phase transformer is

The purposes of providing tertiary winding in a three phase transformer is to carry extra load added to the transformer which for some reason must be kept isolated, to inter connect three supply systems operating at different voltages but most important purpose is to suppress harmonic voltages and to limit voltage unbalance when the main load is asymmetrical.

6. When operating, two or more transformers in parallel, % impedance of these transformers should be

Load sharing by transformers operating in parallel are as per their MVA rating if % impedances of the transformers are same.

7. Tap changer is generally provided in

Tap changer is used to control the output voltage of a transformer. As power transformer is connected to the load whose voltage level to be maintained at desired level to serve the consumer a quality power, tap changer is generally used in power transformer. Current, voltage and earthing transformers have no direct dealing with consumer load hence there may not be any tap changer.

8. The heat generated in the transformer is dissipated mainly by

Heat generated in transformer is mainly dissipated in atmosphere from radiator outer surface mainly due to convectional flow of air surrounds the radiators.

9. Transformer cooling and insulating oil must be of

The transformer oil subjects to convectional flow for cooling the transformer. As low viscous liquid flow easily than high viscous one, transformer insulating oil should be of low viscosity.

10. Normally for the design of transformer the ambient temperature is taken as

Normally for the design of transformer the ambient temperature is taken as 45°C.

11. Which of the following machine will have maximum efficiency?

Transformer has maximum efficiency as it is static device means it does not have any moving parts in the main body of a transformer hence no mechanical loss is counted in transformer.

12. The heat transfer by convection in a transformer least depends on

Heat dissipated in air from surface of transformer radiator depends upon obviously the temperature difference between oil and atmospheric temperature, velocity of flowing air through the surface of radiators, and surface area of radiator. It also depend upon color of paint of radiator surface since many colors obstruct radiation of heat to the atmosphere but the effect of color is minor compared to other conditions of heat dissipation.

13. Which of the following is not a part of transformer?

Commutator is a part of DC machine.

14. Maximum allowable acidity level of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the maximum allowable acidity level of transformer oil is 0.3 mg of KOH / gm.

15. Maximum allowable tan – delta ( tanδ ) of dielectric dispassion factor of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the maximum allowable tan – delta ( tanδ ) of dielectric dispassion factor of power transformer insulating oil is 1 at 90°C.

16. Minimum allowable resistivity of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the minimum allowable resistivity of transformer oil is 1 × 1012 ohm – cm at 27°C and 0.1 × 1012 ohm – cm at 90°C.

17. Maximum allowable moisture content of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the maximum allowable moisture content of power transformer insulating oil is 35 ppm.

18. Minimum allowable flash point of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the maximum allowable flash point of power transformer insulating oil is 140°.

19. Dissolve gas analysis of transformer oil is carried out to determine

Whenever there is any thermal fault occurs inside the transformer the oil gets decomposed and produces different gases which are dissolved in transformer insulating oil. By quantitative analyzing these dissolved gases one can predict nature of internal fault of transformer and also the internal condition of transformer.

20. Fufuraldehyde analysis is done to determine the

When oil soaked paper insulation in transformer is damaged by heat, some unique oil soluble compounds are realized and dissolved in the oil. These compounds belong to the Furfuraldehyde group. Hence, Fufuraldehyde analysis is done to determine the condition of paper insulation.