# Multiple Choice Questions on Transformer | Page – 2

1. In a core type transformer

LV winding is wound on the limb first. Then HV winding is placed on it and there is an insulation between LV and HV winding. In thin manner less amount of insulation needed. Also insulation between the core and the inner winding is stressed to low voltage.

2. The primary and secondary windings of a transformer are

Whenever alternating voltage is applied to the primary of a electrical transformer, there will be a alternating flux through the core of transformer. As this flux will link with secondary winding there will be an induced emf in the secondary winding. The phenomenon of inducing voltage by changing flux is called electrical induction, hence it can be concluded that the windings of a transformer are linked inductively.

3. At what power factor, the voltage regulation of a transformer can be zero ?

Now, if Voltage regulation is zero, R2 / X2 = − sinθ2 / cosθ2 = − tanθ2 = tan( − θ2 ). − θ2 means leading power factor.

4. In a three phase star - delta transformer, what is the angle difference between primary and secondary phase voltages?

This is vector group and has + 30° displacement. Therefore, delta side leads by + 30°.

5. What will be the eddy current loss if the supply frequency of a transformer becomes double?

Eddy Current loss in transformer is denoted as,
Pe = Kef2Kf2Bm2 watts.
Where, Ke = Eddy current constant,
Kf = form factor.
Hence apparently, the eddy current loss Pe ∝ f2, but for any given voltage, if f decreases, Bm increases correspondingly and if f increases Bm decreases correspondingly. Hence the eddy current loss Pe at any given voltage, is independent of frequency.

6. The efficiency of a transformer is mainly dependent on

Transformer efficiency = output / input = output / ( output + losses ) = output / ( output + iron loss + copper loss)
Iron / Core loss : - Iron loss is caused by the alternating flux in the core and consists of hysteresis and eddy current losses. These losses occur in the core and yoke due to alternating flux.
Copper losses occur in the primary and secondary windings due to their resistances when the transformer is loaded.
Iron losses ( core losses ) are independent of load, and therefore are called constant losses. Generally at maximum efficiency the core loss of transformer is equal to copper loss. Again the transformers are so designed that maximum efficiency of transformer reaches at full load current. So for all other load copper loss is less than core loss hence core loss dose have maximum value among all other losses in a power transformer and that is why the efficiency of a transformer mainly depends upon core losses.

7. If supply frequency of a transformer decreases,

The total core loss Pc in terms of voltage and frequency is
Pc = hysteresis loss + eddy current loss = KhVxf 1 - x + KeV2.
Hence, eddy current is independent of frequency and hysteresis loss is inversely varies with frequency.

8. Transformers operating in parallel will share the load depending upon their

The load sharing by the transformers must be proportionate with their MVA rating.

9. No load primary current in a transformer

As the winding of the transformer is inductive the magnetizing current in it lags behind the primary applied voltage by exactly 90° but there is a core loss component which will be in phase with applied voltage. The no load current is the vector sum of these two currents hence this must lags behind applied voltage somewhat less than 90°.

10. Which of the following is minimized by laminating the core of a transformer?

By laminating the core of transformer we make the eddy current circulating path narrow, means increasing the resistance of eddy current and proportionately reducing the eddy current. Although the resistance of eddy current path Re increases, but as the eddy current Ie reduces, the overall eddy current loss Ie2Re is reduced.

11. Breather is provided in a transformer to

Whenever transformer oil becomes hot it expands in volume, the air on the top of the oil in conservator tank will come out through the breather and when oil becomes cold the atmospheric air enters in the transformer through this breather. This is called breathing of transformer. Silica gel is provided in the breather pot which absorbs moisture hence air becomes dry during passing through the breather. So breather is provided in a transformer to absorb moisture from air entering in transformer as well as provide breathing to the transformer.

12. Iron loss in a transformer mainly occurs in

Iron loss is also called core loss in transformer which consists of hysteresis loss and eddy current loss. Both of these losses associated with transformer core.

13. Which of the following losses in the transformer varies with load?

Copper loss in a transformer is expressed as IL2R2. Where IL is load current and R2 is equivalent resistance of the transformer referred to secondary. Hence copper loss in transformer fully depends upon load current.

14. In a transformer the magnetic coupling between the primary and secondary winding can be increased by

Magnetic coupling between two windings depends upon flux linkage between them. Flux linkage can be maximized between two coils by placing magnetic core of low reluctance in between them.

15. The short circuit test on a transformer is conducted to obtain

The watt meter reading in short circuit test of a transformer, gives the copper losses and negligible amount of core losses of the transformer. Since, the core losses depend upon applied voltage and voltage applied in short circuit test, is very small compared to rated voltage of transformer. Therefore, from the watt-meter reading in short circuit test, we get mainly copper losses of transformer.

16. In a transformer short circuit test and open circuit test are conducted

For a short circuit test on HV side the instrument required for measurement voltage, ranges 2 % to 12 % of rated voltage and rated current in HV side is also less which can be easily measurable by ordinary measuring instrument. In open circuit test the rated voltage is applied to the transformer terminal keeping other side terminals open. If voltage is applied to the HV side the requirement of voltage source and voltmeter are of higher range but in LV side both of them can be of lower range. That is why, in a transformer short circuit test and open circuit test are conducted in HV side and LV side of transformer respectively.

17. In one 132 / 33 KV transformer the LV resistance is 0.02 per unit. The resistance referred to HV side is

The per unit value of any parameter of a transformer remain same irrespective of whether it is referred to primary or secondary.

18. Lamination of transformer core is made of

The lamination of transformer core is made of cold rolled grain oriented silicon steel ( CRGO silicon steel ) to reduced its hysteresis loss.

19. Oil is provided in an oil filled transformer for

Transformer insulating oil is used in transformer for both cooling and insulating purpose.

20. Air core transformer as compared to iron core transformer, has

Magnetic core losses consist of hysteresis loss and eddy current loss. Hysteresis loss is associated with ferromagnetic property of iron and eddy current loss is associated with conducting property of iron. As the air is not a ferromagnetic substance as well as it is not a conductor so there will not be any core loss in the air core transformer.