# Multiple Choice Questions on Transformer | Page – 3

1. CRGO steel is used in the core of transformer to reduce

CRGO means cold rolled grain orientation. Hysteresis loss mainly occurs in ferromagnetic material due to its random grain ( small magnetic region ) orientation. In CRGO steel these grain are properly oriented so that hysteresis loss can be minimized.

2. If input of a transformer is square wave the output will be

In a square wave the changes in amplitude, takes place only at the side edge of wave form hence transformation action takes place at these edges only. Because of that, the output voltage will be in shape of pulsed wave.

3. Two transformers with identical rating, are designed with flux densities of 1.2 and 1.4 Wb / m2 respectively. The weight of first transformer per KVA is

The voltage equation of a transformer is V = 4.44.φfT Volt. Hence for identical voltage rating flux Φ of both transformers must be same if we consider f and T of both transformer are same. Again φ = BmA. Where Bm is maximum flux density and A is area of cross section of transformer core. As φ is same for both transformers, the area of cross section of transformer core of first transformer is more than that of second transformer means weight of first transformer is more.

4. The flux involved in the emf equation of a transformer has

Derivation of emf equation of a transformer involves derivative of function of alternating flux wave in the core. The expression of flux function is the product of maximum flux and a cosine function that is φcosωt. Obviously emf equation in its expression, involves maximum value of flux.

5. The no load current in a transformer lags the applied voltage by

Ideally a transformer draws the magnetizing current, lags primary applied voltage by 90°. But the transformer also has core loss current component which will be in phase with applied voltage. And no load current is nothing but the vector summation of these two currents. Hence, the no load current will not lag behind applied voltage by exactly 90° but it lags somewhat less than 90o. It is in practice generally about 75°.

6. The leakage flux in a transformer depends upon

Whenever the transformer is loaded, load current starts to flow in secondary winding which produces secondary flux in the core of transformer. For neutralizing this secondary flux primary winding draws current from source to produce counter flux in the core. But in this phenomenon some of the flux produced in the secondary will not linked with primary winding and similarly some of the flux produced in primary will not link with secondary winding, as all the flux will not pass through the core. Those fluxes pass outside the core and do not link with other winding, are called leakage flux. As this phenomenon of leakage flux comes into picture only when load is connected to the transformer and obviously varies with load.

7. For satisfactory parallel operation of two transformers which of the following conditions is most essential,

Percentage impedance of all transformers operated in parallel should be same but if there is a difference, parallel operation is still possible but load sharing by the transformers may not be as desired. If voltage ratio of all transformers operated in parallel is not same, still parallel operation of transformers is possible but there will be local circulating current in-between transformers. We can always put different MVA rated transformers in parallel but load sharing would be according to there MVA ratings if other conditions are properly fulfilled. But if polarities two transformers are not properly connected there will not be any output at all since total current will locally circulated in between transformers no current will come to the load. We can compare this phenomenon with two oppositely connected battery.

8. If a transformer has turns ratio K, the primary and secondary current are I1 and I2 respectively and magnetizing current and core loss component of no load current Iμ and Iw respectively, then

Total primary current drawn from source by an electrically loaded transformer has to supply the load at secondary, has to magnetize the core and also has to compensate the core loss of transformer. So primary current of electrically loaded transformer has three components, primary equivalent of secondary load current i.e. product of turns ratio and secondary current, second one is magnetizing component and third one is obviously core loss component.

9. Which of the following transformer insulating oil generally used in India?

Transformer insulating oil consists of four major classes of organic compounds, namely, paraffin, naphtha, aromatics and olefins. Generally paraffin based and naphtha based insulating oil are used in transformer. Paraffin based insulating oil has faster deterioration rate than naphtha oil and the sludge of paraffin oil is not soluble, hence can obstruct the convectional flow of oil during cooling, but still in India this oil is hugely being used due to its easy availability and insolubility of sludge is somewhat overcome due to warm weather condition of India.

10. Winding conductors of an electrical power transformer are mainly insulated by

Normally crepe Kraft papers are used for insulation purpose inside oil immersed transformer as because the deterioration rate of paper inside oil is very slow compared to other insulating materials.

11. The overall power factor of an on load transformer

Whenever we calculate over all power factor of a transformer, we consider the impedance of the electrically loaded transformer which consists of impedance of that transformer and impedance of its load. As power factor of load depends upon its nature of impedance, obviously overall power factor of an on load transformers depends on the power factor of the load.

12. If in a sub – station there is one 132/33KV transformer whose secondary is connected with primary of one 33 / 11 KV transformer, the total transformation ratio of the sub – station will be,

If two transformers are connected in such a way that secondary of first one is connected with primary of second, and if transformation ratios of these two transformers are K1 and K2, total or overall transformation ratio of this system will be K1K2.

13. A short circuit test on a transformer gives

In a short circuit test we apply variable voltage at HV terminals of a transformer keeping LV side short circuited. The applied voltage is varied until the input current reaches its rated value or full load value. As the current drawn from source is full load current the test normally measures the copper losses of transformer at full load. But since, by adjusting variable supply voltage we can control the supply current at any value, a short circuit test can give copper loss at any load.

14. A step up transformer increases

A step up electrical transformer transforms a power from one circuit to another without changing its frequency to higher voltage level and proportionate lower current level.

15. In a step down transformer which one of the following statements is true?

In a transformer voltage per turn is same in both primary and secondary winding hence, induced voltage across the winding is proportional to number of turns in the winding. As in step down transformer secondary voltage is less than primary, the number of turns in secondary is obviously less than that of primary.

16. When a transformer is loaded, the secondary terminal voltage drops. That means power factor is

Approximate relation of no load and full load voltages in a transformer is E2 = V2 + (R.cosθ + Xsinθ). Where E2, V2, R, X and cosθ are no load voltage, full load voltage, secondary resistance of transformer, secondary leakage reactance of transformer and power factor respectively. If power factor is lagging, θ is positive which means ( Rcosθ + Xsinθ) > 0. Thus E2 > V2.

17. When a transformer is loaded, the secondary terminal voltage gains. That means power factor is

Approximate relation of no load and full load voltages in a transformer is E2 = V2 + ( Rcosθ + Xsinθ ). Where E2, V2, R, X and cosθ are no load voltage, full load voltage, secondary resistance of transformer, secondary leakage reactance of transformer and power factor respectively. If power factor is leading, θ is negative which means { Rcos( - θ) + Xsin( - θ ) } or, { Rcosθ - Xsinθ } may be less than zero if Rcosθ < Xsinθ. In that case full load secondary voltage V2 may be greater than no load secondary voltage E2.

18. If Kh is hysteresis constant, f is supply frequency, Bm maximum flux density in core of transformer, the expression of hysteresis loss in transformer, is given by

If Kh is hysteresis constant, f is supply frequency, Bm maximum flux density in core of transformer, the expression of hysteresis loss in transformer, is given by KhfBmx watts. Where x ranges from 1.5 to 2.5.

19. If Ke is eddy current constant, Kf is form factor, f is supply frequency, Bm maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by

If Ke is eddy current constant, Kf is form factor, f is supply frequency, Bm maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by Kef2Kf2Bm2.

20. Stray losses in transformer occurs in

Stray loss occurs in the mechanical structures and winding conductors due to stray fluxes.