# Multiple Choice Questions on Transformer | Page – 1

1. When AC supply is given to the transformer, the flux will depend on

V = 4.44φfT.
Where V is applied voltage, φ is the flux in the core, f is the frequency of the supply, T is the number of turns in the winding. Hence from this equation, flux φ depends upon both voltage V and supply frequency f.

2. In transformer, humming sound occurs due to

Due to alternating magnetic field, the magnetic domains of a ferromagnetic material continuously change their shape and orientation. This phenomenon is called magnetostriction. Due to this the overall dimension of each laminated sheet changes according to supply frequency, hence creates a humming sound.

3. As compared the transformer with AC machines it is more efficient because

It is a static device. It is not at all having any mechanical losses hence having more efficiency.

4. The primary and secondary winding of transformer are linked each other by

The function of a transformer depends upon the mutual induction between it's primary and secondary winding. Whenever an alternating source is applied to the primary, a changing alternating flux is produced and passed through magnetic core, and linked with secondary winding.

5. Which parameter does not change during transformation action in transformer?

Transformation action depends upon the rate of change of flux linkage between primary and secondary winding. The applied alternating voltage results alternating current in the primary winding, which ultimately causes alternating flux in the core with same frequency of primary voltage or current. This alternating flux links with secondary winding and induces voltage in it hence the secondary voltage will also have same frequency as primary voltage. So there will be no change of frequency between primary and secondary of a transformer.

6. Secondary current of a step down transformer is

In a transformer input power equals to output power while loss is neglected. Again power is the product of voltage and current if power factor is taken as unity. As the output voltage of step down transformer decreases hence there will be proportionate increase in current at secondary.

7. The core of transformer must have

In a transformer there will be mutual inductance between primary and secondary circuits linked by a common magnetic flux through the core of transformer. It is desirable to pass all the flux produced by magnetizing current, through the core, hence the core of transformer must be of low reluctance.

8. The working principle of transformer depends upon

Transformation action depends upon the rate of change of flux linkage between primary and secondary winding of transformers. Faraday’s laws of electromagnetic induction, states that the rate of change of flux linkage with respect to time is directly proportional to induced emf in a coil. Hence it can be concluded that working principle of transformer depends upon Faraday’s law of electromagnetic induction.

9. An ideal transformer is one which has

An ideal transformer consists of two winding which are purely inductive and wound on a loss free core means there will be no losses in transformer, no leakage reactance of transformer and also no resistance of transformer.

10. In an ideal transformer the magnetizing current lags behind applied primary voltage by an angle of

Since in an ideal transformer the primary coil is purely inductive, it draws the magnetizing current lags primary applied voltage by 90°.

11. In an ideal transformers primary and secondary induced emfs are

The changing flux in the core due to primary magnetizing current is linked with both the windings. Therefore it produces self induced counter emf in the primary. As the changing flux produced by the current in primary winding links with the secondary winding, the induced emf in the secondary winding is mutually induced emf.

12. The input power under no load condition of transformer consists of

Ideally the output power under no load condition, gives only core losses in transformer but in practice as the magnetizing current flows through the primary winding there will be a small amount of copper loss in the winding due its resistance. This negligible amount of copper loss in transformer also be included in input power under no load condition of the transformer.

13. In a transformer the coils of primary and secondary are placed concentrically to achieve

If the primary and secondary windings are kept separate and widely spaced, there will be much room for leakage flux. Leakage between primary and secondary could be eliminated if the windings could be made to occupy the same space. This of course, is physically impossible, but an approximation to it achieved if the coils of primary and secondary are placed concentrically.

14. Iron loss in transformer can be determined by

In open circuit test the primary current drawn by the transformer is only its magnetizing current. As the output of the transformer is open circuited there will be no load current in the windings hence the total power indicated in the wattmeter connected to the input of transformer is mainly associated with the losses occur in the core that is iron loss in transformer.

15. No load losses in a transformer can be minimized by using steel of

The no load losses in transformer consist of hysteresis loss and eddy current loss. Hysteresis loss can be minimized by using high content of silicon in steel and eddy current loss can be minimized by using very thin lamination of that silicon steel.

16. Which of the following losses remain constant during normal operation of transformer?

Core loss in transformer includes both hysteresis loss and eddy current loss. As the core flux in a transformer remains practically constant at all loads, the core loss is also constant at all loads. The copper loss is mainly due to ohmic resistance of transformer winding hence it varies with load current. Stray loss occurs in the mechanical structure and winding conductor of a transformer due to stray fluxes. So it also depends upon load. Hence only core loss of transformer remains constant during normal operation of transformer.

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

Core loss in transformer includes both hysteresis loss and eddy current loss. As the core flux in a transformer remains practically constant at all loads, the core loss is also constant at all loads. The copper loss is I2R loss in transformer mainly due to ohmic resistance of transformer winding hence it varies with load current.

18. The maximum efficiency of a transformer occurs when

Maximum efficiency occurs when iron loss is equal to copper loss in transformer.

19. Which one of the following is not a condition of parallel operation of transformer?

The parallel operation of transformers does not mean that all the transformers operated in parallel will share equal load. It means the load sharing by the transformers must be proportionate with their MVA rating. So two unequal MVA rated transformers can always be operated in parallel but they should share the load according to their MVA ratings.

20. If the supply frequency of a transformer increases then output voltage of the transformer will

If the supply voltage is fixed, then for all supply frequencies the induced voltage at primary and secondary will remain same.