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Transformer’s interview questions paper-2

21. What is the phase relationship between the primary and secondary voltages of a transformer?

Ans:180⁰ out of phase .

22. What is called turn ratio of a transformer ?

Ans: The ratio of number of turns in the primary to the number of turns in the secondary winding is called the turn ratio or the ratio of transformation of the transformer which is indicated by a constant ҅K ҆ .Than (N1/N2 ) = E1/E2 = K .

23. What is a voltage ratio of a transformer?

Ans : Voltage ratio is the ratio of the voltage between the line terminals of one winding to that between the terminals of another winding at no load .

24. What is the relation between voltage ratio and turn ratio of (a) D/D ,(b) D/Y,(c) Y/Y &(d) Y/D transformer ?

25. What do you mean by true ratio of transformation and commercial ratio transformations ?

Ans: The true ratio of transformation is the ratio of phase voltages of the primary and secondary winding whereas the commercial ratio of transformation is the ratio of the primary and secondary line voltages .

26. What is the relation between primary ampere-turns to the secondary ampere-turns ?

Ans : The primary and secondary ampere-turns are equal and opposite i.e. I1 N1= I2 N2 Then I1/I2 = N2/N2 i.e the primary and secondary currents are inverse as the respective turns .

27. How is the secondary voltage calculated by the turns of the winding ?

Ans: The voltage per turn is practically constant in particular transformer so the secondary voltage is calculated by multiplying the input primary voltage per turn to the number of secondary turns .

28.What current flows in the transformer primary when its secondary is open ?

Ans: When the secondary is open there is no current in the secondary of the transformer. The primary takes a small current (I0)from the source called as no load current which has two components, a magnetising component (I0 sin⁡ ∅0 )producing the magnetic flux and a working component (I0 cos⁡ ∅0 ) supplying real power for iron losses .

29. What is the formula for calculating no load current ?

30. Why is the frequency not changed during transformation of electrical energy in a transformer ?

Ans: As the same flux having a definite frequency is responsible for the production of e.m.fs on both the primary and secondary windings so there is no question of changing the frequency .

31. How is the induced e.m.f related with the flux linkage ?

Ans : The induced e.m.f is directly proportional to the flux linkage .

32. What do you mean by the counter e.m.f in the primary winding of a transformer ?

Ans :when an alternating voltage to the primary winding it causes a flow of alternating current which creates alternating flux. This alternating flux is linked with both the primary and secondary winding and hence induces self-induces e.m.f on the primary winding and a mutually-induced e.m.f on the secondary winding . Since the primary self-induced e.m.f is in the opposite direction to the applied voltage , it is known as counter e.m.f of the transformer .

33.What is the e.m.f equation of a transformer ?

Ans: E(r.m.s) =4.44 ∅m.f.T. volts Wherem=maximum value of the flux linking with both the winding . F=frequency of the supply. T=number of turns.

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