Transformer Objective Type Questions and Answers

1. A 1 kVA, 230/100 V, single phase, 50 Hz transformer having negligible winding resistance and leakage inductance is operating under saturation, while 250 V, 50 Hz sinusoidal supply is connected to the high voltage winding. A resistive load is connected to the low voltage winding which draws a rated current. Which one of the following quantities will not be sinusoidal?

(a) Voltage induced across the l_v winding.
(b) Core flux.
(c) Load.
(d) Current drawn from the source. [GATE E.E. 2002]

2. For the purpose of analysis exact equivalent circuit of a transformer is usually replaced by an approximate equivalent circuit. In doing so errors introduced due to winding copper loss and core loss are of different nature. Due to this, the analysis by approximate equivalent circuit gives fairly satisfactory results. Under the circumstances, which one of the following statements is correct in respect of losses referred to approx. equivalent circuit as compared to exact equivalent circuit ?

(a) This accounts for somewhat lesser primary winding copper loss and less core loss.
(b) This accounts for somewhat lesser primary winding copper loss and more core loss.
(c) This accounts for somewhat greater secondary winding copper loss and less core loss.
(d) This accounts for somewhat lesser secondary winding copper loss and core loss. [U.PS.C. IES. E. E.-II 1995]

3. Full-load voltage regulation of a power transformer is zero when power factor of the load is near

(a) unity and leading.
(b) zero and leading.
(c) zero and lagging.
(d) unity and lagging.

4. In a transformer, zero voltage regulation at full load is:

(a) not possible.
(b) possible at leading power factor load.
(c) possible at lagging power factor load.
(d) possible at unity power factor load. [GATE E.E. 2007; U.P.S.C. IES. E.E.-II, 2011]

5. The losses in a transformer are :

l. Copper loss
ll. Eddy current loss
III. Hysteresis loss.
The constant power loss of a transformer is given by
(a) I only.
(b) I and Il only.
(c) II and III only.
(d) I, II and III [U.P S.C. I.E.S. E.E.- II, 1992]

6. In a transformer, the hysteresis and eddy current losses depend upon

l. load current
ll. maximum flux density in the core
lll. type of lamination
lV. supply frequency

The correct statements are
(a) I and III.
(b) II and III.
(c) l and IV.
(d) II and IV.

7. In a power transformer iron losses remain practically constant from no load to full load. This is because

(a) core flux remains constant.
(b) leakage flux remains constant.
(c) both (a) and (b).
(d) neither (a) nor (b).

8. In a power transformer, if in place of sinusoidal wave, a peaked wave voltage is fed to the primary

(a) copper losses will be less.
(b) noise level will be reduced.
(c) iron losses will be more.
(d) iron losses will be less.

9. On which of the following factors does hysteresis loss depend?

(1) Flux density.
(2) Frequency.
(3) Thickness of laminations.
(4) Time.
Select the correct answer using the code given below:
(a) 2 and 3.
(b) 1 and 2.
(c) 3 and 4.
(d) 1 and 4. [U.P.S.C. I.E.S. E.E.-II, 2007]

10. The full-load copper loss and iron loss of a transformer are 6,400 W and 5,000 W respectively. The copper loss and iron loss at half load will be, respectively

(a) 3,200 W and 2,500 W.
(b) 3,200 W and 5,200 W.
(c) 1,600 W and 1,250 W.
(d) 1,600 W and 5,000 W. [U.P.S.C. I.E.S. E.E.-II, 2002, 2008]

11. If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then

(a) both hysteresis loss and eddy current loss in the core will increase.
(b) hysteresis loss will increase but eddy current loss will decrease.
(c) hysteresis loss will decrease but eddy current loss will increase.
(d) hysteresis loss will decrease but eddy current loss will remain unchanged. [U.P.S.C. I.E.S. E.E. II 1994]

12. The total iron loss in a transformer core at normal flux density was measured at 25 Hz and at 50 Hz and was found to be 250 W and 800 W respectively. The hysteresis loss at 50 Hz would be

(a) 100 W.
(b) 150 W.
(c) 200 W.
(d) 600 W. [I.E.S. E.E.-I, 1999]

13. The main purpose of performing open-circuit on a transformer is to measure its

(a) copper loss.
(b) core loss.
(c) total loss.
(d) insulation resistance.

14. Open-circuit test in a transformer is performed with

(a) rated transformer voltage.
(b) rated transformer current.
(c) direct current.
(d) high frequency supply.

15. The open circuit test on a transformer is usually performed by exciting the low voltage winding. This is because

(a) it draws sufficiently large no-load current which can be conveniently measured.
(b) the required power input is low.
(c) it is not advisable to work on high voltage side.
(d) the voltage required is low.

16. The open-circuit test on a transformer gives

(a) equivalent resistance and leakage reactance.
(b) magnetizing current and core loss at rated voltage.
(c) copper losses.
(d) both (b) and (c).

17. In transformers, which of the following statements is valid?

(a) In an open-circuit test, copper losses are obtained while in short-circuit test, core losses are obtained.
(b) In an open-circuit test, current is drawn at high power factor.
(c) In a short-circuit, current is drawn at zero power factor.
(d) In an open-circuit test, current is drawn at low power factor. [GATE, E.E. 2006]

18. Consider the following tests:

1. Load test
2. Short-circuit test
3. OC test
4. Retardation test

Which of the above tests are to be conducted for the determination of voltage regulation of a transformer?
(a) 1 only.
(b) 2 only.
(c) 2 and 3.
(d) 3 and 4. [U.P.S.C. I.E.S. E.E.-II, 2009]

19. When a no-load test is carried out on hv side of 1,000/100 V single phase transformer with 100 V winding open, the wattmeter reads 110 watts. If the test is repeated on lv side with 1,000 V winding open. The wattmeter will read.

(a) 11 W
(b) 110 W
(c) 1.1 W
(d) 1,100 W

20. It is desired to measure parameters of 230 V/115 V, 2 kVA, single-phase transformer. The following wattmeters are available in a laboratory:

W1 : 250 V, 10 A, Low Power Factor.
W2 : 250 V, 5 A, Low Power Factor.
W3 : 150 V, 10 A, High Power Factor.
W4 : 150 V, 5 A, High Power Factor.
The wattmeters used in open-circuit test and short-circuit test of the transformer will respectively be
(a) W1 and W2
(b) W2 and W4
(c) W1 and W4
(d) W2 and W3 [GATE E.E. 2008]

21. The nominal short-circuit voltage (V_SCN) of a transformer means the

(a) voltage across the primary terminals when the secondary terminals are short circuited.
(b) voltage appearing across the secondary terminals when they are short circuited.
(c) primary voltage when the secondary terminals are short circuited and the rated current flows through the primary winding.
(d) none of the above.

22. Short-circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short-circuit test is now performed with the same magnitude of impressed voltage, but at a frequency higher than the rated frequency, then

(a) the magnitude of current and power factor will both increase.
(b) the magnitude of current will decrease bat the power factor will increase.
(c) the magnitude of current will increase but power factor will decrease.
(d) the magnitude of current as well as the power factor will decrease. [U.P.S.C. I.E.S. E.E.-II, 1993]

23. For a transformer to be tested at full load conditions but consuming only losses from the mains, we do

(a) load test.
(b) open-circuit and short-circuit tests.
(c) back-to-back test.
(d) none of the above. [A.M.I.E. Sec B. Elec. Machines Summer 1995]

24. The efficiency of two identical transformers under load conditions can be determined by

(a) back to back test.
(b) open-circuit test.
(c) short-circuit test.
(d) any of the above. [U.P.S.C I.E.S. E.E.-II, 1992]

25. Two transformers, each having iron loss of P_i watts and full-load copper loss of P_c are put to back to back test and full-load current is allowed to flow through the secondaries, the total input will be

(a) 2P_i.
(b) 2P_c
(c) P_i + P_c
(d) 2(P_i + P_c)

26. The efficiency of a power transformer can be determined indirectly by

(a) open-circuit test alone.
(b) short-circuit test alone.
(c) open-circuit and short-circuit tests.
(d) back-to-back test.

27. The transformer efficiency at relatively light loads is quite low. This is due to

(a) small copper losses.
(b) small secondary output.
(c) high fixed loss in comparison to the output.
(d) poor power factor.

28. The transformer efficiency, under heavy loads, is comparatively low due to

(a) large increase in copper losses in comparison to the output.
(b) large increase in iron losses.
(c) drop in power factor.
(d) none of the above.

29. The efficiency of a 100 kVA transformer is 0.98 at full as well as half load. For this transformer at full-load the copper loss

(a) is less than core less.
(b) is equal to core loss.
(c) is more than core loss.
(d) none of the above. [GATE E.E. 1998]

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