21. A transformer may have negative voltage regulation if the loadp powerfactor (pf) is
(a)Leading for some values of pf
(b) Unity pf
(c) lagging but not zero pf
(d) Only zero pf lag
Ans : (a)Leading for some values of pf
22.Positive voltage regulation is an indication of_______load
(a) inductive
(b) capacitive(
c) either inductive or capacitive
(d) pure resistive
Ans : (c) either inductive or capacitive
23. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 lagging and 0.8 leading, respectively?
(a) 4% and -0.8%
(c) 1.6% and -3.2%
(b) 3.2% and -1.6%
(d) 4.8% and -0.6%
Ans : (a) 4% and -0.8%
24. The percentage resistance and reactance of a transformer are 2% and 4% respectively. The approximate regulation on full load at 0.8 pf lag is
(a) 12%
(b) 8%
(c) 6%
(d) 4%
Ans : (d) 4%
25. The voltage regulation of a transformer having 2% resistance and 5% reactance, at full load, 0.8 pf lagging is
(a) 4.6%
(b) -4.6%
(c) -1.4%
(d) 6.4%
Ans : (a) 4.6%
26.The regulation of transformer in which ohmic loss is 1% of output and reactance drop is 5% of the voltage, when operating the at 0.8 power factor lagging is
(a) 3.8%
(b) 4.8%
(c) 5.2%
(d) 5.8%
Ans : (a) 3.8%
27. 10kVA, 400 V/200 V single phase transformer with a resistance of 3% and reactance of 6% is supplying a current of 50 A to a resistive load. The voltage across the load is
(a) 194 V
(b) 196 V
(c) 198 V
(d) 390 V
Ans : (a) 194 V
28. A single-phase 100 kVA, 1000 V/100 V, 50 Hz transformer has a voltage drop of 5% across its series impedance at full load. O this, 3% is due to resistance. The percentage regulation of th transformer at full load with 0.8 lagging power factor is
(a) 4.8
(b) 6.8
(c) 8.8
(d) 10.8
Ans : (a) 4.8
29. The losses in a transformer are :
(I) Copper loss
(II) Eddy current loss
(III) Hysteresis loss.
The constant power loss of a transformer is given by
(a) I only.
(b) I and II only.
(c) II and III only.
(d) I,II and III.
Ans : (c) II and III only.
30. On which of the following factors does hysteresis loss depend?
(1) Flux density.
(2) Frequency.
(3) Thickness of laminations.
(4) Time.
(a) 2 and 3.
(b) 1 and 2.
(c) 3 and 4.
(d) 1 and 4.
Ans : (b) 1 and 2.
31.In a power transformer iron losses remain, practically constant Form 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).
Ans : (a) core flux remains constant.
32. 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.
Ans : (d) iron losses will be less.
33. 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.
Ans : (d)1,600 W and 5,000 W.
34.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
Ans : (d) hysteresis loss will decrease but eddy current loss will remain unchanged
35. Open-circuit test in a transformer is performed with
(a) rated transformer voltage.
(b) rated transformer current
(c) direct current
(d) high frequency supply.
Ans : (a) rated transformer voltage.
36. 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.
Ans : (a) it draws sufficiently large no-load current which can be
37. Consider the following statements: The open-circuit test in a transformer can be used to obtain
1. core losses.
2. magnitude of exciting current.
3. copper losses.
4. equivalent series impedance.
Correct statements are
(a) 1, 2, 3 and 4.
(b) 1 and 3 only.
(c) 1 and 2 only.
(d) 2 and 4 only.
Ans : (c) 1 and 2 only.
38. Consider the following losses for short circuit test on a transformer
1. Copper loss.
2. Copper and iron losses.
3. Eddy current and hysteresis losses.
4. Friction and windage losses.
(a) 1 only.
(b) 2 only.
(c) 3 only.
(d) 2,3 and 4.
Ans : (a) 1 only.
39.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.
Ans : d) In an open-circuit test, current is drawn at low power factor.
40. 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.
Ans : (b) 2 only.
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