Electrical Machines Objective Type Questions For Gate Preparation-2

26.the percentage resistance and percentage reactance of a10kVA,400 V/200 V, 3-phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%, the maximum possible percentage efficiency of the transformer is
a)98.32 b)97.25
c)96.85 d)96.12
Answer:97.25

27.A 10 kVA,400 V/200 Single phase transformer with percentage with a percentage resistance of 3% and percentage reactance of 6% is supplying a current of 50 A to a resistive load. The value of the load voltage is
a)194V b)390V
c)192V d)196V
Answer:194V

28.A 244/240 V,200kVA single phase transformer has a core loss of 1.8 kW at rated voltage. Its equivalent resistance is 1.1 percent. then, the transformer efficiency at 0.9 pf and on full-load is
a)97.57% b)98.05%
c)97.82% d)96.56%
Answer:97.82%

29.A 400/100 V,5kVA, single-phase two winding transformer is converted into an auto-transformer to supply 400 volts from a 500 V voltage source. When tested as a two winding transformer at rated load and 0.85 pp. lagging, efficiency is 0.95, then its efficiency as auto- transformer at rated-load and 0.8 pp. lagging will be
a)96.02%
b)98.95%
c)98.35%
d)96.37%
Answer:98.95%

30.A 220 V, 60 Hz single-phase transformer has hysteresis loss of 340 watts and eddy current loss of 120 watts. If the transformer is operated from,230V,50Hz supply mains, then total core-loss, assuming stein Metz’s constant equal to 1.6 will be
a)539.3 watt b)408 watt
c)349.4 watt d)308 watt
Answer:539.3 watt
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31.A 300 kVA,3300/400V, three-phase transformer has its primary and secondary windings connected in delta for a dummy load test, for circulating full-load current, the magnitude of the injected voltage in open delta of HV windings is 360 Voltage leakage impedance in per –unit system is
a)0.0396 b)0.424
c)0.0364 d)0.1092
Answer:0.0364

32.Two single-phase transformer rated 1000kVA and 500kVA have per unit leakage impedance of (0.02+j0.06) and (0.025+j0.08) respectively. then, the largest kVA load delivered by the parallel combination of these two transformers without overloading any one is
a)1825.3kVA b)1377.44kVA
c)1500kVA d)3202.74kVA
Answer:1377.44kVA

33.A 100kVA, 2400/240V, 50Hz single phase transformer has an exciting current of 0.64A and a core loss of 700 watts, when its high-voltage side is energized at rated voltage and frequency. If load current is 40 A at 0.8 power factor of on the LV side, then the primary current will be
a)4.58A b)4A
c)4.64A d) none of these
Answer:4.58A

34.Given the maximum efficiency of a transformer of a 500 kVA, 3300/500V,50Hz single-phase transformer is 97% and occurs at ¾ of full-load, unity power factor. If the impedance is 10%, the regulation at full-load, power factor 0.8 lagging will be
a)15.42% b)9.43%
c)7.52% d)10.12%
Answer:7.52%

Also see the First set: Electrical machines Objective Type Questions For Gate Preparation-1

35.Voltage regulation of a large transformer is mainly influenced by
a) no-load current and load power factor
b) winding resistances and load power factor
c)leakage fluxes and load power factor
d)winding resistances and core loss
Answer: winding resistances and load power factor

36.If height to width ratio of the window of core-type transformer is increased, then
a) its leakage reactance and percentage voltage regulation will decrease
b) its leakage reactance and percentage voltage regulation will increase
c)its leakage reactance will be unaffected but total space in the window will increase to accommodate the windings better
d)efficiency will increase and temperature rise will decrease
Answer: its leakage reactance and percentage voltage regulation will decrease

Also see: Transformer Vector Group
37.A single-phase transformer when supplied from 220V,50 Hz has eddy current loss of 330V,50Hz, then eddy current loss will be
a)168.75W b)112.5W
c)75. W d)50W
Answer:112.5W
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38.The torque of a reluctance motor can be effectively increased by
a) increasing reluctance of the magnetic circuit along the direct axis
b) decreasing reluctance of magnetic circuit along the quadrature
c)increasing ratio of the quadrature axis reluctance to the direct axis reluctance
d)decreasing ratio of the quadrature axis reluctance to the direct axis reluctance
Answer: decreasing reluctance of magnetic circuit along the quadrature

39.Full-load voltage regulation of a power transformer is zero when power factor of the load near
a) unity and leading b) zero and leading
c)zero and lagging d) unity and lagging
Answer: unity and leading

40.If the sinusoidal voltage source is connected to a power transformer, its no-load current would be
a) sinusoidal and lagging the voltage by 90degree
b) sinusoidal and lagging the voltage by less than 90degree
c)rich in third harmonic and its fundamental would lag the voltage by 90 degree
d)rich in third harmonic and its fundamental would lag the voltage by less than 90 degree
Answer: rich in third harmonic and its fundamental would lag the voltage by 90degree

41.If all other requirements for parallel operation of transformer are fulfilled, which one of the following pairs of three-phase transformers, with the given VECTOR GROUPS, can be operated in parallel?
a)Yd 1 and Yy 0 b)Yd 1 and Dy 11
c)Dd 6 and Dy 1 d)Dd 0 and Dy 11
Answer: Yd 1 and Dy 11

42.The utility of delta-connected tertiary windings in star –star connected power transformer is that it
a) makes supply available for small loads
b) provides low reactance paths for zero sequence currents
c)is used to suppress harmonic voltage
d)all of these
Answer: all of these

43.A 3-phase alternator is connected to Did transformer. The hysteresis and eddy current losses of the transformer are respectively 300W and 400W.if speed of the alternator is reduced by 10%, then hysteresis and eddy current losses of the transformer will be respectively
a)228 W and 262.44 W
b)243 W and 324 W
c)243 W and 360 W
d)270 W and 400 W
Answer:243 W and 360 W

44.A 400/200 V transformer has a PU impedance of 0.05.the HV side voltage required to circulate full load current during short circuit test is
a)40 V b)20 V
c)10 V d)80 V
Answer:20 V

45.A 200/100 V 50Hz transformer is to be excited at 40 Hz from the 100 side. For the exciting current to remain the same, the applied voltage should be
a)150 V b)125 V
c)100 V d)80 V
Answer:80 V

46.For successful parallel operation of two single-phase transformers, the most essential condition is that them
a) percentage impedance is equal
b) polarities are properly connected
c)turns ratios are exactly equal
d)kVA ratings are equal
Answer: kVA ratings are equal

47.In a transformer, if the iron losses and copper losses are 40.5kW and 50kW respectively, then at what fraction of load will the efficiency be maximum
a)0.8 b)0.57
c)0.70 d)0.9
Answer:0.9

48.The magnetizing current in a transformer is rich in
a)3rd harmonics b)5th harmonics
c)7th harmonics d)13th harmonics
Answer:3rd harmonics

49.A transformer has a voltage regulation of 3% at full load,0.8 power factor lagging.at full load unit power factor, the regulation will be
a)3% b) negative
c)less than 3% d) more than 3%
Answer:3%

50.In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
a) overload
b) poor insulations
c)iron loss
d)saturation of core
Answer: saturation of core