Important MCQ on Electrical Machines- 6

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Important Multiple Choice Questions on Electrical Machines
1.       Can a 50Hz transformer be used for a 25Hz if the input voltage is maintained constant at the rated value corresponding to 50Hz?
a)      Yes. Since the voltage is constant, current levels will not change.
b)      No. Flux will be doubled which will drive the core excessive saturation.
c)       No. Owing to decreased reactance of transformer, input current will be doubled at load.
d)      Yes. At constant voltage, insulation will not be overstressed.
2.       In a transformer, if the iron losses and copper losses are 40.5 kW and 50 kW respectively, then at what fraction of load will be the efficiency be maximum?
a)      0.8
b)      0.57
c)       0.7
d)      0.9
3.       If the frequency of input voltage of the transformer is increased keeping the magnitude of voltage unchanged, then
a)      Both the hysteresis and eddy current losses in the core will increase.
b)      Hysteresis losses will increase but eddy current losses will decrease.
c)       Hysteresis losses will decrease but eddy current losses will increase.
d)      Hysteresis losses will increase but eddy current losses will remain unchanged.
4.       A 2KVA transformer has iron losses of 150W and full-load copper loss of 250W. The maximum efficiency of the transformer would occur when the total loss is
a)      500W
b)      400W
c)       300W
d)      275W
5.       A 50Hz operation, a single-phase transformer has hysteresis loss of 200W and eddy current loss of 100W. Its core loss at 60 Hz operation will be
a)      432W
b)      408W
c)       384W
d)      360W
6.       In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
a)      Over load.
b)      Poor insulation.
c)       Iron loss.
d)      Saturation of the core.
7.       An auto-transformer having a transformation ratio of 0.8 supplies a load of 10kW. The power transferred inductively from the primary to the secondary is
a)      10kW
b)      8kW
c)       2kW
d)      Zero
8.       When a transformer winding suffers a short-circuit, the adjoining turns of the same winding experience
a)      An attractive force
b)      A repulsive force
c)       No force
d)      None of these
9.       Two transformers of identical voltages but of different capacities are operating in parallel. For satisfactory load sharing
a)      Impedances must be equal.
b)      Per-unit impedances must be equal.
c)       Per-unit impedances and X/R ratios must be equal.
d)      Impedances and X/R ratios must be equal.
10.   A transformer designed for operation on 60Hz supply is worked on 50Hz supply system without changing its voltage and current ratings. When compared with full load, efficiency at 60Hz, the transformer efficiency on full load at 50Hz will
a)      Increase marginally
b)      Increase by a factor of 1.2
c)       Remain unaltered.
d)      Decrease marginally.

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