Important MCQ on Electrical Machines- 6
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.
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