Electrical machines miscellaneous Easy Questions and Answers | Page - 43

Electrical machines miscellaneous

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 the efficiency be maximum?

1. 0.8
1. 0.57
1. 0.70
1. 0.9

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Efficiency is maximum, when
x²P_c = P_i
where, x = fraction of loading
P_c = full load copper loss
P_i = normal voltage iron loss.
It gives x = 0.9.

Correct Option: D

Efficiency is maximum, when
x²P_c = P_i
where, x = fraction of loading
P_c = full load copper loss
P_i = normal voltage iron loss.
It gives x = 0.9.

Can a 50 Hz transformer be used for 25 Hz, if the input voltage is maintained constant at the rated vale corresponding to 50 Hz?

1. Yes. Since the voltage is constant, current levels will not changed.
1. No. Flux will be doubled which will drive the care to excessive saturation
1. No. Owing to decreased reactance of transformer, input current will be doubled atload
1. Yes. At constant voltage, insulation will not be overstressed.

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V = √2 n ƒB_mA₁N
This expression shows that if V is unchanged and ƒ gets reduced to half, B_m will have to get doubled, thereby driving the core into saturation.

Correct Option: B

V = √2 n ƒB_mA₁N
This expression shows that if V is unchanged and ƒ gets reduced to half, B_m will have to get doubled, thereby driving the core into saturation.

While performing the open-circuit and shortcircuit tests on a transformer to determine its parameters the status of the low voltage (L.V.) and high voltage (H.V.) winding will be such that

1. in O.C., LV is open and in SC, H.V. is shorted
1. in O.C., HV is open and in SC, L.V. is shorted
1. in O.C., LV is open and in SC, L.V. is shorted
1. in O.C., HV is open and in SC, H.V. is shorted

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From availability of proper range instruments point of view and from voltage considerations, OC test is performed on LV side with HV side open and sc test is performed on HV side with low voltage side shorted.

Correct Option: B

From availability of proper range instruments point of view and from voltage considerations, OC test is performed on LV side with HV side open and sc test is performed on HV side with low voltage side shorted.

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

1. the magnitude of current and power factor will both increase
1. the magnitude of current will decrease but the power factor will increase
1. the magnitude of current will increase, but power factor will decrease
1. the magnitude of current as well as the power factor will decrease.

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Since frequency has been increased, the leakage reactance and hence impedance will increase. Thus current will be less and power factor will be poorer.

Correct Option: D

Since frequency has been increased, the leakage reactance and hence impedance will increase. Thus current will be less and power factor will be poorer.

A transformer has a core loss of 61 W and copper loss of 144 W. When it is carrying 20% over load current, the load at which this transformer will operates at the maximum efficiency

1. 80%
1. 66%
1. 120%
1. 44%

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NA

Correct Option: A

NA