Multiple Choice Questions on Electrical Machines -3

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Important Multiple Choice Questions in Electrical Machines

1.       A single phase transformer when supplied from 220V, 50 Hz has eddy current loss of 50 W. If the transformer is connected to a voltage of 330V, 50 Hz, the eddy current loss will be
A)     168.75 W
B)      112.5 W
C)      75 W
D)     50 W
2.       A coil wound on magnetic core is excited from an AC voltage source. The source voltage and its frequency are both doubled. The eddy current loss in the core will become
A)     Half
B)      Remains same
C)      Double
D)     Four times
3.       Building steel core out of stampings reduces eddy current loss because it
A)     Increases core resistivity
B)      Increases the effective length of eddy currents paths thereby increasing effective resistance to the flow of eddy currents
C)      Increases core permeability
D)     Reduces the effective length of eddy current path, thereby reducing effective resistance to the flow of eddy currents.
4.       A 200/100 V, 50 Hz transformer is to be excited at 40 Hz from 100V side. For the exciting current to remain the same, the applied voltage will be
A)     150 V
B)      125 V
C)      100 V
D)     80V
5.       The applied voltage of a certain transformer is increases by 50% while the frequency reduced to 50%. The maximum core flux density will be
A)      Becomes three times
B)      Becomes 1.5 times
C)      Becomes half
D)     Remains the same
6.       Which of the following tests must be performed on a transformer to determine its leakage reactance
A)     SC test only
B)      OC test only
C)       Both OC and SC tests
D)     Test by an impedance bridge
7.       Power input to a transformer on no load at rated voltage comprises predominantly
A)     Copper loss
B)      Hysteresis loss
C)      Core loss
D)     Eddy current loss
8.       Hysteresis losses are directly proportional to the
A)     Square of frequency and flux density to the power 1.6
B)      Frequency and flux density to the power 1.6
C)      The square of frequency and proportional to the square of flux density
D)     Frequency and proportional to flux density
9.       For a fixed primary voltage how does the secondary terminal voltage of a transformer change if the power factor of inductive load is raised from 0.7 to 0.9 with load current remaining fixed?
A)     No change in voltage
B)      Voltage decreases
C)      Voltage rises
D)     Voltage will rise, go through  maximum and then decrease
10.   For a fixed primary voltage how does the secondary terminal voltage of a transformer change if the power factor of capacitive load rises from 0.7 to 0.9 with load current remaining fixed?
A.      No change in voltage
B.      Voltage decreases
C.      Voltage rises

D.      Voltage will rise, go through  maximum and then decrease

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