transformer

1.     A transformer steps up the voltage by a factor 100. The ratio of current in primary to secondary is

a)     1                  b) 100                   c) 0.01                  d) 0.1

Ans:b

2.     The  power factor  of a power transformer on no load will be about

a)     Unity     b) 0.75                  c) 0.5          d)0.35

Ans:d

3.     The flux in transformer core

a)     Increases with load

b)    Decreses with load

c)     Remains constant irrespective of load

d)    None of  the above

Ans:c

4.     The primary ampere-turns of  are counter balanced by

a)secondary ampere-turns

b)primary flux

c)increase in mutual flux

d)increase in secondary current

Ans:a

5.     If the applied voltage to a transformer primary is increased by keeping the v/f  ratio fixed, then the magnetizing current and the corelessloss will respectively

a)     decrease and remain the same

b)    increase & decrease

c)     remains same & increase

d)    both remains same

Ans:d

6.     If applied voltage of a certain transformer is increased b 50%  & frequency is reduced to 50% (assuming that the magnetic circuit remains unsaturated),the maximum core flux density will

a)change to three time the original value
b)change to 1.5 times the original value

c) change to 0.5 times the original value

d) remains the same as the original value

Ans:a

7.     The applied voltage of a certain transformer s increased by 75% while the frequency of  applied voltage is educed by 25%. The maximum core flux density will

a)     Increase by seven time

b)    Increase by three times

c)     Reduce to one quarter

d)    Remains the same

Ans:b

8.      A single phase transformer reted or 220/240,  50hz operates at no load at 220V , 40Hz. This frequency operation at rated  voltage results in which one of following?

a)     Increase of both eddy current & hysteresis losses

b)    Reduction of hysteresisloss & increase of eddy current lodd

c)     Increase of hysteresis loss &no change in eddy current loss

d)    None of the above

             Ans:c

9.     A 50hz transformer having equal hysteresis and eddy current  losses at rated excitation is operated at 45hz at 90%  of its rated voltage. Compared to rated operating point , the core losses under this condition.

a)     Reduce by 10%

b)    Reduce by 19%

c)     Reduce by14.5%

d)    Remains unchanged

Ans:c

10.    A single phase transformer when suppilied from 220v, 50hz has eddy current loss of 50hhz , the eddy current loss will be

a)     168.75W

b)    112.5W

c)     75W

d)    50W

Ans:b

11.    1KVA,230V,50HZ, single phase transformer has an eddy current loss of 30 watts, the eddy current loss when the transformer is excited by a dc source of same voltage will be

a)     30 watts

b)    More than 30 watts

c)     Less than 30 watts

d)    Zero watt.

Ans:d

12.         At 50hz operation , a single phase transformer has hysteresis loss of 200W and eddy current loss of 100w its core loss at 60hz operation will be

a)     432 w

b)    408w

c)     384w

d)    360w

Ans:c

13.           Can a 50 hz transformer  be used for 25hz, if the input voltage is maintained constant at the rated value corresponding to 50hz?

a)     Yes, since the voltage is constant current level will not change

b)    No,flux will be doubled which will drive the core to excessive saturation

c)     No,owing to decreaed resistance of  transformer , input current will be doubled at the load

d)    Yes,at constant voltage, insulation will not be overstressed

Ans:b

14.                                                                                                                                                 On load phasor diagram of transformer the core loss component of current remains in phase with

a)     No-load current

b)    Primary supply voltage

c)     Core flux

d)    Primary induced voltage

Ans:b

15.          The induced reactance of a transformer depends on

a)     Electromotive force

b)    Magnetomotive force

c)     Magnetic flux

d)    Leakage flux

Ans:d

16.        Open circuit test performed on a transformer with

a)     Rated secondary terminal voltage

b)    Rated transformer current

c)     Direct current

d)    High frequency supply

Ans:a

17.     In transformer circuit mode the core losses is represented as a

a)     Series resistance

b)    Shunt resistance

c)     Series inductive

d)    Shunt inductive

          Ans:b

18.              When a transformer is operating on no load , the primary applied voltage is approximately balanced by

a)     Primary induced emf

b)    Secondary induced emf

c)     Terminal voltage across the secondary

d)    Voltage drop across the resistance and reactance

Ans:a

19.      Full load voltage regulation of  a power transformer is zero when power factor of the load is near

a)     Unity and lagging

b)    Zero and leading

c)     Zero and lagging

d)    Unity and lagging

Ans:a

20.          In transformer , zero voltage regulation at full load is

a)     Not possible

b)    Possible at leading power factor load

c)     Possible at unity power factor load

d)    Possible at lagging power factor load

Ans:b