1. The primary and secondary windings of a power transformer always have
(a) a common magnetic circuit.
(b) separate magnetic circuits.
(c) wire of same size.
(d) same number of turns.
2. If rated dc voltage is applied instead of ac to the primary of a transformer
(a) secondary of transformer will burn.
(b) primary of transformer will burn.
(c) secondary voltage will be excessively high.
(d) there will be no secondary voltage.
3. A transformer transforms
(c) voltage and current.
4. The core flux in transformer depends mainly on
(a) supply voltage.
(b) supply voltage and frequency.
(c) supply voltage, frequency and load.
(d) supply voltage and load but independent of frequency.
5. When voltage is transformed from primary to secondary then it is
(a) multiplied by K2
(b) multiplied by K
(c) divided by K2
(d) divided by K
6. A transformer steps up the voltage by a factor 100. The ratio of current in the primary to that in the secondary is
7. If the applied voltage to a transformer primary is increased by keeping the V/f ratio fixed, then the magnetizing current and the core loss will, respectively:
(a) decrease and remain the same.
(b) increase and decrease.
(c) both remain the same.
(d) remain the same and increase.
8. If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced to 50% (assuming that the magnetic circuit remains unsaturated), the maximum core flux density will
(a) change to three times the original value.
(b) change to 1.5 times the original value.
(c) change to 0.5 times the original value.
(d) remain the same as the original value.
9. The applied voltage of a certain transformer is increased by 75%; while the frequency of the applied voltage is reduced by 25%. The maximum core flux density will:
(a) increase by seven times.
(b) increase by three times.
(c) reduce to one quarter.
(d) remain the same.
10. If the supply frequency of a transformer is increased, keeping the supply voltage constant, then
(a) both magnetising component and core loss component of current will decrease.
(b) both magnetising component and core loss component of current will increase.
(c) magnetising component of current will increase while core loss component of current will decrease.
(d) magnetising component of current will decrease while core loss component of current will increase.
11. The low voltage winding of a 400/230 V single phase 50 Hz transformer is to be connected to a 25 Hz supply. In order to keep the magnetization current at the same level in both the cases the voltage at 25 Hz should be:
(a) 230 V
(b) 460 V
(c) 115 V
(d) 155 V
12. A single-phase transformer rated for 220/240 V, 50 Hz operates at no load at 220 V, 40 Hz. This frequency operation at rated voltage results in which one of the following?
(a) Increase of both eddy-current and hysteresis losses.
(b) Reduction of both eddy-current and hysteresis losses.
(c) Reduction of hysteresis loss and increase in eddy-current loss.
(d) Increase of hysteresis loss and no change in the eddy-current loss.
13. A 50 Hz transformer having equal hysteresis and eddy current losses at rated excitation is operated at 45 Hz 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 by 14.5%.
(d) remain unchanged.
14. 1 kVA, 230 V, 50 Hz. single phase nansformer has an eddy current ioss 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 W.
(c) less than 30 watts.
(d) zero watt.
15. Can a 50 Hz transformer be used for 25 Hz, if the input voltage is maintained constant at the rated value corresponding to 50 Hz ?
(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 decreased resistance of transformer, input current will be doubled at the load.
(d) Yes, at constant voltage, insulation will not be overstressed.
16. On no-load phasor diagram of transformer, the core loss component of the current remains in phase with
(a) no-load current.
(b) primary supply voltage.
(c) core flux.
(d) primary induced
17. If a sinusoidal voltage source is connected to a power transformer, its no-load current would be
(a) sinusoidal and lagging the voltage by 90°.
(b) sinusoidal and lagging the voltage by less than 90°.
(c) rich in third harmonic and its fundamental would lag the voltage 90°.
(d) rich in third harmonic and its fundamental would lag the voltage by less than 90°.
18. The transformer exciting current has two components, magnetizing component and core-loss component. Neglecting impedance drop
(a) both of them are in phase with the supply voltage.
(b) the former lags the supply voltage by 90° while the latter is in phase with supply voltage.
(c) both of them lag the supply voltage by 90°.
(d) the former is in phase with the supply voltage while the latter lags the supply voltage by 90°.
19. The power factor of a power transformer on no load will be about
20. The power factor of a transformer on no load is poor due to
(a) magnetizing reactance of the transformer.
(b) open-circuited secondary.
(c) low primary winding resistance.
(d) low no-load current.
21. Primary winding of a transformer comprises of two identical windings in parallel. If one winding is removed, magnetizing current will be
(b) the same.
(d) increased four times.
22. The core flux of a practical transformer with a resistance load
(a) is strictly constant with load changes.
(b) increases linearly with load.
(c) increases as the square root of the load.
(d) decreases with increase of load.
23. The inductive reactance of a transformer depends on
(a) electromotive force.
(b) magnetomotive force.
(c) magnetic flux.
(d) leakage flux.
24. The flux in transformer core
(a) increases with load.
(b) decreases with load.
(c) remains constant irrespective of load.
(d) none of the above.
25. The mutual flux in a loaded transformer can be varied by varying
(a) primary current.
(b) load impedance.
(c) secondary current.
(d) reluctance of the magnetic path.
26. The primary ampere-turns are counter balanced by
(a) secondary ampere-turns.
(b) primary flux.
(c) increase in mutual flux.
(d) increase in secondary current.
27. In a transformer ………… decreases with the increase in leakage flux.
(a) primary induced emf
(b) secondary induced emf
(c) secondary terminal voltage
(d) none of the above.
28. Power transformed from primary to secondary depends upon
(a) number of primary turns.
(b) number of secondary turns.
(c) current transformation ratio.
(d) magnetic coupling between primary and secondary windings.
29. A transformer is supplying pure resistive (unity pf) load. The power factor on primary side will be
(a) about 0.95 (lead).
(b) about 0.95 (lag).
30. In a transformer supplying inductive load
(a) the secondary current results in equivalent primary current in phase opposition.
(b) the secondary terminal voltage is less than the secondary induced emf.
(c) the power factor on primary side will be lower than that of load.
(d) all of the above.
31. The phasor diagram of a transformer on load can be drawn only if we know
(a) equivalent circuit parameters of the transformer.
(b) load current.
(c) load pf.
(d) all of the above.
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