II Year PEE Unit-7 Transformers bits

UNIT-7: TRANSFORMERS and Their Performance

1. 1. If rated Dc voltage is applied instead of AC to the primary of a transformer thenprimary winding of the transformer will burn.

2. If Pi be the iron losses and Pc be the copper losses on full load, then the condition for obtaining the maximum efficiency at 3/4th full load is 

Pc= (16/9) Pi

3. A transformer can have Zero regulation at leading power factor.

4. the chemical used in breather is silica gel

5. The no load current in a transformer lags the applied voltage by 75˚

6. Condition for maximum efficiency of Transformer is Constant losses =variable losses

7. In a Transformer Core is laminated to reduce Eddy current loss

8. A transformer has an efficiency of 80% and works at 100V, 4KW. If the secondary voltage is 240V, find primary current [a ]

(a)40A (b)30A (c)20A (d)10A

9. The no load current of a transformer is generally of the order of _______the full load current [a ]

(a)less than 5% (b)more than 5% (c)equal to that (d)zero

10. The core losses in a transformer consists of Hysteresis and Eddy current losses

11. The voltage regulation of a transformer can be determined by conducting OC and SC tests.

12. A transformer transfers electrical energy from primary to secondary usually with a change in [ a]

(a) frequency (b) power (c) voltage (d) time period

13. The open-circuit test on a transformer is always made on [ a]

(a) low-voltage winding (b) high-voltage winding

(c) either low or high voltage winding (d) none of the above

14. The open-circuit test on a transformer gives Iron losses

15. A transformer has full-load copper loss of 400 W. The copper loss at half-full load will be 100W

16. Transformer operates on AC only.

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

18. If Pi be the iron losses and Pc be the full load losses, the load current at which maximum efficiency occurs is ____________ times of full load current

19. Efficiency of a transformer is maximum if Iron losses=cu losses

20. The Iron core in a transformer provides a Low reluctance path to the main flux.

21. The Principle of operation of a Transformer is [ B]

(A) Electromagnetic Induction (B)Mutual Induction

(C) Varying a conductor in a magnetic field (D) Electrostatic induction

22. The flux involved in the emf equation of a transformer has [ D]

(A) RMS value (B) Average value (C) Total value (D) Max. value

23. In a single phase transformer, with subscripts 1 and 2 for primary and secondary windings[a ]

(A) E1N2= E2N1 and I1N1= I2N2 (B) E1N1= E2N2 and I1N1= I2N2

(C) E1N2= E2N1 and I1N2= I2N1 (D) E1N1= E2N2 and I1N2= I2N1s

24. The Regulation of a Transformer is zero, when the Power factor isleading

25. Eddy current losses in a transformer core can be reduced by _____ [a ]

(a) Reducing the thickness (b) decreasing air gap in the magnetic circuit

(c) Increasing air gap in the magnetic circuit. (d) Increasing the thickness