3-phase Transformer MCQ

1. Power transformers are designed to have maximum efficiency around ……… full load.

(a) nearly
(b) 70% of
(c) 50% of
(d) 25% of

2. The transformer efficiency will be maximum at a power factor of

(a) 0.8 lead.
(b) unity.
(c) 0.8 lag.
(d) 0.5 lag or lead.

3. If P₁ and P₂ be the iron and copper losses of a transformer at full-load and the maximum efficiency is at 75% of the full-load, then what is the ratio of P₁ and P₂?

(a) 9/16
(b) 10/16
(c) 3/4
(d) 3/16 [U.P.S.C. I.E.S. E.E.- ll, 2006]

4. The all day efficiency of a distribution transformer will be high with low

(a) copper losses.
(b) iron losses.
(c) operating temperature.
(d) copper as well as iron losses.

5. The all-day efficiency of atransformer depends primarily on

(a) its copper loss.
(b) the amount of load.
(c) the duration of load.
(d) both (b) and (c).

6. A single phase transformer is to be switched to the supply to have minimum inrush current. The switch should be closed at

(a) maximum supply voltage.
(b) zero supply voltage.
(c) maximum supply voltage ÷ √2.
(d) maximum supply voltage ÷ 2. [GATE E.E. 2001]

7. The use of higher flux density in the transformer design

(a) reduces the weight per kVA.
(b) increases the weight per kVA.
(c) has no relation with the weight of transformer.
(d) increases the weight per kW. [U.P.S.C. I.E.S. E.E.-II, 2003]

8. What does the use of higher flux density value in a transformer design lead to ?

(a) Increase in weight per kVA.
(b) Decrease in weight per kVA.
(c) Reduced iron losses.
(d) Reduced copper losses. [U.PS.C. I.E.S. E.E.-II, 2007]

9. The size cf the transformer core depends upon

(a) frequency.
(b) permissible flux density in the core material.
(c) area of the core.
(d) both (a) and (b).

10. Ferrite cores are employed in high frequency transformers due to their

(a) low resistance.
(b) high resistance.
(c) low permeability.
(d) high hysteresis loss.

11. What is the core loss in a high frequency ferrite core transformer used in SMPS power supply?

(a) 10% of rated power.
(b) 5% of rated power.
(c) 2% of rated power.
(d) I% of rated power.

12. Cores of large power transformers are made from which one of the following?

(a) Hot-rolled steel
(b) Cold-rolled non-grain oriented steel.
(c) Cold-rolled grain oriented steel.
(d) Ferrite. [U.P.S.C. I.E.S. 2008]

13. Grain oriented laminated sheet steel in transformer reduces

(a) copper loss.
(b) eddy current loss.
(c) hysteresis loss.
(d) none of the above.

14. Silicon content in iron laminations is kept within 5% as it

(a) increases hysteresis loss.
(b) increases cost.
(c) makes the material brittle.
(d) reduces the curie point. [AMIE. Sec B. Winter 1993]

15. The permissible flux density in cold rolled grain oriented steel is about

(a) 1.8 T
(b) 2.5 T
(c) 3.5 T
(d) 4.5 T

16. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will

(a) increase.
(b) decrease.
(c) remain unchanged.
(d) become zero. [U.P.S.C. I.E.S. E.E.-II, 2006]

17. Which of the following will improve the mutual coupling between primary and secondary circuits ?

(a) Transformer oil of high breakdown voltage.
(b) High reluctance magnetic core.
(c) Winding material of high resistivity.
(d) Low reluctance magnetic core. [UPSC I.E.S. E.E.-II, 1992]

18. The joints in the transformer core laminations are staggered so as to

(a) avoid cmitinuous gap causing increase in magnetizing current.
(b) increase the mechanical strength of the assembled core.
(c) avoid undue humming noise.
(d) facilitate assembly of core after putting preformed coils on the core.
(e) all of the above.

19. For CRGOS mitred overlap is preferred for the core-yoke joints as it

(a) reduces the magnetizing current and also the core losses.
(b) improves the mechanical strength.
(c) reduces magnetostriction.
(d) makes better use of core space.

20. The degree of mechanical vibrations caused by core laminations in a transformer depends on

(a) size of laminations.
(b) gauge of laminations.
(c) tightness of clamping.
(d) all of the above.

21. The primary and secondary windings are interlaced for

(a) easiness of coil making.
(b) reduced leakage reactance.
(c) reduced cost.
(d) uniform heating. [A.M.I.E. Sec B. Summer 1993]

22. The leakage flux of primary and secondary windings can be reduced to the minimum by

(a) winding primary and secondary coils on separate limbs.
(b) winding primary and secondary coils one upon the other coaxially.
(c) increasing the number of turns.
(d) employing low permeability magnetic material core.

23. The concentric windings are used in core type transformers with ……… winding placed next to the core.

(a) Iv
(b) primary
(c) hv
(d) secondary

24. Low voltage windings are placed next to the core in the case of concentric windings as in this case ……… is/are reduced.

(a) hysteresis loss
(b) leakage fluxes
(c) eddy current loss
(d) copper loss
(e) insulation requirement

25. In transformers, the primary and secondary are interlaced so that

(a) there may be maximum flux linkage between the two windings.
(b) copper is saved.
(c) both (a) and (b) .
(d) statement is false.

26. When a transformer winding suffers a short circuit, the adjoining turns of the same winding experience

(a) an attractive force.
(b) a repulsive force.
(c) no force. [GATE E.E. 1994]

27. Under short-circuit condition, the windings of a transformer having 5% impedance will experience a radial force

(a) 5 times of full load value.
(b) 20 times of full load value.
(c) 25 times of full load value.
(d) 400 times of full load value. [A.M.I.E. Sec B. Summer 1993]

28. Major insulation in a transformer is the insulation between the

(a) lv winding and core.
(b) Iv winding and hv winding.
(c) turns of the winding turns.
(d) both (a) and (b).

29. Minor insulation in a transformer is the insulation between the

(a) lv winding and core.
(b) lv and hv windings.
(c) turns of the windings.
(d) layers of the windings.
(e) both (c) and (d).

30. In a transformer, an insulating material may fail due to

(a) moisture.
(b) dust.
(c) voids in the winding.
(d) any one or more of the above.

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