This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Transformer Losses and Efficiency”.

1. I. Maximum voltage regulation occurs at the leading p.f.

II. Maximum voltage regulation occurs when load p.f. angle and impedance angle of the leakage impedance are same.

III. V.R. at zero p.f. is always zero.

IV. V.R. of a transformer may be negative at leading p.f.

Which of the statements made here are correct regarding the transformer?

a) I,III

b) II,IV

c) 1,III

d) I,IV,III

View Answer

Explanation: V.R. is always negative at leading p.f. and the load p.f. angle and

impedance angle of the leakage impedance should be same for maximum V.R.

2. I. Maximum voltage regulation occurs at the leading p.f.

II. Maximum voltage regulation occurs when load p.f. angle and impedance angle of the leakage impedance are same.

III. V.R. at zero p.f. is always zero.

IV. V.R. of a transformer may be negative at leading p.f.

Which of the statements made here are correct regarding the transformer?

a) I,II,III

b) II,IV

c) 1,III

d) I,IV,III

View Answer

Explanation: V.R. is always negative at leading p.f. and the load p.f. angle.

3. I. Maximum voltage regulation occurs at the leading p.f.

II. Maximum voltage regulation occurs when load p.f. angle and impedance angle of the leakage impedance are same.

III. V.R. at zero p.f. is always zero.

IV. V.R. of a transformer may be negative at leading p.f.

Which of the statements made here are incorrect regarding the transformer?

a) I,III

b) II,IV

c) 1,III

d) I,IV,III

View Answer

Explanation: V.R. is always negative at leading p.f. and the load p.f. angle and impedance angle of the leakage impedance should be same for maximum V.R.

4. I. Maximum voltage regulation occurs at the leading p.f.

II. Maximum voltage regulation occurs when load p.f. angle and impedance angle of the leakage impedance are same.

III. V.R. at zero p.f. is always zero.

IV. V.R. of a transformer may be negative at leading p.f.

Which of the statements made here are incorrect regarding the transformer?

a) I,II,III

b) IV

c) 1,III

d) I,IV,III

View Answer

Explanation: V.R. is always negative at leading p.f. and the load p.f. angle.

5. Which of the below mentioned losses occur in a transformer?

a) Hysteresis losses ;Eddy current losses; Dielectric losses; Stray load losses

b) Hysteresis losses ;Eddy current losses;

c) Dielectric losses; Stray load losses

d) Hysteresis losses ;Eddy current losses; Stray load losses

View Answer

Explanation: Hysteresis losses-due to magnetic material;Eddy current losses-on the core area; Dielectric losses- due to insulation material; Stray load losses- due to leakgae through the parts of the transformer.

6. Mutual flux ______ at the lagging loading and it _____ at the leading power factor.

a) decreases, increases

b) increases,increases

c) decreases, decreases

d) increases, decreases

View Answer

Explanation: The mutual flux falls when the transformer is working at lagging p.f. and it increases at leading p.f.

7. It is possible to attain maximum efficiency in a transformer when the

a) core losses are equal to rated full load copper losses

b) core losses are more than rated full load copper losses

c) core losses and full load copper losses are constant

d) copper loss also becomes constant

View Answer

Explanation: Maximum efficiency is achieved at the condition when fixed core losses and copper losses at rated condition are equal.

8. The efficiency of a 20 KVA, 2000/200 V, single phase transformer at unity pf is 98%. The the total losses at this condition is

a) 408W

b) 4.08kW

c) 204W

d) 2.04kW

View Answer

Explanation: Efficiency = 1-(losses/input) = 1-(losses/output+losses)

0.98 = 1-(Losses/20000+losses)

Losses = 408 W.

9. The efficiency of a 20 KVA, 2000/200 V, single phase transformer at unity pf is 98%. The given total losses at full load is 200 W. The pu resistance is

a) 0.01

b) 0.1

c) 1.0

d) 0.0196

View Answer

Explanation: pu resistance = Ohmic losses/KVA = 200/20000 = 0.01 pu.

10. The load current at which maximum efficiency occurs is independent of the load power factor.

a) True

b) False

View Answer

Explanation: Because Core losses and the effective resistance of the secondary are almost unaffected by load p.f.

11. The full load voltage drop in a 1-phase transformer is 2% and 4% respectively due to resistance and leakage reactance. Then the voltage drop is maximum at

a) 0.45 lagging

b) 0.45 leading

c) 0.9 lagging

d) 0.9 leading

View Answer

Explanation: Z = 4.47; cosθ = percentage r/percentage z = 2/(4.47) = 0.45.

12. The full load voltage drop in a 1-phase transformer is 2% and 4% respectively due to resistance and leakage reactance. Then the voltage drop is zero at

a) 0.45 lagging

b) 0.45 leading

c) 0.9 lagging

d) 0.9 leading

View Answer

Explanation: Z = 4.47; cosθ = percentage x/percentage z = 4/4.47 = 0.89 .

13. Core losses of a transformer as a function of frequency is best approximated by which of the below characteristics?

a)

b)

c)

d)

View Answer

Explanation: We can represent the core losses per cycle eq. as Pc/f = A+B*f, where A and B are constants.

14. From the above graph of the core losses per cycle is, the hysteresis losses per phase for a transformer at a operating frequency of 40 Hz is

a) 20W

b) 30W

c) 80W

d) 32W

View Answer

Explanation: Ph = k1*f = 0.5*40 = 20W.

15. From the above graph of the core losses per cycle is, the eddy losses per phase for a transformer at a operating frequency of 40 Hz is

a) 15 W

b) 20W

c) 35W

d) 32W

View Answer

Explanation: Pe = k2*f

^{2};

Using the slope of the line, (k2)*50 =0.8-0.5 = 0.3

(k2) = 0.006

Pe = 50*50*0.006 = 15 W

16. The efficiency of a transformer can be calculated accurately from

a) Open circuit test, Short circuit test

b) Open circuit test, Short circuit test, Sumpner’s test

c) Sumpner’s test

d) load test

View Answer

Explanation: OC and SC tests together can help to estimate most correct value of the efficiency for the given transformer.

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