501 MCQ Questions on Transformer

1. In a 3-phase power transformer, 5-limbed construction is adopted to

(a) suppress the 5th and 7th harmonics.
(b) suppress the 3rd and its multiple harmonics.
(c) suppress all the orders of harmonics.
(d) increase the capacity of the transformer.

2. Consider the following statements: The use of Delta-connected tertiary windings in star-star connected power transformers

1. makes available supply for single-phase loads.
2. suppresses harmonic voltages.
3. allows flow of earth fault current for operation of protective devices.
4. provides low-reactance paths for zero- sequence currents.

Of the statements are correct:
(a) 1, 2 and 3 are correct
(b) 1, 3 and 4 are correct
(c) 2 and 4 are correct
(d) 1, 2, 3 and 4 are correct

3. If the height to width ratio of the window of core-type transformer is increased, then

(a) its leakage reactance and percentage voltage regulation will decrease.
(b) its leakage reactance will decrease and percentage voltage regulation will increase.
(c) its leakage reactance will be unaffected but total space in the window will increase to accommodate the windings better.
(d) efficiency will increase and temperature rise will decrease.

4. A 220/110V, 50 Hz single-phase transformer having a negligible winding resistances operates from a variable voltage, variable frequency supply such that V₁/f (V₁ = primary applied voltage, f = source frequency) is constant. This will bringing the given range of frequencies,

(a) variation in both eddy current loss and hysteresis loss.
(b) variation only in the eddy current loss, hysteresis loss remaining constant.
(c) variation only in the hysteresis loss, eddy current loss remaining constant.
(d) no change either in the eddy current loss or the hysteresis loss.

5. A 3-phase Y/Δ transformer has per phase turn ratio ‘K’ and line voltage ratio K₁. If the voltage of Δ winding lags the voltage of Y winding by an angle ɸ then K, K₁, and are related as

(a) K₁ = K and ɸ = -30°
(b) K₁ = K/√3 and ɸ = -30°
(c) K₁ = √3K and ɸ = -30°
(d) K₁ = K and ɸ = 30°

6. Two transformers X and Y with identical ratings and dimensions have 0.8 mm and 1.2 mm thick laminations respectively. If R_m and X_m are the magnetizing branch parameters in the equivalent circuit, then

(a) R_m values in both are likely to be equal, but X_m of X is likely to be higher than X_m of Y.
(b) X_m values in both are likely to be equal, but R_m of X is likely to be higher than R_m of Y.
(c) X_m values in both are likely to be equal, but R_m of X is likely to be lower than R of Y.
(d) R_m values in both are likely to be equal, but X_m of X is likely to be lower than X_m of Y.

7. Hysteresis loss of a transformer designed at voltage ‘V’ and frequency ‘f’ is P_h If the transformer is operated from a voltage ‘xV’ and frequency ‘xf’ the hysteresis loss for a Steinmetz constant of will be

(a) x²P_h      
(b) xP_h
(c) P_h         
(d) x³P_h

8. Two magnetic materials. A and B have their electrical conductivities in the ratio 4 : 9. Eddy current losses in the materials A and B are in the ratio

(a) 2 : 3     
(c) 9 : 4
(b) 3 : 2     
(d) 4 : 9

9. For a distribution sub-station power transformer, operating at normal voltage and frequency, and with usual 24-hour urban domestic load-cycle, let the commercial efficiency be ɳ_c and the all- day efficiency be ɳ_a, Under the operating conditions stated above, which one of the following is correct?

(a) ɳ_c = ɳ_a           
(b) ɳ_c > ɳ_a           
(c) ɳ_c <ɳ_a
(d) ɳ_c and ɳ_a can not be compared, as they have different units

10. Consider the following statements concerning the ferromagnetic materials :

1. There are easy and hard magnetization directions in the ferromagnetic Crystal.
2. Eddy current losses are smaller for materials having low electrical conductivity values.
3. All ferromagnetic materials can be used at high frequencies.

Which of the statements given above are correct?
(a) 1, 2 and 3               
(c) 2 and 3
(b) 1 and 2                   
(d) 1 and 3

11. Two transformers with identical voltage ratings are working in parallel to supply a common load. The percentage impedance of one transformer higher compared to that of the other. The load sharing between the two transformers will

(a) be proportionate to their percentage impedances.
(b) be independent of their percentage impedances.
(c) be inversely proportional to their respective impedances.
(d) depend on the resistance to leakage reactance ratio of each transformer.

12. Consider the following statements: In a transformer, an iron core is used to

1. Increase the useful flux.
2. Reduce the leakage flux.
3. Reduce the useful flux.

Which of the above is/are correct?
(a) 2 only  
(b) 1 and 2
(c) 2 and 3         
(d) 3 only

13. Three single-phase transformers, each of 100 KVA rating are connected in delta, if one of the transformers is taken out of service, the capacity of the system will be

(a) 200 kVA                 
(b) 173.2 kVA
(c) 115.5 kVA              
(d) 36.6 kVA

14. Three units of 1 :  5 transformers are connected in Δ -Y to supply a 3-phase load from a 400 V 3-phase source. The line voltage on the load side is

(a) 1000V           
(c) 3464 V
(b) 80V               
(d) 803 V

15. A 15 MVA, 33/11 kV, 3-phase transformer having an impedance of 16 on the high tension side is supplied from a generator rated 30 MVA, 11 kV. The p.u. impedance of the transformer calculated on its own base and on the generator base will be in the ratio

(a) 1 : 1              
(c) 1 : 9
(b) 1 : 3              
(d) 1 : 18

16. The full-load, copper loss of a transformer is twice its core loss. At what percent of the full-load, will the efficiency be maximum?

(a) 25%               
(b) 50%
(c) 70.7%           
(d) 141 %

17. Three single-phase transformers with turn ratio 1 : 10 are connected in delta-star to supply a 3-phase load. The supply voltage is 400 V (line to line). The line voltage on the load side is

(a) 400V             
(b) 400 √3 V
(c) 4000V           
(d) 4000 √3 V

18. Which of the following statements are incorrect?

1. Maximum voltage regulation of transformer occurs at leading power factor.
2. Voltage regulation of a transformer is the maximum when load power factor (lagging) angle has the same value as the angle of equivalent impedance.
3. Voltage regulation of a transformer at leading power factor is always zero.
4. Voltage regulation of a transformer can be negative at leading power factor.

Select the correct answer using the codes given below:
(a) 1 and 3         
(c) 2 and 4
(b) 2 and 3         
(d) 1 and 4

19. Two 3-phase transformers are connected to the same supply of balanced 3-phase voltages on the primary side. The first transformer is connected in Yd1 on the second in Dy11. The phase difference between the line-to-line voltages of the corresponding phases on the secondary sides of the two transformers of

(a) 0°                  
(c) 60°
(b) 30°                
(d) 90°

20. The leakage flux in a transformer depends upon the

(a) Applied voltage              
(b) Frequency
(c) Load current          
(d) Mutual flux

21. The voltage regulation of a large transformer is mainly influenced by

(a) no-load current and load power factor.
(b) winding resistances and load power factor.
(c) leakage fluxes and load power factor.
(d) winding resistances and core loss.

22. A bank of three identical singe-phase 250 kVA, 11 kV/230 V transformers is used to provide 400 V low tension supply from a 11 kV, 3-phase sub-station. The effective kVA rating of the bank will be

(a) 250               
(c) 500
(b) 250 √3
(d) 750

23. A transformer designed for operation on 60 Hz supply is worked on 50 Hz supply system without changing its voltage and current ratings. When compared with full load efficiency at 60 Hz, the transformer efficiency on full load at 50 Hz will

(a) Increase marginally
(b) Increase by a factor of 1.2
(c) Remain unaltered
(d) Decrease marginally

24. A transformer has a core loss of 200W and a full load copper loss of 800W. The maximum efficiency of the transformer will occur at

(a) 0.5 times full load current
(b) 0.6 times full load current
(c) 0.7 times full load current
(d) 0.8 times full load current

25. In a transformer P_i; is iron loss at rated voltage and P_c. is full load copper loss. If the maximum efficiency occurs at 75% of full load, then which one of the following is correct?

(a) P_i = P_c
(b) 3P_i/4 = P_c
(c) P_i = 9P_c/16
(d) P_i = 16P_c/9

26. An open circuit test was done on a transformer with rated voltage at rated frequency. If the open circuit test is now done on the transformer with double the rated voltage with double the rated frequency, then which one of the following is correct?

(a) Both current and power factor increase.
(b) Both current and power factor decrease.
(c) Current decreases but power factor increases.
(d) Current increases but power factor decreases.

27. The main purpose of the conservator in a (Driving system) transformer is to

(a) store extra oil to compensate for the loss of oil due to leakage.
(b) achieve better cooling of the transformer.
(c) take up the expansion of oil due to heating.
(d) have the Bucholtz relay fitted.

28. Which one of the following can be obtained by the equivalent circuit of an electrical machine?

(a) Temperature rise in the cores.
(b) Complete performance characteristic of the machine.
(c) Type of protection used in the machine
(d) Design parameters of the windings.

29. Which one of the following statements regarding a transformer is correct?

(a) In transformers, a laminated core is used to reduce copper losses.
(b) Core losses in a transformer can be determined by open-circuit test.
(c) Short-circuit test is conducted on a transformer to determine constant losses.
(d) In a shell type transformer the primary and secondary windings are wound on separate limbs.

30. A 300 kVA transformer has 95% efficiency at full load, 0.8 pf lagging and 96% efficiency at half load, unity pf. The iron loss W_i; and copper loss W_cu in kW, under full-load operation are

(a) 4.12, 8.51              
(b) 6.59, 9.21
(c) 8.51, 4.12              
(d) 12.72, 307

31. A single-phase transformer has a maximum efficiency of 90% at full load and unity power factor. Efficiency at half load at the same power factor is

(a) 87.7%  
(b) 88.26%         
(c) 95%               
(d) 96%

32. The percentage resistance and percentage reactance of a 10 kVA, 400 V/200 V, single- phase transformer are 3% and 6%. The transformer is supplying a current of 50 A to a resistive load. The value of the voltage across the load is

(a) 194 V            
(b) 198 V
(c) 192 V            
(d) 196 V

33. The efficiency of a 100 kVA transformer is 0.98 at full as well as at half load. For this transformer at full load, the copper loss

(a) is less than core loss.
(b) is equal to core loss.
(c) is more than core loss.
(d) none of the above.

34. The low-voltage winding of a 400 V/230 V, 1 phase, 50 Hz transformer is to be connected to a 25 Hz supply. In order to keep the magnetisation current at the same level as that for normal 50 Hz supply, at 25 Hz supply the supply voltage should be

(a) 230 V            
(b) 460 V
(c) 115 V            
(d) 65 V

35. A voltage V = 400 sin 314.16t is applied to a 1- phase transformer on no-load. If the no-load current of the transformer is 2 sin (314.16 t – 85°), the magnetisation branch impedance will be approximately equal to

(a) 141∠90°       
(b) 200∠-85°
(c) 200∠85°       
(d) 282∠-80°