Tranformers_objectives

1.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

2.The low-voltage winding of a core-type transformer is subdivided into two equal halves, each of half the original width of the single winding with the high-voltage winding in between these two halves of low-voltage winding (instead of having usual construction of low-voltage winding adjacent to the core and surrounded b the high-voltage winding.) Such an interlacing of coils would make the combined primary and secondary leakage reactance (in terms of the primary) nearly
a. Twice
b. Equal
c. Half
d. One-fourth

3.Two 3-limb, 3-phase delta-star connected transformers are supplied from’ the same source. One of the transformers is of Dy 1and the other is of Dy 11 connection. The phase difference between the corresponding phase voltages of the two second Aries would be
a. 0°
b. 30°
c. 60°
d. 120°

4.In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
a. Overload
b. Poor insulation
c. Iron loss
d. Saturation of core

5.A 40k VA transformer has a core loss of 400W and. a full-load copper loss of 800W. The proportion of full-load at maximum efficiency is
a. 50%
b. 62.3%
c. 70.7%
d. 100%

6.A single-phase transformer has a. rating of 15 k VA, 600/120V. It is reconnected as an auto- transformer to supply at 720 V from a 600 V primary source. The maximum load it can supply is
a. 90 kVA
b. 18 kVA
c. 15 kVA
d. 12 kVA

7. Two single-phase transformers with turns ratios 1 and 2 respectively are connected to a 3-phase supply on the primary side as shown in the figure. The voltmeter V2, will read


a. 100 V
b. 173 V
c. 200 V
d. 265 V
8. In a 3-phase transformer shown in the figure, the phase displacement of secondary line voltages with corresponding primary line voltages will be


a. Zero
b. 300 lag
c. 30° lead
d. 180°


9. Three 3-phase transformers each rated at 75 MVA, 132 kV/1 kV, have the following different methods of cooling:
1. Self oil cooled
2. Forced oil cooled.
3. Forced air cooled.
The correct sequence in ascending order in terms of the weights of these transformers is
a. 1,2,3
b. 2, 3, 1
c. 3, 1, 2
d. 3, 2, 1

10. A single-phase induction regulator is a constant input transformer to obtain smooth variation of the output voltage by varying the
a. ratio of turns between primary and secondary windings
b. frequency
c. flux density in the core
d. angle between the magnetic axes of the primary and secondary windings

11. An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power transferred inductively from the primary to the secondary is
a. 10 kW
b. 8 k W
c. 2kW
d. Zero

12. Full-load voltage regulation of a power transformer is zero, when power factor of the load is near
a. unity and leading
b. zero and leading
c. zero and lagging
d. unity and lagging

13. In a single-phase transformer, the magnitude of leakage reactance is twice that of resistance of both primary and secondary. With secondary short-circuited, the input power factor is
a. 1/ 2
b. 1/ 5
c. 2/ 5
d. 1/3

14. 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 by 90°
d. rich in third harmonic and its fundamental would lag the voltage by less than 90°

15. The zero power factor characteristic for the Potier’s diagram can be obtained by loading the alternator using
a. lamp load
b. synchronous motor
c. water load
d. dc motor

16. If all other requirements for parallel operation of transformer are fulfilled, which one of following pairs of three phase transformers, with given VECTOR GROUPS, can be operated in parallel?
a. Y d 1 and Yy 0
b. Y d 1 and Dy 11
c. Dd 6 and Dy 1
d. Dd 0 and Dy 11

17. When short-circuit test on a transformer is performed at 25 V, 50 Hz, the drawn current ‘I1’ is at a lagging p.f. angle of Φ1. If the test is performed at 25 V, 25 Hz and the drawn current “I2’ is at a lagging p.f. angle of Φ2, then
a. I2 > I1 and Φ2 < Φ1
b. I2 > I1 and Φ2< Φ1
c. I2 < I1 and Φ2 > Φ1
d. I2 < I1 and Φ2 > Φ1


18. A 10 kVA, 400 V/ 200 V single-phase transformer with a resistance of 3% and a reactance of 6% is supplying a current of 50 A to a resistive load. The voltage across the load is
a. 194 V
b. 196 V
c. 198 V
d. 390 V

19. A 3-phase alternator is connected to a Dd transformer. The hysteresis and eddy current losses of the transformer are respectively 300 W and 400 W. If the speed of the alternator is reduced by 10%, then the hysteresis and eddy current losses of the transformer will be respectively
a. 228 W and 262.44 W
b. 243 W and 324 W
c. 243 W and 360 W
d. 270 W and 400 W

20. Match List I with List II and select the correct answer:
List I
A. Shell type with wound core
B. Core type with core of laminated sheets
C. Shell type with laminated core
D. Core type with wound core
List II



A B C D
a. 4 3 1 2
b. 4 2 1 3
c. 1 2 4 3
d. 1 3 4 2

21.In an autotransformer of voltage ratio V1 / V2 and V1 > V2, the fraction of power transferred inductively is
a.V1/(V1+V2 )

b.V2/V1

c.(V1-V2)/(V1+V2 )

d.(V1-V2)/V1



22. The windings of a Q kVA, V1 / V2 volts, three phase Dd core type transformer are reconnected to work as a single-phase transformer, the power rating and the maximum voltage ratio of the new configuration will be respectively
a. Q/3 kVA and V1/ V2 volts
b. 3Q kVA and 3 V1 / 3 V2 volts
c. 2Q kVA and 3 V1 / 3 V2 volts
d. 2/3Q kVA and 2 V1 / 2 V2 volts


23. Which one of the following relays has the capability of anticipating the possible major fault in a transformer?
a. Over current relay
b. Differential relay
c. Buchholz relay
d. Over fluxing relay

24. A single-phase transformer when supplied from 220 V, 50 Hz has eddy current loss of 50 W. If the transformer is connected to a voltage of 330V, 50 Hz, the eddy current loss will be
a. 168.75 W
b. 112.5 W
c. 75W
d. 50W

25. In case of auto-transformers, which of the following statements are correct?
1. An auto-transformer requires less copper as compared to a conventional, 2-winding transformer of the same capacity.
2. An auto-transformer provides isolation between the primary and secondary windings.
3. An auto-transformer has less leakage reactance as compared to the conventional, 2-winding transformer of the same capacity.
Select the correct answer using .the codes given below:
a. 1, 2 and 3
b. 1 and 2
c. 1 and 3
d. 2 and 3

26. Match List I with List II and select the correct answer
List I (Transformer)
A. Power transformer
B. Auto transformer
C. Welding transformer
D. Isolation transformer
List II (Voltage ratio)
1. 230 V / 230 V
2. 220 V / 240V
3. 400 V / 100V
4. 132 k V / 11 kV
Codes;
A B C D
a. 4 2 3 1
b. 4 2 1 3
c. 2 4 1 3
d. 2 4 3 1

27. A two-winding transformer is used as an auto-transformer. The kVA rating of the auto-transformer compared to the twowinding transformer will be
a. 3 times
b. 2 times
c. 1.5 times
d. same

28. A 20 kVA, 2000/200 V, 1-phase transformer has nameplate leakage impedance of 8% Voltage required to be applied on the high - voltage side to circulate full - load current with the low voltage winding short-circuited will be
a. 16 V
b. 56.56 V
c. 160 V
d. 568.68 V

29. The full-load copper-Loss and iron-loss of a transformer are 6400 W and 5000 W respectively. The copper-loss and iron-loss at half load will be, respectively
a. 3200 W and 2500 W
b. 3200 W and 5200 W
c. 16O0 W and 1250 W
d. 1600 W and 5000 W

30. In a 100 kVA, 1100/220 V, 50 Hz single -phase trans former with 2000 turns on the high -voltage side, the open- circuit test result gives 220 V, 91 A, 5 kW on low voltage side. The core-loss component of current is, approximately
a. 9.1 A
b. 22.7 A
c. 45.0 A
d. 91 A

31. 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

32. The function of oil in a transformer is to provide
a. Insulation and cooling
b. Protection against lightning
c. Protection against short circuit
d. Lubrication

33. A coil of 1000 turns is wound on a core. A current of 1 A flowing through the coil creates a core flux of 1 mWb. The energy stored in the magnetic field is
a. 0.25 J
b. 0.5 J
c. 1 J
d. 2 J

34. Consider the following statements relating to the constructional features of a large power transformer:
1. The conservator is used to maintain the level of oil in the transformer tank 2. The bushing is used to protect transformer insulation against lightning over-voltages
3. The Buchholz relay is an over current relay
4. Silica gel is used to absorb moisture.
Which of these statements are correct?
a. 1, 2, 3 and 4
b. 2 and 3
c. 1 and 4
d. 1, 2 and 4

35. Two transformers operating in parallel will share the load depending upon their
a. Ratings
b. Leakage reactance
c. Efficiency
d. Per unit impedance

36. A 1 kVa, 200/100 V, 50 Hz, single-phase transformer gave the following test results on 50 Hz :
OC (LV side) : 100 V, 20 watts
SC (HV side) : 5 A, 25 watts
It is assumed that no-load loss components are equally divided. The above tests were then conducted on the same transformer at 40 Hz.
Tests results were:
OC (HV):160 V, W1 watts.
SC (LV): 10 A, W2 watts neglecting skin effect, W1 and W2 will be
a. W1 = 16 watts, W2 = 25 watts
b. W1 = 25 watts, W2 = 31.25 watts
c. W1 = 20 watts, W2 = 20 watts
d. W1 = 14.4 watts, W2 = 25 watts

37.The diagram given below shows the connection of a four–wire delta bank for obtaining a 3–phase 4–wire distribution system. The secondary voltages between the terminals are as indicated, a. What is the voltage between the terminals b and d in the above system when the primary side is energized from an appropriate symmetrical 3–phase system?



38. If per unit impedances of two transformers connected in parallel are not equal, then which one of the following statements is correct?
a. The power factor of the two transformers will be different from that of the connected load
b. Transformers will get overloaded
c. Dead short circuit occurs
d. The transformer with higher per unit impedance will share more load

39. A coil of 1000 turns is wound on a core. A current of 1 A flowing through the coil creates a core flux of 1 mWb. What is the energy stored in the magnetic field?
a. 1/4J
b. 1/2J
c. 1 J
d. 2 J

40. Match List I (Type of Coil) with List II (Use of Coil) and select the correct answer using the code given below the lists:
List I
A. Sandwitch coils
B. Disc coils
C. Cross-over coils
D. Spiral type
List II
1. Low voltage coils for currents above 100 A
2. High voltage windings of small transformers
3. Cooling oil is in contact with each turn of the winding
4. Shell-type transformer core
Codes;
A B C D
a. 2 3 4 1
b. 4 1 2 3
c. 2 1 4 3
d. 4 3 2 1

41. Match List I (Test) with List II (Quantities) and select the correct answer using the code given below the lists:
List I
A. O C Test
B. S C Test
C. Sumpner’s Test
D. Load Test
List II
1. Copper loss and iron loss
2. Total losses
3. Iron loss
4. Copper loss
Codes;
A B C D
a. 3 4 1 2
b. 2 1 4 3
c. 3 1 4 2
d. 2 4 1 3
42. The exact equivalent circuit of a two winding transformer is given in the figure given above. For affecting simplification, the parallel magnetizing branch, consisting of RC and XQ is shifted to’ the left of the primary leakage impedance (r1 + jx1). This simplification introduces the inaccuracy, in the neglect of:



a. Voltage-drop in ‘the primary impedance due to the secondary current
b. Voltage-drop in the primary impedance due to the exciting current
c. Voltage-drop in the secondary impedance due to the exciting current
d. Reduction in values of RC and XQ of the exciting circuit.

43. A single-phase, 10 kVA, 2000/200 V, 50 Hz transformer is connected to form an auto transformer as shown in the figure given above. What are the values of V1and I2 respectively?



a. 2200 V, 55 A
b. 2200 V, 45 A
c. 2000 V, 45 A
d. 1800 V, 45 A

44. Three single phase 11000/220 V transformers are connected to form 3-phase transformer bank. High voltage side is connected in star, and low voltage side is in delta. What are the voltage ratings and turn ratio of 3-phase transformer?
a. 19052/220V, 50
b. 19052/220V, 50 √3
c. 11000/381 V, 50 √3
d. 11000/220 V, 50



45. If the wave form of the voltage impressed on the primary of a star- delta bank contains 5th harmonics, what are the wave forms of the resultant voltages of the primary and the secondary?
Primary Secondary
a. Peaked Peaked
b. Peaked Flat-topped
c. Flat-topped Peaked
d. Flat-topped Flat-topped

46. Two single phase transformers A and B are connected in parallel, observing all requirements of a parallel operation, except that the induced voltage Ea is slightly greater than Eb; Zea and Zeb being the equivalent impedances of A and B, both referred to the secondary side.



Under this operating condition with the primary bus-bars being energized, a circulating current will flow:
a. Only in the secondary windings of A and B
b. In both the primary and the secondary windings of A and B
c. In both the primary and the secondary windings of A and B, as well as in the primary side network
d. In the primary and the secondary windings of A and B, and boost the voltages on the secondary side of both A and B


47. Which three-phase connection can be used in a transformer to introduce a phase difference of 300 between its output and corresponding input line voltages?
a. Star-delta
b. Star-star
c. Delta-delta
d. Delta-zigzag

48. What is the efficiency of an autotransformer in comparison to that of a two winding transformer of the same rating?
a. Slightly less than that of a two-winding transformer
b. Same as that of a two-winding transformer
c. More than that of a two-winding transformer
d. As low as 1/5th of the efficiency of a two-winding transformer

49. A two-winding transformer is converted into an auto-transformer. If we apply additive polarity and subtractive polarity for the connections, then the secondary voltage is 2640 V and 2160 V, respectively. What is the ratio of primary to secondary voltage on the original transformer?
a. 66 : 54
b. 54 : 66
c. 10 : 1
d. 1 : 10

50. Percentage resistance and percentage reactance of a transformer are 1% and 4%, respectively. What is voltage regulation at power factor 0.8 lagging and 0.8 leading?
a. 2.4% and –0.8%, respectively
b. 3.2% and —1.6%, respectively
c. 3.2 % and —3.2%, respectively
d. 4.8% and 1.6%, respectively

51. If P1 and P2 are the iron and copper losses of a transformer at full load, and the maximum efficiency of the transformer 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

52. 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

53. The equivalent circuit of a transformer has the leakage reactance X1, X '2 and the magnetizing reactance Xm. What is the relationship between their magnitudes?
a. X1 >> X'2 >> Xm
b. X1 << X'2 << Xm
c. X1≈ X2 >> Xm
d. X1≈ X'2 << Xm


54. If the voltage applied to a transformer primary is increased by keeping the V/f ration fixed, then the magnetizing current and the core loss will, respectively,
a. decrease and remain the same
b. increase and decrease
c. remains the same and remains the same
d. remains the same and increase

55. Sludge formation in transformer oil is due to which one of the following?
a. Ingress of dust particles and moisture in the oil.
b. Appearance of small fragments of paper, varnish, cotton and other organic materials in the oil
c. Chemical reaction of transformer oil with the insulating materials
d. Oxidation of transformer oil

56. A single-phase transformer rated for 220/440 V, 50 Hz. This frequency operation at rated voltage results in which one of the following?
a. Increases 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

57. What is the load at which maximum efficiency occurs in case of a 100 kVA transformer with iron loss of 1 kW and full–load copper loss of 2 kW?
a. 100 kVA
b. 70.7 kVA
c. 50.5 kVA
d. 25.2 kVA

58. 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

59. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 leading, respectively?
a. 4% and - 0.8%
b. 3.2% and – 1.6%
c. 1.6% and – 3.2 %
d. 4.8% and – 0.6%