41. For the parallel operation of transformers, which of the following conditions must be satisfied?
- Same voltage ratios
- Must be connected in proper polarities
- Re/Xe Ratio should be same
- Same kVA rating
42. What is the core loss in a high frequency ferrite core transformer used in SMPS power supply?
- 10% of rated power
- 5% of rated power
- 2% of rated power
- 1% of rated power
43. Consider the following tests:
- Load test
- Short circuit test
- OC test
- Retardation test
Which of the above tests are to be conducted for the determination of voltage regulation of a transformer?
- ii only
- i only
- iii and iv
- ii and iii
44. When are eddy-current losses in a transformer reduced?
- If laminations are thick.
- If the number of turns in primary winding is reduced.
- If the number of turns in secondary winding is reduced.
- If laminations are thin.
45. The full-load copper loss and iron loss of a transformer are 6400 W and 5000 W, respectively. What are the above copper loss and iron loss, respectively at half-load?
- 3200 W, 2500 W
- 3200 W, 5000 W
- 1600 W, 1250 W
- 1600 W, 5000 W
46. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 lagging and 0.8 leading, respectively?
- 3.2% and -1.6%
- 4% and -0.8%
- 1.6% and 3.2%
- 4.8% and 0.6%
47. Cores of large power transformers are made from which one of the following?
- Hot-rolled steel
- Cold-rolled non-grain oriented steel
- Cold-rolled grain-oriented steel
48. 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?
- 100 kVA
- 70.7 kVA
- 50.5 kVA
- 25.2 kVA
49. A single-phase transformer rated for 220/440 V, 50 Hz operates at no load at 220 V, 40 Hz. This frequency operation at rated voltage results in which one of the following?
- Increase of both eddy-current and hysteresis losses
- Reduction of both eddy-current and hysteresis losses
- Reduction of hysteresis loss and increase in eddy-current loss
- Increase of hysteresis loss and no change in the eddy-current loss
50. Sludge formation in transformer oil is due to which one of the following?
- Ingress of dust particles and moisture in the oil.
- Appearance of small fragments of paper, varnish, cotton, and other organic materials in the oil.
- Chemical reaction of transformer oil with the insulating materials.
- Oxidation of transformer oil.
51. Two transformers when operating in parallel will share the load depending upon which of the following?
- Magnetizing current
- Leakage reactance
- Per unit impedance
52. Consider the following statements regarding transformers:
- The function of the magnetizing component of no load current is to sustain the alternating flux in the core.
- Short circuit test is performed to find core losses only.
- The function of the breather in transformer is to arrest flow of moisture when outside air enters the transformer.
Which of these statements are correct?
- i and ii
- i and iii
- i, ii and iii
- ii and iii
53. On which of the following factors does hysteresis loss depend?
- Flux density
- Thickness of lamination
Select the correct answer using the code given below:
- ii and iii
- i and ii
- iii and iv
- i and iv
54. A 500 kVA transformer has constant losses of 500 W and copper losses at full load are 2000 W. Then at what load, is the efficiency maximum?
- 500 kVA
- 250 kVA
- 125 kVA
- 1000 kVA
55. In a large power transformer, a conservator drum is provided above the completely oil filled transformer tank and connected to it by a short pipe. The conservator drum is linked to external air through a breather. What is the purpose of providing the conservator?
- To store reserve oil to make up oil losses due to leakage
- To prevent explosion due to rise in oil pressure inside the tank during a fault.
- To accommodate change in oil-level during the “load-cycle” of the transformer load.
- To exert additional pressure by the conservator-oil on the oil inside the main tank to prevent disintegration of oil at high temperature.
56. If the voltage applied to a transformer primary is increased by keeping the V/f ratio fixed, then the magnetizing current and the core loss will, respectively,
- decreases and remain the same
- increases and decreases
- remain the same and remain the same
- remain the same and increases
57. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will
- remain unchanged
- become zero
58. If P1 and P2 be 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?
59. 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?
- 2.4% and 0.8%, respectively
- 3.2% and -1.6%, respectively
- 3.2% and -3.2%, respectively
- 4.8% and 1.6%, respectively
60. Two single-phase transformers A and B with equal turn’s ratio have reactances of j3 Ω and j9 Ω referred to secondary. When operated in parallel, the load-sharing of 100 kW at 0.8 pf lag between A and B transformers would respectively be
- 75 kW and 25 kW
- 60 kW and 40 kW
- 20 kW and 80 kW
- 25 kW and 75 kW