21. A single-phase two winding transformer is designed to operate at 400/200V, 50 Hz. If the h.v. side is now energized from a 400 V, 40 Hz source, the no-load l.v. side voltage would be
- 250 V
- 300 V
- 150 V
22. A 200/100V, 50 Hz transformer is to be excited at 40 Hz from the 100 V side. For the exciting current to remain same, the applied voltage should be
- 125 V
- 150 V
- 80 V
- 100 V
23. Two single-phase 100 kVA transformers each having different leakage impedance are connected in parallel. When a load of 150 kVA at 0.8 power factor lagging is applied
- both transformers will operate at power factor more than 0.8 lagging.
- both transformers will operate at power factor less than 0.8 lagging.
- one of the transformers will operate at power factor more than 0.8 lagging
- both transformers will operate at identical power factors.
24. The most essential condition for parallel operation of two 1-phase transformers is that they should have the same
- kVA rating
- percentage impedance
- voltage ratio
25. For a two-winding power transformer, with the effects of the no-load current being neglected, the ‘Voltage Regulation’ can be zero at load, when the load power factor is
- lagging only
- leading only
- either lagging or leading, depending upon power rating of the transformer
26. A 50 kVA transformer has a core loss of 500 W and full-load core loss of 900 W. The load at which the efficiency is maximum is
- 37.75 kVA
- 27.45 kVA
- 47.5 kVA
- 45.5 kVA
27. Short circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short circuit test is now performed with the same magnitude of impressed voltage, but at a frequency higher than the rated frequency, then the magnitude of current
- and power-factor will both increase
- will decrease but the power factor will increase
- will increase, but power factor will decrease
- and power factor will both decrease
28. Tapping are normally provided on the high voltage winding of a transformer only, because
- it has larger number of turns which allows smoother variation of voltage.
- it has to handle low currents.
- it is easily accessible physically
- it has larger number of turns, has to handle low currents and is also easily accessible physically.
29. For power transformers of larger ratings, the tappings are located in the middle portion of the winding to
- increase the break-down strength of the winding insulation
- enable better cooling of the windings
- enable better distribution of inter-turn voltage
- reduce the mechanical forces affecting the windings during short-circuits
30. When a transformer is first energized, the transient current during first few cycles, is
- less than full load current
- equal to full load current
- equal to no load current
- much higher than full load current
31. A 100 kVA, 2400 V/240 V, Hz single phase transformer has an exciting current of 0.64 A and core loss 700 W when its high voltage side is energized at rated voltage and frequency. If load current is 40 A at 0.8 p.f. lagging on the LV side then magnitude of the primary current will be
- 4.58 A
- 4 A
- 4.85 A
- 4.64 A
32. The open-circuit test in a transformer can be used to obtain.
- Core losses
- Magnitude of exciting current
- Copper losses
- Equivalent series impedance
Correct statements are
- i, ii, iii, and iv
- i and iii only
- ii and iv only
- i and ii only
33. A 10 kVA, 2200/220 V transformer gave the following test results:
Open circuit test, High voltage side open
V= 220 V, I = 1.5 A, W= 150 W.
Short-circuit test. Low voltage side short-circuited
V= 115 V, I = rated, W = 200 W.
The half full-load efficiency of the transformer operating at unity power factor is
34. A 200 V/100 V, 50 Hz transformer is to be excited at 40 Hz from 100 V side. For the exciting current to be the same, the applied voltage should be
- 150 V
- 100 V
- 125 V
35. Conditions for parallel operation of transformer are:
- The same voltage ratio
- The same per unit impedance
- The same polarity
- The same phase sequence
- The relative phase displacement
Which are the absolutely essential conditions?
- i, ii, iii, iv and v
- ii, iii and iv
- iii, iv and v
- i, ii and iii
36. In a transformer, zero voltage regulation at full load is
- not possible.
- possible at leading power factor load.
- possible at lagging power factor load.
- possible at unity power factor load.
37. The full-load copper loss and iron loss of a transformer are 6400 W and 500 W, respectively. The above copper loss and iron loss at half load will be
- 33200 W and 250 W respectively
- 3200 W and 500 W respectively
- 1600 W and 125 W respectively
- 1600 W and 500 W respectively
38. A 4 kVA, 400 V/200 V single phase transtormer has resistance of 0.02 pu and reactance of 0.06 pu. The resistance and reactance referred to high voltage side are:
- 0.2 Ω and 0.6 Ω
- 0.8 Ω and 2.4 Ω
- 0.08 Ω and 0.24 Ω
- 1 Ω and 3 Ω
39. A 10 kVA, 2500/250V single phase transformer has the following results:
|O.C. Test||250 Volts||0.8 amp||50 W|
|S.C. Test||60 Volts||3 amp||45 W|
Then efficiency at half full load at 0.8 power factor will be
40. The applied voltage of a certain transformer is increased by 75%, while the frequency of the applied voltage is reduced by 25%. The maximum core flux density will
- Increase by seven times
- Increase by three times
- Reduce to one quarter
- Remain the same