1. If a saw-cut is made in the iron core of a single-phase transformer as shown in the given figure, with secondary terminals open it will result in
(a) no voltage across secondary terminals due to discontinuity in magnetic circuit.
(b) excessive large current drawn by primary.
(c) no appreciable change in the operation of the transformer.
(d) no transformer action and the primary current will be greatly reduced.
Answer: (b) excessive large current drawn by primary.
2. A star-connected generator supplies power to a star/star connected transformer as shown in the given figure.
The generator voltages are perfectly balanced. As a result,
(a) third harmonic exciting currents will flow in the primaries of star.
(b) third harmonic exciting currents cannot flow in the primaries of star.
(c) the fluxes in the three transformer cores are sinusoidal in time.
(d) the voltage between ‘0’ and ‘n’ is zero.
Answer: (b) third harmonic exciting currents cannot flow in the primaries of star.
3. If the secondary winding of the ideal transformer of the circuit shown in the given figure has 40 turns, then for maximum power transfer to the 2-ohm resistor, the number of turns required in the primary winding will be
(a) 20
(b) 40
(c) 80
(d) 120
Answer: (c) 80
4. A two-winding transformer (200 V/120 V) has a load of 2.4 kVA as shown in Figure-1. The windings of transformers are reconnected as an auto transformer shown in Figure- II.
Assuming the same current rating for both the windings as before and neglecting the losses, the KVA rating of the auto transformer will be
(a) 6.4 kVA
(b) 4.0 kVA
(c) 3.84 kVA
(d) 2.4 kVA
Answer: (a) 6.4 kVA
5. Which one of the following figures represents the correct direction of flow of currents in the different sections of the windings of an auto transformer ?
Answer: (b).
6. The above diagrams show a two-winding transformer (Fig. 1) and an auto transformer (Fig. 2) formed by re-connection of the terminals of the former. The terminal markings are the conventional-ones with regard to the respective polarities. When operated as an auto-transformer with an input voltage of 500V the output load current would be
(a) 50 A
(c) 30 A
(b) 40 A
(d) 10 A
Answer: (a) 50 A
7. The given figure represents a transformer with two windings 1 and 2 wound on the core as shown.
By applying a voltage V1, across winding
1. A voltage V2 induced across winding
2. In an idealized condition V2 would lag V1 by
(a) Zero degrees
(b) 90 degrees
(c) 180 degrees
(d) 270 degrees
Answer: (c) 180 degrees
8. The series equivalent impedance of a single phase transformer referred to its secondary side is Ze2 = Re2 + jXe2 The voltage regulation of this transformer will be zero when the power factor of the load is
Answer: (b)
9. Identify the symbol of isolation transformer among the following.
Answer: (a)
10. Two single-phase transformers A and B with equal turn ratio have reactances of j3 Ω and j9 Ω referred to secondary. When operated in parallel, the load-sharing of 200 kW at 0.8 pf lag between A and B transformers would respectively be:
(a) 75 kW & 25 kW
(b) 100 kW & 100 kW
(c) 150 kW & 50 kW
(d) 120 kW & 80 kW [UPPCL AE 2016]
Answer: (c) 150 kW & 50 kW
11. In a transformer, if the iron losses and copper losses are 32.8 kW and 50 kW respectively, then at what fraction of load will the efficiency be maximum?
(a) 0.81
(b) 0.57
(c) 0.7
(d) 0.9
Answer: (a) 0.81
12. A 1 φ, 10 kVA, 2400/240V, 60 Hz distribution transformer has the following characteristics: Core loss at full voltage is 100W and copper loss at half is 60W. What is the efficiency of the transformer when it delivers full load at 0.8 p.f. lagging?
(a) 93.54%
(b) 94.52%
(c) 95.92%
(d) 97.53%
Answer: (c) 95.92%
13. A transformer rated 250 kVA, 4160 V/480 V, 60 Hz has an impedance of 5.1%. Calculate the total impedance of the transformer referred to the primary. [Jammu & Kashmir JE – 2016]
(a) 69 ohm
(b) 0.92 ohm
(c) 3.52 ohm
(d) 0.51 ohm
Answer: (c) 3.52 ohm
14. A current transformer has double turn primary and a 100 turn secondary winding. The secondary draws current of 6A and magnetising ampere-turns is 60A. What is actual transformation ratio?
(a) 100.5
(c) 50.24
(b) 101
(d) 25.12
Answer: (c) 50.24
15. A 10 kVA, 400 V/200 V single phase transformer with 10 percent impedance draws a steady short circuit line current of
(a) 50 A
(b) 150 A
(c) 250 A
(d) 350 A
Answer: (c) 250 A
16. The iron losses in a 100 kVA transformer are 1 kW and the full load copper losses are 2 kW, then maximum efficiency occurs at a load of:
(a) 100 √2 kVA
(b) 100/√2 kVA
(c) 50 kVA
(d) None of these [FCI 2015]
Answer: (b) 100/√2 kVA
17. If the transformer frequency is changed from 50 Hz to 60 Hz, the ratio of eddy current loss at 60 Hz to 50 Hz at constant voltage will be
(a) 5/6
(b) 6/5
(c) 25/36
(d) 36/25 [Uttarakhand AE – 2013]
Answer: (d) 36/25
18. The no-load current of a transformer is 20 A at a power factor of 0.20 lagging, when connected to 400 V, 50 Hz supply. Calculate the magnetizing component of no-load current.
(a) 13.4 A
(b) 16.8 A
(c) 19.6 A
(d) 18.2 A
Answer: (c) 19.6 A
19. A-60 kVA Transformer at full load the copper losses 1000 W while the core losses 500 W. What is the proportion of full load at maximum efficiency? [UPPCL AE – 2016]
(a) 70.71%
(b) 63.40%
(c) 50%
(d) 41.50%
Answer: (a) 70.71%
