Transformers Objective Questions: Part-3

Transformers Solved Objective Questions from Competitive Exams(Gate, IES)
Each Question Carries Two Marks:

Common Data for Questions 1 and 2: [GATE 2009]

The star-delta transformer shown above is excited on the star side with a balanced,4-wire,3-phase,sinusoidal voltage supply of rated magnitude. The transformer is under no load condition.

[1] With both S1 and S2 open, the core flux waveform will be [GATE 2009]
A. A sinusoidal at fundamental frequency
B. Flat-topped with third harmonic
C. Peaky with third-harmonic
D. None of these

Ans:B

[2] With S2 closed and S1 open, the current waveform in the delta winding will be  [GATE 2009]
A. A sinusoidal at fundamental frequency
B. Flat-topped with third harmonic
C. Only third-harmonic
D. None of these

Ans:C

Common Data for Questions 3 and 4:
The circuit diagram shows a two-winding, losses transformer with no leakage flux, excited from a current source,i(t),whose waveform is also shown. The transformer has a magnetizing inductance of 400/ πmH.

[3] The peak voltage across A and B, with S open is [GATE 2009]
A. 400/ πV
B. 800 πV
C. 4000/π V
D. 800/ πV

Ans: D

[4] If the waveform of i(t) is changed to i(t)= 10 sin(100πt)A, the peak voltage across A and B with S closed is [GATE 2009]
A. 400V
B. 240V
C. 320V
D. 160V

Ans: A

Statement for Linked Answer Questions 5 and 6: [GATE 2009]

The figure above shows coils 1 and 2, with dot markings as shown, having 4000 and 6000 turns respectively. Both the coils have a rated current of 25A. Coil 1 is excited with single phase, 400V, 50Hz supply.

[5] The coils are to be connected to obtain a single phase, 400/1000V, auto-transformer to drive a load of 10kVA. Which of the options given should be exercised to realize the required auto-transformer? [GATE 2009]
A. Connect A and D; Common B
B. Connect B and D; Common C
C. Connect A and C; Common B
D. Connect A and C; Common D

Ans: A

[6] In the autotransformer obtained in Question 57, the current in each coil is [GATE 2009]
A. Coil-1 is 25A and Coil-2 is 10A
B. Coil-1 is 10A and Coil-2 is 25A
C. Coil-1 is 10A and Coil-2 is 15A
D. Coil-1 is 15A and Coil-2 is 10A

Ans: A

[7]The single phase, 50Hz iron core transformer in the circuit has both the vertical arms of cross sectional area 20cm² and both the horizontal arms of cross sectional area 10cm². If the two windings shown were wound instead on opposite horizontal arms, the mutual inductance will [GATE 2009]

A. Double

B. Remain same

C. Be halved

D. Become one quarter

Ans:C

Solution Hint:
Vertical arms A = 20cm²
Horizontal arms A = 10cm²
Wound in the horizontal arms means, Area reduces from 20cm² to 10cm².... So Inductance will be halved.

Thanks for reading