# Electrical Machines Questions and Answers – EMF Induced in Transformer

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “EMF Induced in Transformer”.

1. If the transformer is excited with square wave voltage source then the shape of induced EMF will be __
a) Sinusoidal Wave
b) Square Wave
c) Triangular Wave
d) Impulse Wave

Explanation: Let Vs = Supply voltage , Iµ = Magnetizing Current , then
Vs = Ld(Iµ)/dt
Slope of Iµ = 1/L[∫Vs dt] Shape of Induced emf, e = -L dIµ/dt
Given, Vs = Square Voltage Source
Shape of Iµ = 1/L ∫(square wave)
= Triangular wave
Shape of induced emf = – L dIµ/dt
= -L d(Triangular wave)/dt
= Square wave.

2. If a transformer is excited with triangular wave voltage source then shape of induced emf will be _________
a) Triangular wave with 90o displacement
b) Triangular wave with 180o displacement
c) Pulsed wave with 90o displacement
d) Pulsed wave with 180o displacement

Explanation: Given, Vs = Triangular wave,
Iµ = 1/L [∫Triangular] = Parabolic
Induced emf = -Ld(Parabolic)/dt
= Triangular Wave
Displacement is 180◦ due to fact that induced emf opposes the voltage source according to electromagnetic principle given by faraday.

3. EMF induced in secondary winding of transformer is __________
a) Statically induced EMF
b) Self induced EMF
c) Mutual induced EMF
d) Dynamically induced EMF

Explanation: An EMF induced in a coil due to current passing through another coil which is magnetically coupled to a same flux is called mutual induced EMF.

4. EMF induced in primary winding of transformer is _________
a) Statically induced EMF
b) Self-induced EMF
c) Mutual induced EMF
d) Dynamically induced EMF

Explanation: An EMF induced in a coil due to current passes through its own coil is known as self-induced EMF which is independent of other current flowing coils.

5. A single phase 50 Hz transformer has number of turns of 100 and 200 in primary and secondary respectively. Variation of flux with respect to time is shown in figure below. Find EMF induced in primary and secondary winding respectively in the interval 0 to 1?

a) -12V, -24V
b) 0V, 0V
c) -0.12V, 24V
d) 5V, 10V

Explanation: EMF= -N(dØ/dt)
dØ/dt = slope in 0 to 1
= 0.12
EMF primary = -100*0.12 = -12V
EMF secondary = -200*0.12 = -24V.
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6. A single phase 50 Hz transformer has number of turns of 100 and 200 in primary and secondary respectively. Variation of flux with respect to time is shown in figure below. Find EMF induced in primary and secondary winding respectively in the interval 1 to 2?

a) -12V,-24V
b) 0V, 0V
c) -0.12V, 24V
d) 5V, 10V

Explanation: EMF= -N(dØ/dt)
dØ/dt = slope in 1 to 2
= 0
EMF primary = -100*0 = 0V
EMF secondary = -200*0= 0V

7. A single phase 50 Hz transformer has number of turns of 100 and 200 in primary and secondary respectively. Variation of flux with respect to time is shown in figure below. Find EMF induced in primary and secondary winding respectively in the interval 2 to 3?

a) -12V,-24V
b) 0V, 0V
c) 12V, 24V
d) 5V, 10V

Explanation: EMF= -N(dØ/dt)
dØ/dt = slope in 2 to 3
= -0.12
EMF primary = -100*-0.12 = 12V
EMF secondary = -200*-0.12 = 24V

8. EMF induced in transformer is _________
a) Statically induced EMF
b) Self-induced EMF
c) Mutually induced EMF
d) Dynamically induced EMF

Explanation: An EMD induced in a coil due to relative time variation between magnetic field and set of conductors is called statically induced EMF.

9. In a circuit two winding loss less transformer with no leakage flux, excited from a current source i(t) whose waveform is shown in given figure. The transformer has a magnetizing inductance of 400/∏ mH. Turns ration is 1:1.

The peak voltage in the above figure across secondary side with switch S2 open is __________
a) 400/∏ V
b) 800V
c) 4000/∏ V
d) 800/∏ V

Explanation: Voltage across secondary side = E2
E2 = – M d(Iµ)/dt
Where M=400/∏ mH
d(Iµ)/dt = slope of line = 10-0/5*10-3
E2 = -400/∏ * 10-3*10/5*10-3
= 800/∏ V

10.In a circuit two winding loss less transformer with no leakage flux, excited from a current source i(t) whose waveform is shown in given figure. The transformer has a magnetizing inductance of 400/∏ mH. Turns ration is 1:1. The waveform I(t) = 10 sin (100∏t) A. The peak voltage across A and B with switch S2 closed is _______________

a) 240V
b) 400 V
c) 320 V
d) 160 V

Explanation: E2 = -M d(Iµ)/dt
= -M d(10 sin(100∏t))/dt
= -400V
Voltage drop = 400 V.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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