What is the synchronous speed of a 6-pole, 3-phase induction motor operating at 50 Hz frequency?

The correct answer is B: 1000 RPM. Synchronous speed Ns = 120f/P = (120 × 50)/6 = 1000 RPM, where f = frequency and P = number of poles.

For an industrial induction motor variable frequency drive (VFD) using vector control, what is the typical output frequency range in 2024-25 applications?

The correct answer is C: 0.1-500Hz. Modern VFDs support 0.1Hz to 500Hz+ output, enabling ultra-low speed precision applications and extended-speed motor operation.

A transformer is operating at 110% of rated voltage. The core loss will be approximately:

The correct answer is C: 21% more than rated. Core loss Pc ∝ B² ∝ V². At 1.1V, loss = (1.1)² × rated loss = 1.21 × rated ≈ 21% increase

The power factor of an AC circuit is defined as:

The correct answer is A: cos(φ) where φ is the phase angle between voltage and current. Power factor is the cosine of the phase angle between voltage and current, representing the fraction of apparent power that is real power.

The power factor of a transformer at no-load is low because:

The correct answer is B: Magnetizing current lags the applied voltage. At no-load, the transformer draws mainly magnetizing current which lags the applied voltage by approximately 90°, resulting in low power factor (typically 0.1-0.2).

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Electrical Engg (EEE)
Power Electronics

Electrical machines, power systems, circuits

25 Q 7 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–25 of 25

Topics in Electrical Engg (EEE)

All Circuit Analysis 100 Electrical Machines 100 Power Systems 100 Control Systems 100 Power Electronics 100 Electrical Measurements 40 Digital Electronics 100

Q.21 Easy Power Electronics

In MOSFET, the gate-source voltage at which current starts to flow is called:

A Pinch-off voltage

B Threshold voltage

C Breakdown voltage

D Saturation voltage

Correct Answer: B. Threshold voltage

EXPLANATION

Threshold voltage (Vth) is the gate-source voltage required to create an inversion layer and allow current flow in MOSFET.

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Q.22 Easy Power Electronics

The peak inverse voltage (PIV) rating of a diode is important to prevent:

A Forward conduction

B Reverse breakdown

C Forward voltage drop

D Thermal runaway

Correct Answer: B. Reverse breakdown

EXPLANATION

PIV rating defines the maximum reverse voltage a diode can withstand without breaking down. Exceeding this causes reverse breakdown and device failure.

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Q.23 Easy Power Electronics

Which device is used to convert DC to AC power?

A Rectifier

B Inverter

C Chopper

D Converter

Correct Answer: B. Inverter

EXPLANATION

An inverter converts DC power to AC power. Rectifiers do the opposite (AC to DC). Choppers convert DC to DC.

Test

Q.24 Easy Power Electronics

The forward voltage drop across a silicon diode at room temperature is approximately:

A 0.2 V

B 0.7 V

C 1.5 V

D 2.0 V

Correct Answer: B. 0.7 V

EXPLANATION

Silicon diodes have a typical forward voltage drop of 0.6-0.7 V at room temperature. Germanium diodes have ~0.3 V.

Test

Q.25 Easy Power Electronics

Which of the following is a unidirectional device in power electronics?

A Diode

B Thyristor

C MOSFET

D Both A and B

Correct Answer: D. Both A and B

EXPLANATION

Both diodes and thyristors are unidirectional devices that conduct current in only one direction. MOSFETs are bidirectional.

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Power Electronics