# Induction Motor MCQs Part 7

Induction Motor MCQs Part 7

## The speed of the rotating flux of a 3-phase, 4 pole induction motor having 50 Hz supply will be

The speed of the rotating flux of a 3-phase, 4 pole induction motor having 50 Hz supply will be:

1. 1500 RPM
2. 1000 RPM
3. 750 RPM
4. 3000 RPM

Solution: Rotating flux Speed = 120*f/P = 120*50/4 = 1500 RPM

## 3-phase wound rotor motors are sometimes also referred as

3-phase wound rotor motors are sometimes also referred as:

1. Synchronous
2. Series
3. Commutator
4. Slip ring

## A wound rotor motor is used in practical applications that require

A wound rotor motor is used in practical applications that require:

1. Less costly motor is not required
2. High rotor resistance is required during
3. High starting torque is required
4. Speed control is required

Correct answer: 3. High starting torque is required

## The maximum voltage is induced in the rotor of a 3-phase induction motor for which of following condition

The maximum voltage is induced in the rotor of a 3-phase induction motor for which of following condition:

1. When it is blocked
2. When it runs at no load
3. When it runs at full load
4. None of the above

Correct answer: 1. When it is blocked

## Under running conditions, the maximum torque of a 3-phase induction motor is

Under running conditions, the maximum torque of a 3-phase induction motor is:

1. Inversely proportional to supply voltage
2. Directly proportional to rotor resistance
3. Inversely proportional to rotor reactance at standstill
4. None of the above

Correct answer: 3. Inversely proportional to rotor reactance at standstill

## The approximate efficiency of a 3-phase induction motor running at slip s (in decimal) is given by

The approximate efficiency of a 3-phase induction motor running at slip s (in decimal) is given by:

1. 1-s
2. 1+s
3. s-1
4. s^2

## Design Engineers manufacture 3 phase induction motor is such a way that under running conditions the rotor should have

Design Engineers manufacture 3 phase induction motor is such a way that under running conditions the rotor should have:

1. High resistance
2. High reactance
3. Low resistance
4. Large slip

## In a wound rotor motor, the rotor winding is wound for poles that are ________ that of stator winding

In a wound rotor motor, the rotor winding is wound for poles that are ________ that of stator winding:

1. Greater than
2. Small than
3. Equal to
4. None of these

## If a 3-phase, 4 pole, 50 Hz induction motor is at standstill position, what is value of rotor frequency

If a 3-phase, 4 pole, 50 Hz induction motor is at standstill position, what is value of rotor frequency:

1. Zero
2. 25 Hz
3. 50 Hz
4. Anywhere between 10 and 50 Hz

Solution: Speed of magnetic field = 120*f/P = 120*50/4 = 1500 RPM

Slip Speed = Speed of magnetic field – Mechanical shaft speed = 1500 – 0 = 1500

Slip = Slip speed/Speed of magnetic fields *100% = 1500/1500*100 = 100% = 1

Rotor frequency (f’) = sf = 1 * 50 Hz = 50 Hz

## When an induction motor is running at full load, its rotor reactance is __________ rotor resistance

When an induction motor is running at full load, its rotor reactance is __________ rotor resistance:

1. Comparable to
2. Very large as compared to
3. Large compared to
4. Very small