# Induction Motor Interview Questions and Answers – Q 1-10

Q.1 What is an Electric Motor?

An electric motor converts electrical energy into mechanical energy. The motor drives the mechanical load.

Q.2 Who invented Induction Motor?

Nikola Tesla invented the induction motor

Q. 3 What is an Induction Motor?

The induction motor has a stator and rotor as two major parts. The stator produces a magnetic field that links to the rotor conductor. The interaction of the flux and the rotor current produces torque in the motor.

Q. 4 Why is the speed of the rotor and the speed of the synchronous speed different in an induction motor?

The synchronous speed is the speed of a rotating magnetic field. The rotor of the motor can not rotate at a synchronous speed because of copper loss in the rotor. If the speed of the rotor is equal to synchronous speed, the motor produces no torque because the emf induced in the rotor is zero. Therefore, the motoring operation is possible if the rotor speed is less than the synchronous speed.

Q.5 What is the synchronous speed of an induction motor?

When we feed a three-phase supply to the stator, a rotating magnetic field is produced. The speed at which the magnetic field rotates in the air gap of the motor is the synchronous speed of the motor. The synchronous speed depends on;

• Number of poles
• frequency of the supply voltage

Ns = 120f/P

Where Ns = Synchronous  Speed(RPM)

f  = Supply frequency(Hz)

P  = Number of Poles

Q.6 What are the various speed control method of squirrel cage induction motor?

The following methods are used for speed control;

•  Stator Voltage Control ( Limited speed control possible)
• By pole changing
• By frequency-changing – Voltage and frequency simultaneously changed to maintain a constant v/f ratio. Thus, the flux in the motor remains constant.

Q.7 What is the working principle of a three-phase induction motor?

Rotating magnetic field produces when we feed a three-phase supply to the stator. The rotating magnetic field links to the stationary rotor. The linked magnetic field in the rotor induces EMF in the rotor by mutual magnetic induction. The rotor has short-circuited conductor. Therefore, the current starts flowing in the rotor conductor. The magnetic flux and the rotor current interacts and torque is produced.

(i) Simple and rugged construction.

(ii) Less costly

(iii) Almost no maintenance

(iv) Higher efficiency and reasonably good power factor

(v) Self-starting feature

(vi) Constant speed motor

(i)  Starting torque is low compared to DC motor

(ii) Low starting torque at starting

Q.9 What is the construction of the induction motor?

The induction motor has two major parts (i) Stator (ii) rotor. The stator is the stationary part and the rotor is the rotating part. The stator and the rotor are separated by the air gap which ranges from 0.4 to 2.5 mm. The power factor of the motor depends on the air gap length.

Q.10 What is the effect of the number of poles on the speed of the induction motor?

The speed of the induction motor is inversely proportional to the number of poles.  The Synchronous speed of 2 poles, 50 Hz induction motor is 3000 RPM, The synchronous speed of 4 poles motor is 1500 RPM.