Short Answer:

The working principle of an induction motor is based on electromagnetic induction, where a rotating magnetic field in the stator induces a current in the rotor. This induced current creates its own magnetic field, which interacts with the stator’s field to produce torque and cause rotation.

When three-phase AC supply is given to the stator, it generates a rotating magnetic field. The rotor, which is not connected to any external power, receives current by induction. This interaction between the magnetic fields of stator and rotor results in continuous rotation of the rotor at a speed slightly less than the synchronous speed.

Detailed Explanation:

Working principle of an induction motor

An induction motor is a widely used AC motor that operates on the principle of electromagnetic induction, discovered by Faraday and later applied to rotating machinery by Nikola Tesla. It is called an “induction” motor because electric current is induced in the rotor without any physical contact between the stator and rotor windings.

This motor works by using a rotating magnetic field to generate torque. Unlike synchronous motors, the rotor in an induction motor never rotates at the same speed as the stator’s rotating field—it always lags slightly, and this difference is called slip.

1. Creation of Rotating Magnetic Field (RMF):

When a three-phase AC supply is given to the stator winding, it creates a rotating magnetic field inside the motor. This field rotates at a constant speed known as the synchronous speed (Ns), calculated by the formula:

Ns = (120 × f) / P
Where:

• f is the supply frequency in hertz
• P is the number of poles in the stator

This rotating magnetic field sweeps across the rotor conductors.

2. Induction of Current in Rotor:

The rotor of the induction motor can be either a squirrel cage or wound type, but both work on the same principle. Since the rotor is initially at rest, the rotating magnetic field cuts across the rotor conductors, and by Faraday’s Law, this changing magnetic field induces an EMF (electromotive force) in the rotor.

As per Lenz’s Law, the induced current in the rotor flows in such a direction that it tries to oppose the cause—in this case, the relative motion between stator field and rotor. So, the rotor starts to rotate in the same direction as the magnetic field.

3. Development of Torque:

The interaction between the stator’s rotating magnetic field and the magnetic field created by the induced rotor current produces a force. This force generates torque, which causes the rotor to spin.

However, the rotor can never catch up to the synchronous speed because if it did, there would be no relative motion, hence no induction of current. This small difference in speed is called slip and is necessary for continuous operation.

Slip (%) = [(Ns – Nr) / Ns] × 100
Where:

• Ns = Synchronous speed
• Nr = Rotor speed

4. Summary of Operation:

• AC supply creates rotating magnetic field in stator
• Field induces current in rotor conductors
• Rotor current produces its own magnetic field
• Magnetic fields interact to produce torque
• Rotor starts rotating but always lags behind the stator field (slip)

Applications:

• Fans, pumps, blowers
• Elevators and conveyors
• Compressors
• Domestic and industrial machinery
  Induction motors are preferred due to their robust design, low cost, and ease of maintenance.

Conclusion:

The working principle of an induction motor is based on the electromagnetic induction process, where the stator’s rotating magnetic field induces a current in the rotor. This current interacts with the magnetic field to produce torque and rotate the rotor. The motor runs reliably with simple construction and no need for electrical connection to the rotor, making it the most widely used motor in both domestic and industrial applications.