Question

How does a three-phase induction motor work?

Answer 1

A three-phase induction motor is a type of electric motor that works on the principle of electromagnetic induction. Here's how it works step-by-step:

1. Three-Phase Power Supply  

In a three-phase induction motor, the motor is powered by a three-phase AC (alternating current) supply. This means that the current in each phase is slightly out of phase with the others (by 120 degrees). This creates a rotating magnetic field when the current flows through the motor’s stator.

2. Stator – The Stationary Part  

The stator is the stationary part of the motor and has three sets of windings, each connected to one phase of the power supply. When the three-phase AC current flows through these windings, it creates a rotating magnetic field. This rotating magnetic field is the key to the motor’s operation.

3. Rotor – The Rotating Part  

The rotor is the rotating part of the motor. It is placed inside the stator and does not have any direct electrical connection to the power supply. The rotor can either be a squirrel-cage rotor (which is most common) or wound rotor.

The rotating magnetic field created by the stator induces an electric current in the rotor due to electromagnetic induction. According to Faraday’s Law of Induction, any change in the magnetic flux through a conductor will induce a current in it. So, as the stator’s magnetic field rotates, it cuts through the rotor, inducing a current in the rotor.

4. Electromagnetic Force and Torque  

The current induced in the rotor creates its own magnetic field. This field interacts with the rotating magnetic field of the stator. According to Lenz’s Law, the rotor tries to oppose the rotation of the magnetic field. This interaction generates a force on the rotor, causing it to rotate. The rotation of the rotor is what produces the mechanical output (torque) that drives the load connected to the motor.

5. Slip – The Difference in Speed  

The rotor never catches up to the speed of the rotating magnetic field. The speed of the magnetic field is called the synchronous speed, and it depends on the number of poles in the stator and the frequency of the AC supply. The rotor always rotates a little slower than the synchronous speed, and this difference in speed is called slip. The slip is necessary for inducing current in the rotor, which is why the rotor cannot reach the synchronous speed.

Summary:

1. The stator creates a rotating magnetic field using three-phase current.
   

1. The rotating magnetic field induces a current in the rotor.
   

1. The interaction between the rotor’s magnetic field and the stator’s rotating field produces a force that makes the rotor rotate.
   

1. The motor’s speed is slightly less than the synchronous speed to maintain this interaction and induce current in the rotor.
   

That’s how a three-phase induction motor works!