Working principle of three-phase asynchronous motor

Three-phase asynchronous motors, also known as induction motors, operate on the principle of electromagnetic induction. Here’s how they work:

1. Stator: The stator is the stationary part of the motor and consists of a core made of laminated steel sheets with slots for winding. Three sets of windings (typically copper or aluminum) are placed 120 degrees apart around the inner periphery of the stator.

2. Supply: When a three-phase AC supply is connected to these windings, an alternating magnetic field is generated in the stator. The magnetic field rotates at a synchronous speed determined by the frequency of the AC supply and the number of poles in the motor.

3. Rotor: The rotor is the rotating part of the motor and is either wound type or squirrel cage type. In squirrel cage motors (the most common type):

– It consists of a cylindrical laminated core with conductive bars placed in slots parallel to the motor axis and short-circuited at both ends by conducting rings (end rings).

– When the stator windings are energized with a three-phase AC supply, they create a rotating magnetic field.

– This rotating magnetic field induces currents in the rotor bars due to electromagnetic induction.

– The interaction of the rotating magnetic field and the induced currents in the rotor bars generates a torque, causing the rotor to rotate in the direction of the rotating magnetic field.

4. Slip: In practical operation, the rotor never reaches the synchronous speed of the rotating magnetic field due to a phenomenon called slip. Slip is necessary for torque production and varies with the mechanical load on the motor.

5. Speed Control: The speed of a three-phase asynchronous motor is primarily determined by the frequency of the AC supply and the number of poles in the motor. It can be varied to some extent by changing the frequency of the AC supply or by using variable frequency drives (VFDs).

6. Applications: Three-phase asynchronous motors are widely used in industrial and commercial applications due to their robustness, reliability, and ability to operate over a wide range of speeds and loads.

In summary, three-phase asynchronous motors work by generating a rotating magnetic field in the stator that induces currents in the rotor, producing torque and causing the rotor to rotate.