A three-phase induction motor is a type of AC (alternating current) electric motor widely used for various industrial and commercial applications due to its efficiency, reliability, and simplicity. It operates on the principle of electromagnetic induction and consists of a stator and a rotor.
Here's how a three-phase induction motor operates:
Stator: The stator is the stationary part of the motor and is comprised of three sets of windings, spaced 120 degrees apart, connected to three-phase AC power. When three-phase AC voltage is applied to these windings, it generates a rotating magnetic field in the motor's air gap. This magnetic field rotates at the same frequency as the AC power supply and induces voltage in the rotor.
Rotor: The rotor is the moving part of the motor, typically made of a series of stacked laminations to reduce eddy current losses. The rotor can be of two types: squirrel-cage rotor and wound rotor.
Squirrel-Cage Rotor: This is the most common type of rotor. It consists of a cylindrical core with evenly spaced conductive bars (aluminum or copper) running parallel to the rotor's axis. The ends of these bars are short-circuited by two end rings. When the rotating magnetic field from the stator cuts across the conductive bars, it induces a voltage in them. This induced voltage causes currents to flow in the bars, generating a magnetic field that interacts with the stator's magnetic field, producing torque and causing the rotor to turn.
Wound Rotor: In this type of rotor, the conductive bars are replaced with windings. These windings are connected to slip rings instead of short-circuited rings. External resistance can be connected to these windings through the slip rings, allowing control over the motor's starting torque and speed.
Operation: As the rotating magnetic field from the stator sweeps across the rotor, it creates a relative motion between the two magnetic fields. According to the principle of electromagnetic induction, this relative motion induces voltage in the rotor windings or conductive bars. The induced voltage drives currents in the rotor, which, in turn, generates a magnetic field. This interaction between the stator's rotating magnetic field and the rotor's magnetic field creates a torque that causes the rotor to start rotating.
The speed at which the rotor rotates is known as the synchronous speed and is determined by the frequency of the AC power supply and the number of poles in the motor. However, due to factors such as slip (the difference between synchronous speed and rotor speed) and losses, the rotor speed is slightly lower than the synchronous speed. This difference in speed allows the motor to produce useful mechanical work.
In summary, a three-phase induction motor operates by generating a rotating magnetic field in the stator, which induces voltage and currents in the rotor, resulting in the generation of torque and rotational motion.