The stator winding of a three-phase induction motor is powered by a three-phase AC power supply, and the current in the rotor winding is generated by electromagnetic induction, thus converting electrical energy into mechanical energy.
When the three-phase winding of the stator is supplied with symmetrical three-phase current, a rotating magnetic field is produced inside the motor. When the magnetic field rotates clockwise, it is equivalent to the rotor cage bars cutting the magnetic induction lines counterclockwise, thereby generating induced electromotive force and induced current in the rotor cage bars. Because the rotor current is not supplied by energy current but generated by induction, it is called a three-phase induction motor.
The direction of the magnetic force acting on the rotor cage bars can be determined by Ampere's law. Under the action of this electromagnetic torque, the rotor will also rotate clockwise, and its rotation direction will be consistent with the rotation direction of the magnetic field. The speed of the rotor is related to the load on the rotor shaft, but the rotor speed always needs to be lower than the speed of the rotating magnetic field. Otherwise, it will slow down due to the lack of electromagnetic torque and the action of resistance torque. Therefore, the three-phase induction motor is also known as a three-phase asynchronous motor.
Since the rotor rotation is consistent with the rotation of the magnetic field, and the rotation direction of the magnetic field is determined by the phase sequence of the current, when two power lines are interchanged, the change in the phase sequence of the current leads to a reversal of the rotation direction of the rotating magnetic field, and therefore the direction of the rotor also changes. It can be seen that a three-phase induction motor can conveniently change the direction of the rotor shaft rotation by interchanging any two power lines.