The induction motor uses the principle of electromagnetic induction to generate a rotating magnetic field through the three-phase current of the stator, and interacts with the induced current in the rotor winding to generate electromagnetic torque for energy conversion. Under normal circumstances, the rotor speed of an induction motor is always slightly lower or slightly higher than the speed of the rotating magnetic field (synchronous speed), so the induction motor is also called "asynchronous motor".
When the load of the induction motor changes, the rotor speed and slip will change accordingly, so that the electromotive force, current and electromagnetic torque in the rotor conductor will change accordingly to meet the needs of the load. According to the positive, negative and magnitude of slip, induction motors have three operating states: motor, generator and electromagnetic brake.
When the rotor speed is lower than that of the rotating magnetic field (ns>n>0), the slip rate is 0<s<l. Set the air gap rotating magnetic field generated by the sub-three-phase current to turn counterclockwise and follow the right-hand rule. That can determine the direction of the induced electromotive force after the rotor conductor "cuts" the air gap magnetic field. Since the rotor winding is short-circuited, current flows through the rotor conductor. The rotor induced current interacts with the air gap magnetic field to generate electromagnetic force and electromagnetic torque; according to the left-hand rule, the direction of the electromagnetic torque is the same as the rotation of the rotor, that is, the electromagnetic torque is the driving torque. At this time, the motor is input from the grid Power, through electromagnetic induction, the rotor outputs mechanical power, and the motor is in the state of a motor.
If the motor is driven by a prime mover and the rotor speed is higher than the rotating magnetic field speed (n>ns), then the slip ratio s<0. At this time, the induced electromotive force in the rotor conductor and the active component of the current will be opposite to the state of the motor, so the direction of the electromagnetic torque will be opposite to both the rotating magnetic field and the rotor steering, that is, the electromagnetic torque is a braking torque. In order to make the rotor continue to rotate at a speed higher than the rotating magnetic field. The driving torque of the prime mover must overcome the electromagnetic torque of the brake; at this time, the rotor inputs mechanical power from the prime mover, and the output electric power is determined by electromagnetic induction, and the motor is in the generator state.
