The premise and principle of three-phase asynchronous motor rotation

Update:17 Aug 2021
Summary: The prerequisite for the rotation of a three-phase asynchronous motor is to have a rotating magnetic field, and the sta...
The prerequisite for the rotation of a three-phase asynchronous motor is to have a rotating magnetic field, and the stator winding of the three-phase asynchronous motor is used to generate the rotating magnetic field. As we all know, the voltage difference between the phase power supply and the phase is 120 degrees, and the three windings in the stator of the three-phase asynchronous motor are also 120 degrees different in the spatial direction. Therefore, when the three-phase power is introduced into the stator winding, the stator winding will generate a rotating magnetic field. When the current changes every cycle, the rotating magnetic field rotates once in space, that is, the speed of the rotating magnetic field is synchronized with the change of the current. The speed of the rotating magnetic field: n=60f/P where f is the power frequency, P is the number of pole pairs of the magnetic field, and the unit of n is the number of revolutions per minute. According to this formula, we know that the speed of the motor is related to the number of poles and the power supply frequency.
The single-phase AC motor has only one winding, and the rotor is a squirrel cage. When a single-phase sinusoidal current passes through the stator windings, the motor generates an alternating magnetic field. The strength and direction of this magnetic field change in a sinusoidal law at all times, but its orientation in space is fixed, so this magnetic field is also called an alternating pulsating magnetic field. The alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with the same speed and opposite rotation directions.
When the rotor of a three-phase asynchronous motor is stationary, the two rotating magnetic fields generate two torques of the same magnitude and opposite directions in the rotor, so that the combined torque is zero, so the motor cannot rotate. When we use external force to rotate the three-phase asynchronous motor in a certain direction (such as clockwise rotation), the movement of the cutting magnetic field lines between the rotor and the clockwise rotating magnetic field becomes smaller; the cutting magnetic field lines between the rotor and the rotating magnetic field are reversed The movement in the clockwise direction becomes larger. In this way, the balance is broken, the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the pushing direction.