The prerequisite for three-phase induction motor to rotate is to have a rotating magnetic field. The stator winding of three-phase induction motor is used to generate rotating magnetic field. As we know, the voltage between the phases of the phase power supply is 120 degrees different in phase, and the three windings in the stator of the three-phase induction motor are also 120 degrees different from each other in spatial orientation. In this way, when the three-phase power supply is connected to the stator winding, the stator winding will produce a rotating magnetic field. Every time the current changes a cycle, the rotating magnetic field rotates one cycle in space, that is, the rotation speed of the rotating magnetic field is synchronized with the change of the current. The rotating speed of the rotating magnetic field is: n = 60F / P, where f is the power frequency, P is the number of magnetic poles of the magnetic field, and the unit of n is revolutions per minute. According to this formula, we know that the speed of the motor is related to the number of magnetic poles and the frequency of the power supply.
Single phase AC motor has only one winding and the rotor is squirrel cage type. When the single-phase sinusoidal current passes through the stator winding, the motor will produce an alternating magnetic field. The strength and direction of the magnetic field change sinusoidally with time, but it is fixed in spatial orientation, so it is also called alternating pulsating magnetic field. The alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with the same speed and opposite rotation direction. When the rotor is stationary, the two rotating magnetic fields produce two torques with the same size and opposite direction in the rotor, so that the constituent torque is zero, so the motor cannot rotate. When we use external force to make the motor rotate in a certain direction (such as clockwise rotation), the motion of the magnetic line of force between the rotor and the rotating magnetic field in the clockwise rotation direction becomes smaller; The motion of the magnetic line of force between the rotor and the rotating magnetic field in the counterclockwise rotation 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 driving direction.
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