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Rotor rotation principle of three-phase asynchronous motor

Date:2023-08-31   Author:XINDA MOTOR

The number of power phases required by the motor during operation is different, and the classification of the motor is also different. Three-phase asynchronous motors are classified according to the number of power phases. Among the asynchronous motors, three-phase asynchronous motors are the most widely used type. If you want to understand the three-phase asynchronous motor, you can start from the rotor rotation principle of the three-phase asynchronous motor!

The stator core slot of the three-phase asynchronous motor is embedded with three-phase symmetrical windings U1-U2, V1-V2, and W1-W2. The rotor is a closed polyphase winding cage motor. The small circles on the stator and rotor of the three-phase asynchronous motor represent the stator windings and rotor conductors.

When the stator winding is connected to a three-phase AC power supply, the three-phase symmetrical current flowing into the stator winding generates a magnetic field rotating at synchronous speed in the air gap of the motor. The rotor conductor is embedded in the rotor core slot, and both ends are short-circuited by conductive rings. When the rotating magnetic field rotates in the clockwise direction, the rotor conductor cuts the magnetic field lines to generate an induced electromotive force, and its direction is determined by the right-hand rule.

The direction of the electromotive force in the upper half of the three-phase asynchronous motor rotor goes out of the paper, and the direction of the electromotive force in the lower half of the conductor enters the paper. When the rotor circuit is closed, current flows in the rotor conductors. If the rotor winding inductance is not considered, the direction of the current is the same as the direction of the electromotive force.

The current-carrying conductor of the rotor of a three-phase asynchronous motor will be subjected to electromagnetic force in the rotating magnetic field. The direction of the electromagnetic force on the conductor can be judged by the left-hand rule. Each guide bar on the rotor is subject to a clockwise force. Rotating in a clockwise direction, its rotation speed is n, which is the same direction as the rotating magnetic field. Since the rotor conductor current is generated by electromagnetic induction , this type of motor can also be called an induction motor.

Under normal circumstances, the rotor speed of the three-phase asynchronous motor cannot reach the speed of the rotating magnetic field. This is because the rotor conductor does not cut the stator rotating magnetic field when n=n1, and there is no induced electromotive force and current in the rotor, and no torque is generated. , so the rotor speed and the stator rotating magnetic field speed cannot be consistent, so it is called a three-phase asynchronous motor.

For example, the synchronous speed of a four-pole asynchronous motor is n1=1500r/min. At rated load, its speed is about 1400r/min. Usually we call the ratio of the difference between the synchronous speed n1 and the rotor speed n to the synchronous speed n1 as the slip rate, equation 2, expressed by s. Slip s is a basic physical quantity of three-phase asynchronous motor.

Reflects various operating conditions of three-phase asynchronous motors. When the motor starts, n=0 and slip rate s=1. Under ideal no-load torque, n=n1, then s=0. The higher the rotor speed, the smaller the slip. Usually the slip rate of a three-phase asynchronous motor at rated load is usually between 0.01 and 0.06. That is, the speed of the three-phase asynchronous motor is very close to the synchronous speed.