Hengli Motor takes you to understand the speed and slip of three-phase asynchronous motors

The rotor of an electric motor generates induced current and rotates under the action of electromagnetic torque, mainly due to the relative motion between the rotor and the rotating magnetic field, which means that there is a certain speed difference between the two. This is a necessary condition for the normal operation of a constant force asynchronous motor. If the rotor speed reaches the speed of the rotating magnetic field, that is, the synchronous speed, the constant force motor and the rotor are relatively stationary, and the rotor conductor no longer cuts the magnetic field lines. Therefore, the rotor conductor cannot generate induced electromotive force, and of course, there cannot be induced current and electromagnetic torque. The rotor speed will start to slow down. On the other hand, once the rotor speed slows down, relative motion will occur again between the rotor and the rotating magnetic field, causing the rotor to be subjected to electromagnetic torque again. forcing the rotor to accelerate its speed, The contradictory result will inevitably stabilize the rotor speed at a certain speed n. Since the speed n of this type of motor is always lower than the synchronous speed ns, it is called an asynchronous motor constant force motor. Also, because the rotor current of this type of motor is generated by electromagnetic induction, it is also called an induction motor.

To represent the relationship between the speed and synchronous speed of a three-phase asynchronous motor, we introduce the concept of slip rate. The constant force motor slip rate is the ratio of the difference between the synchronous speed ns and the rotor speed n to the synchronous speed ns, represented by s, that is:

s=(ns-n)/ns

The slip rate is an important parameter of an electric motor, and we need to remember its three specific operating points:

(1) At the moment when the motor starts, although the rotating magnetic field has been generated, the rotor has not yet rotated. At this time, n=0, then s=1;

(2) When the motor runs without load, the no-load resistance is very small and the speed is high. n0 is about equal to ns, so s is very small, usually around 0.005;

(3) When the motor operates at rated speed, there is a rated speed nN and a rated slip sN, with 0<sN<1, generally between 0.01 and 0.07.