Detailed explanation of the inching control principle of three-phase asynchronous motor

The three-phase asynchronous motor is a type of induction motor, which is powered by 380V three-phase AC current (phase difference of 120 degrees) at the same time. Since the rotor and stator rotating magnetic field of the three-phase asynchronous motor rotate in the same direction and at different speeds, there is a slip rate, so it is called a three-phase asynchronous motor. The speed of the rotor of the three-phase asynchronous motor is lower than the speed of the rotating magnetic field. The rotor winding generates electromotive force and current due to the relative motion between the rotor winding and the magnetic field, and interacts with the magnetic field to generate electromagnetic torque to achieve energy conversion.

The three-phase asynchronous motor consists of two basic parts: a fixed stator and a rotating rotor. The rotor is installed in the inner cavity of the stator and supported on two end covers by bearings. In order to ensure that the rotor can rotate freely in the stator, there must be a gap between the stator and the rotor, called the air gap. The air gap of the motor is a very important parameter, and its size and symmetry have a great influence on the performance of the motor.

The Principle of Inching Control of Three-phase Asynchronous Motor

The inching forward control circuit is the simplest forward control circuit that uses buttons and contactors to control the operation of the motor. The so-called inching control means that when the button is pressed, the motor will be powered and run; when the button is released, the motor will lose power and stop.

The schematic diagram of the typical three-phase asynchronous motor inching control is shown in Figure (a). The inching forward control circuit is composed of a transfer switch QS, a fuse FU, a start button SB, a contactor KM and a motor M. The transfer switch QS is used as a power isolation switch, the fuse FU is used as a short-circuit protection, the button SB controls the coil of the contactor KM to be energized and de-energized, and the main contact of the contactor KM controls the start and stop of the motor M.

Inching control principle: When the motor needs to be inching, first close the conversion switch QS, and the motor M is not yet powered on. Press the start button SB, the coil of the contactor KM is energized, driving the three pairs of main contacts of the contactor KM to close, and the motor M is powered on to start running. When the motor needs to stop, just release the start button SB to de-energize the coil of the contactor KM, driving the three pairs of main contacts of the contactor KM to resume disconnection, and the motor M loses power and stops. In actual production applications, the inching control circuit of the motor is widely used. By replacing the start button SB with a pressure contact, a limit node, a water level contact, etc., various automatic control circuits can be realized to control the automatic operation of small motors.