Introduction to Three-Phase Asynchronous Motors

The three-phase asynchronous motor (or induction motor) is a type of electric motor powered by a simultaneous supply of 380V three-phase alternating current (with a phase difference of 120 degrees). It is termed "asynchronous" because its rotor rotates in the same direction as the stator's rotating magnetic field, yet at a different speed-resulting in a phenomenon known as "slip." The rotor of a three-phase asynchronous motor rotates at a speed lower than that of the rotating magnetic field; this relative motion between the rotor windings and the magnetic field induces an electromotive force (EMF) and current within the windings, which then interact with the magnetic field to generate electromagnetic torque, thereby facilitating energy conversion.

Compared to single-phase asynchronous motors, three-phase asynchronous motors offer superior operating performance and allow for savings in various materials. Based on rotor structure, three-phase asynchronous motors are classified into two types: squirrel-cage and wound-rotor. Squirrel-cage asynchronous motors feature a simple structure, reliable operation, light weight, and low cost, leading to their widespread application; however, their primary disadvantage is the difficulty associated with speed control. In contrast, the wound-rotor three-phase asynchronous motor is equipped with three-phase windings on its rotor-much like its stator-which are connected to an external variable resistor via slip rings and brushes. By adjusting the resistance of this variable resistor, one can improve the motor's starting performance and regulate its operating speed.