AC synchronous motors are constant-speed drive motors whose rotor speed maintains a fixed proportional relationship with the power supply frequency. They are widely adopted in electronic instruments, modern office equipment, textile machinery and other fields.

1. Permanent Magnet Synchronous Motors
Permanent magnet synchronous motors fall under the category of line-start permanent magnet synchronous motors. Their magnetic field system consists of one or more permanent magnets. Typically, magnetic poles embedded with permanent magnets matching the required number of poles are installed inside a squirrel-cage rotor fabricated via cast aluminum or welded copper bars. The stator structure resembles that of an asynchronous motor.
When power is supplied to the stator windings, the motor initiates rotation following the operating principle of an asynchronous motor. Once it accelerates to synchronous speed, the synchronous electromagnetic torque generated by the rotor permanent magnetic field and stator magnetic field (a composite torque of the electromagnetic torque from the rotor permanent magnetic field and the reluctance torque from the stator magnetic field) pulls the rotor into synchronism, and the motor operates in synchronous mode.
2. Reluctance Synchronous Motors
Also known as reaction synchronous motors, reluctance synchronous motors operate by generating reluctance torque resulting from the unequal magnetic reluctance along the quadrature and direct axes of the rotor. Their stators share a similar structure with those of asynchronous motors, while the rotor design differs.
Evolved from squirrel-cage asynchronous motors, reluctance synchronous motors are equipped with cast aluminum squirrel-cage windings on the rotor to provide asynchronous starting torque. The rotor is machined with reaction slots corresponding to the number of stator poles. These slots only serve as salient poles, with no excitation windings or permanent magnets fitted, and function to produce synchronous reluctance torque.
Based on distinct reaction slot configurations, rotors are classified into internal reaction rotors, external reaction rotors and combined internal-external reaction rotors. For external reaction rotors, reaction slots are cut into the rotor outer circumference, creating unequal air gaps between the direct and quadrature axes. Internal reaction rotors feature internal grooves that obstruct magnetic flux along the quadrature axis and increase its magnetic reluctance. Combined internal-external reaction rotors integrate the structural merits of the former two types, yielding a larger disparity between direct and quadrature axes and thus superior motor power density.
Reluctance synchronous motors are further categorized into multiple variants, including single-phase capacitor-run, single-phase capacitor-start and single-phase dual-capacitor types.
3. Hysteresis Synchronous Motors
Hysteresis synchronous motors operate by harnessing hysteresis torque produced from hysteresis materials. They are divided into inner-rotor hysteresis synchronous motors, outer-rotor hysteresis synchronous motors and single-phase shaded-pole hysteresis synchronous motors.
Inner-rotor hysteresis synchronous motors feature a non-salient cylindrical rotor with a smooth outer surface. No windings are mounted on the rotor; instead, an effective annular layer made of hysteresis materials covers the outer circumference of the rotor core.
After energization of the stator windings, the resulting rotating magnetic field generates asynchronous torque to drive the hysteresis rotor into rotation, which then automatically locks into synchronous operation. During asynchronous operation, the stator rotating magnetic field magnetizes the rotor repeatedly at slip frequency. Under synchronous operation, the hysteresis material on the rotor is magnetized to form permanent magnetic poles, thereby generating synchronous torque.