The structure of the stator of the AC servo motor is basically similar to that of the capacitive split-phase single-phase asynchronous motor, and the stator is equipped with two windings with a position difference of 90°, one is the excitation winding Rf, which is always connected to the AC voltage Uf, and the other is the control winding L, which is connected to the control signal voltage Uc. So AC servo motors are also called two servo motors.
The rotor of the AC servo motor is usually made into a squirrel cage type, but in order to make the servo motor have a wide range of speed regulation, linear mechanical characteristics, no "rotation" phenomenon and fast response performance, it should have the characteristics of large rotor resistance and small moment of inertia compared with ordinary motors. There are two forms of rotor structure that is widely used: one is the squirrel cage rotor made of high resistivity conductive material made of high resistivity guide bar, in order to reduce the moment of inertia of the rotor, the rotor is made slender; the other is the coreless cup rotor made of aluminum alloy, the cup wall is very thin, only 0.2-0.3mm, in order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator should be placed in the coreless cup rotor.
When the AC servo motor has no control voltage, there is only a pulsating magnetic field generated by the excitation winding in the stator, and the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, the rotor rotates in the direction of the rotating magnetic field, in the case of constant load, the speed of the motor changes with the size of the control voltage, when the phase of the control voltage is opposite, the servo motor will be reversed.