The stator of an AC servo motor is basically similar in structure to that of a capacitor-split-phase single-phase asynchronous motor . Its stator has two windings positioned 90° apart: one is the excitation winding Rf, which is always connected to the AC voltage Uf; the other is the control winding L, connected to the control signal voltage Uc. Therefore, an AC servo motor is also called a dual-servo motor.
AC servo motors typically use squirrel-cage rotors. However, to achieve a wide speed range, linear mechanical characteristics, no "self-rotation," and rapid response, servo motors, compared to ordinary motors, should possess high rotor resistance and low moment of inertia. Currently, two common rotor structures are used: one is a squirrel-cage rotor with high-resistivity conductors made of high-resistivity conductive materials; to reduce moment of inertia, the rotor is made slender. The other is a hollow cup-shaped rotor made of aluminum alloy with very thin walls (only 0.2-0.3 mm ) . To reduce magnetic resistance, a fixed inner stator is placed inside the hollow cup-shaped rotor . The hollow cup-shaped rotor has very low moment of inertia, rapid response, and smooth operation, hence its widespread adoption.
When there is no control voltage, the stator of an AC servo motor only has a pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the motor speed varies with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
