AC servo motor is mainly composed of a stator and a rotor. Laminated stator core is usually made of silicon. Embedded in the surface of the stator core slot is two phase windings: One phase winding is a field winding, and the other is to control the winding phase and the two phase windings have the difference of 90 ° space electrical angle on a mutual position. Field winding is connected to an AC excitation power supply, and winding k is to control the voltage of the signal when the ac servo motor is working.
There is only pulsating magnetic field generated by magnet winding in the air gap when in the absence of a control voltage, air gap field winding. At this time the rotor keeps stationary with no start torque on itself. There is going to be rotary magnet field to generate magnet torque in the air gap , which makes rotor rotate in the direction of rotary magnet field on the condition that controlling voltage is given and current of control winding and field winding currents is different. However, the requirements for the servo motor is not only the ability to start at a control voltage but to be stopped immediately after the disappearance of the motor voltage. For the working principle of AC servo motors, if the servo motor continues to rotate after the disappearance of the motor voltage like a common single-phase induction motor, it would become uncontrolled. This phenomenon is called self-rotation.
To eliminate self-rotation of AC servo motor, the resistance of rotor r2 should be increased. This is because once the control voltage disappears, the AC servo motor is in single-phase operation mode but if the rotor resistance is so large to ensure the critical slip sm> 1, then there will be two torque curve and torque characteristics of synthetic positive and negative sequence rotating magnetic field generated by the rotor effect curve shown in Figure 1.1 of working principle of AC servo motors. From the figure, the direction of the combined torque and the motor rotation is oppose indicating a braking torque, which ensures that the AC servo motor will be quickly braked to a halt when the control voltage disappears but the rotor is still rotating. After the rotor the increased resistance can not only eliminate the self-rotation, but also obtain the advantage of extended speed range, improved regulating characteristics and increasing the reaction rate.