What is the working principle of three phase servo motor?

Applied with PM servo motor. Because of power generation effect when stops, it is easier to brake. (Rotator: PM Stator: Windings) The magnetic force is be made by permanent magnet and the current is used to generate torque. Then high torque and efficiency are available at low current and small size. It has no brush so there is little noise/vibration and no dirt. And high precision control is available with high resolution encoder. A Servo motor Based on the design of conventional AC Synchronous motor, with addition of an Amplifier and a feedback device.