Dc motor has been gradually phased out, now the mainstream in the market are AC motor, in performance, AC servo is superior to DC motor.

   Since the 1980s, with the development of integrated circuit, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding development. The famous electrical manufacturers of various countries have successively launched their own ac servo motor and servo driver series products and continuously improved and updated them. Ac servo system has become the main development direction of high performance servo system, which makes the original DC servo face the crisis of being eliminated. Since 1990s, the ac servo system which has been commercialized all over the world is the servo drive of sinusoidal motor controlled by digital. Ac servo drive device in the field of transmission development with each passing day. Compared with dc servo motor, permanent magnet AC servo motor has the following main advantages:

　  （1）no brush and commutator, so the work is reliable, low maintenance and maintenance requirements.

　  （2）The stator winding heat dissipation is more convenient.

　  （3）Small inertia, easy to improve the speed of the system.

　  （4）Suitable for high speed and high torque working state

　  （5）Smaller volume and weight at the same power.

　　The working principle of AC servo motor is similar to that of single-phase asynchronous motor, but the rotor resistance of the former is much greater than that of the latter. Therefore, compared with the single-phase asynchronous motor, the servo motor has three significant characteristics:

　　A、High starting torque

　　Due to the large rotor resistance, the torque characteristic curve of the rotor is obviously different from that of the ordinary asynchronous motor. It can make the critical slip S0 > 1, which not only makes the torque characteristic (mechanical characteristic) more linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast starting and high sensitivity.

　　B、Wide range of operation

　　C. No rotation

　　Servo motor in normal operation, as long as the control voltage is lost, the motor will stop running immediately. When the servo motor loses its control voltage, it is in a single-phase operation state. Due to the large rotor resistance, two torque characteristics (T1-S1, T2-S2 curves) and synthetic torque characteristics (T-S curves) are generated by the interaction between the rotating magnetic fields rotating in opposite directions in the stator and the rotor.

　　The output power of ac servo motor is generally 0.1-100W. When the power supply frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; When the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V, etc.

　　Ac servo motor runs smoothly and has low noise. However, the control characteristics are nonlinear, and due to the large rotor resistance, large loss and low efficiency, compared with the dc servo motor with the same capacity, it is large in size and heavy in weight, so it is only suitable for the small power control system of 0.5-100W.