Deriving from Greek ‘slave’, servo motor can be understood as a motor that absolutely obeys the control signal. Before the signal issued, the rotor is stationary which will rotates immediately when the control signal is issued and will stop immediately when the control signal disappears.
Servo motor,also known as an actuator motor, is used as an actuator in automatic control system to convert the received electrical signal into an angular displacement or velocity output via the shaft.
There are two types of servo motor -- DC servo motor and AC servo motor.
AC servo motor
The basic structure of an AC servo motor is similar to that of AC induction motor (also known as asynchronous motor). On the stator, there are excitation windings Wf and control winding WcoWf with a phase space displacement of 90°, which connected with the constant AC voltage to control the operation of motor by the AC voltage or phase applied to the Wc. Features as a stable operation, good controllability and high sensitivity, the non-linearity index of the adjustment characteristics of such motor is strict (requires less than 10% to 15% and less than 15% to 25% , respectively).
Advantages of AC servo motor: good & high speed control -- smooth control in the entire speed zone with almost no oscillation, high efficiency above 90%, low heat generation, high precision position control (depending on encoder accuracy), within rated operating area, constant torque, low inertia & noise, no brush wear, maintenance-free (suitable for dust-free, explosive environme-nts).
Disadvantage: complex control, driver parameters should be on-site adjusted by PID.
DC servo motor
Having a similar structure with normal DC motor, we could get the speed of DC servo motor by the fomula n=E/K1j=(Ua-IaRa)/K1j -- E means armature counter electromotive force, K means an constant, j means flux per pole, Ua and Ia means woltage and current of armature separatly and Ra means resistance of armature. Although readjustment of Ua or φ could change the speed of DC servo motor, and usually we only change Ua since there is an permanent φ in DC servo motor.DC servo motor has a good linear regulation and fast response.
Advantages of DC servo motor: accuracy speed control, hard torque speed characteristic, simple and convenient control and cheap price.
Disadvantage of DC motor: unsmooth commutation ability of brush, limit speed, hard resistance and wear debris would be produced which is danger in dust free and explosive environment.