Short note: AC servo motor

An a.c. servo motor is a two-phase induction motor designed for use in a.c. position and speed control systems.

• Construction: the stator has two windings displaced by 90° in space: the reference winding (fixed voltage, often with a series capacitor) and the control winding (variable voltage from the servo amplifier, 90° out of phase). The rotor is a squirrel cage or a drag-cup type, with high resistance and a long, thin shape for low inertia.
• Working: the two windings produce a rotating field. The torque is proportional to the control voltage, and its direction depends on whether the control voltage leads or lags the reference voltage by 90°. When the control voltage is zero, the torque is zero and the motor stops, and does not run as a single-phase motor, due to the high rotor resistance.
• Characteristics: torque-speed curve is nearly linear with negative slope at different control voltages; good damping, stable operation.
• Uses: instrument servos, radar tracking, aircraft flight controls, and small robotic arms (up to about 100 W).