AC motor are preferred for low power application as they are light weight and rugged. Two phase induction motors are utilized in feedback control system.
Figure: AC Servomotor
The motor contain a stator with two distributed windings displaced 90 electrical degree apart. Under normal operating conditions, a fixed voltage from a constant voltage source is applied to one phase, the other phase is energized by a voltage of variable magnitude and polarity. This voltage is at 90o out of phase with respect to the voltage of fixed phase. The control phase voltage is typically supplied from the servoamplifier. The direction of rotation of the motor reverses if control phase signal modify sign. The rotor construction is usually of squirrel cage or drag-cup type with no electrical access. The torque-speed characteristics of a servomotor (two-phase induction motor) are illustrated in Figure.