--- title: "Short note: AC servo motor" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:short-note-ac-servo-motor" source: https://www.bctengineer.com/q/semester_4/electrical_machine/short-note-ac-servo-motor site: BCT Engineering --- # Short note: AC servo motor Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2071 Magh, 4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/short-note-ac-servo-motor - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Write a short note on AC servo motor. ## Short answer An AC servo motor is a two-phase induction motor with reference and control windings 90° apart. Control voltage varies torque; zero voltage stops it due to high rotor resistance. It has linear torque-speed curves, used in radar tracking, flight controls, and small servos. ## Answer 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are the windings of an a.c. servo motor arranged? A. Reference and control windings 90° apart in space, with control voltage 90° out of phase B. Both windings in phase and in the same slots C. A single winding with slip rings D. Three windings 120° apart Answer: A. It is a two-phase induction motor with a fixed reference winding and a variable control winding 90° out of phase. (Key point: Winding arrangement) 2. How is torque and direction controlled? A. Torque is proportional to the control voltage, and direction depends on whether it leads or lags the reference B. Torque is fixed and direction is set by the rotor C. Torque depends on the reference voltage only D. Direction depends on the rotor resistance Answer: A. Changing the magnitude and phase (lead or lag) of the control voltage sets torque and direction. (Key point: Torque control) 3. Why does the motor stop when the control voltage is zero? A. The high rotor resistance prevents it from running as a single-phase motor B. The rotor loses its squirrel cage C. The reference winding is disconnected D. The capacitor discharges Answer: A. With high rotor resistance the backward torque dominates, so the motor does not keep running on one winding. 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