--- title: "Servo motor operating principle" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:servo-motor-principle" source: https://www.bctengineer.com/q/semester_4/electrical_machine/servo-motor-principle site: BCT Engineering --- # Servo motor operating principle Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2072 Magh - Video: [Servo Motor - Motors - Industrial Electronics](https://www.youtube.com/watch?v=OYMJDFwVj-I) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/servo-motor-principle - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Explain the operating principle of servo motor. ## Short answer A servo motor operates in a closed-loop system, where an error signal between reference and feedback drives the motor. In a two-phase AC servo, a control winding voltage 90° out of phase produces torque proportional to the error. High rotor resistance gives a linear torque-speed curve with negative slope for stable positioning. ## Answer A servo motor is a motor used in a closed-loop control system to give precise control of position, speed or torque, in response to a control signal. It has a low-inertia rotor, a fast response and can start, stop and reverse quickly. Types are d.c. servo motors and a.c. (two-phase) servo motors. #### Operating principle (closed loop) ```text Reference ->(+)-> Error -> Amplifier -> Servo -> Load input ^ (-) signal motor | | | +------ Feedback (tacho / encoder) -+ ``` 1. A reference signal (desired position or speed) is compared with the feedback signal from the output (a potentiometer, tachogenerator or encoder). 2. The difference, the error signal, is amplified and applied to the control winding (a.c. servo) or armature/field (d.c. servo). 3. The motor develops torque proportional to the error signal and turns the load in the direction that reduces the error. 4. When the output reaches the desired value, the error becomes zero and the torque falls to zero, so the motor stops. The torque reverses if the output overshoots. #### Principle of the two-phase a.c. servo motor - The stator has two windings 90° apart. The reference (fixed) winding is excited from a constant a.c. supply, often with a capacitor. The control winding is excited by the amplified error signal, 90° out of phase with the reference voltage. - The rotor is a squirrel cage or drag cup of high resistance (R/X ratio high), to give a torque which falls linearly with speed (negative slope) and to avoid single-phase running. - The torque is proportional to the control voltage, and the direction of rotation depends on the phase (lead or lag) of the control voltage with respect to the reference. ```text Torque |\ Vc = high | \\ Vc = medium | \\\ Vc = low +-----\----> speed (nearly linear, negative slope) ``` **Features:** low inertia (long, thin rotor), linear torque-speed characteristic, quick response, no hunting, stable. **Uses:** robotics, radar and antenna positioning, machine tools, process control and instruments. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is applied to the servo motor as its input? A. The reference signal alone B. The amplified error between the reference and the feedback signal C. The feedback signal alone D. A constant supply with no control Answer: B. The reference is compared with the feedback, and the amplified error drives the motor. (Key point: Closed-loop operation) 2. What happens when the output reaches the desired value? A. The error and torque become zero and the motor stops B. The motor speeds up C. The torque becomes maximum D. The feedback is disconnected and it keeps running Answer: A. Zero error means zero torque. Overshoot gives a reversed error, which reverses the torque. (Key point: Stopping at the target) 3. Why does a two-phase a.c. servo motor have a high-resistance rotor? A. To give a linear torque that falls with speed (negative slope) and avoid single-phase running B. To increase the inertia C. To raise the efficiency to the maximum D. To make the motor self-starting from a single winding at constant speed Answer: A. A high R/X rotor gives a nearly linear, negatively sloped torque-speed curve and prevents the motor running on the reference winding alone. (Key point: Rotor of a.c. servo motor) 4. In an a.c. servo motor, what decides the direction of rotation? A. Whether the control voltage leads or lags the reference voltage B. The magnitude of the reference voltage C. The rotor diameter D. The number of turns of the reference winding Answer: A. Torque is proportional to control voltage, and its direction depends on the phase (lead or lag) of the control voltage. (Key point: Direction of rotation) ## Close variants set in other exams - [DC servo motor and applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-servo-motor-principle-applications) — Electrical Machine, chapter 7, asked 2× ## Related in Electrical Machine - [Stepper motor and servo motor principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Astepper-and-servo-motor-principle) — Electrical Machines, chapter 7 - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5 - [Servo motor construction and working](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aservo-motor-construction-working) — Electrical Machines, chapter 7, asked 2× - [Short note: AC servo motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashort-note-ac-servo-motor) — Electrical Machine, chapter 7 - [Reason: servo motor long, thin](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-long-small-diameter-reason) — Electrical Machine, chapter 7 - [Servo motor length and diameter reason](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aservo-motor-long-small-diameter) — Electrical Machines, chapter 7 ## Related in other subjects - [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2× - [Shaded pole motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashaded-pole-motor) — Electric Machines II, chapter 3 - [Short note: AC servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-ac-servo-motor) — Electric Machines II, chapter 3, asked 3× - [Short note: Servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-servo-motor) — Electric Machines II, chapter 3