---
title: "Short note: Servo motor"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:short-note-servo-motor"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-servo-motor
site: BCT Engineering
---

# Short note: Servo motor

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2082 Baisakh, 4 marks
- Video: [Servo Motors, how do they work?](https://www.youtube.com/watch?v=1WnGv-DPexc) — The Engineering Mindset
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-servo-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Write a short note on servo motor.

## Short answer

A servo motor is a small motor designed for closed-loop position/speed control, requiring low inertia, linear torque, and no drift at zero signal. AC types are two-phase induction; DC types are separately excited or permanent magnet. Common applications include robots, CNC, and radar positioning.

## Answer

A **servo motor** is a small motor designed for use in a closed-loop control system, in which its shaft position or speed follows a command signal accurately and quickly.

#### Requirements

- Torque proportional to control signal, linear torque-speed characteristics.
- **Low inertia** (small diameter, long rotor) for quick start, stop and reversal.
- Stable operation; no running when the control signal is zero.

#### Types

| AC servo motor | DC servo motor |
|---|---|
| Two-phase induction motor | Separately excited / PM DC motor |
| Reference + control winding at $90^\circ$ | Armature or field control |
| High-resistance cage or drag-cup rotor | Low-inertia armature |
| Low power (few W to ~100 W) | Higher power |
| No brushes, rugged | Brushes need maintenance |

#### Working (closed loop)

```text
 Ref -->(+)--> [Amplifier] --> [Servo motor] --> Load
        (-)                          |
         ^------[Feedback sensor]<---+
```

- The error between the command and the feedback (from a potentiometer, encoder or tachogenerator) is amplified and applied to the motor.
- The motor turns until the error becomes zero; the direction of the error decides the direction of rotation.

#### Applications

Robots, CNC machine tools, radar and antenna positioning, aircraft control surfaces, camera auto-focus, printers and instrument servos.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A servo motor is used in:
   A. A closed-loop control system to follow a command accurately
   B. An open-loop fan drive
   C. Power transmission lines
   D. Heavy traction only
   Answer: A. Its position or speed follows the command signal using feedback. (Key point: Definition)

2. Servo motors have small diameter and long rotor to:
   A. Reduce inertia for quick start, stop and reversal
   B. Increase inertia
   C. Increase speed above Ns
   D. Reduce torque
   Answer: A. Low inertia gives fast response. (Key point: Low inertia)

3. In the closed loop, the motor turns until:
   A. The error between command and feedback becomes zero
   B. The amplifier saturates
   C. The supply is cut
   D. The rotor reaches Ns
   Answer: A. The amplified error drives the motor, and its sign decides direction. (Key point: Feedback operation)

4. Which is true of AC servo motors compared with DC ones?
   A. No brushes and rugged but low power
   B. Higher power with brushes
   C. Need commutator maintenance
   D. Use field and armature control
   Answer: A. AC servos are brushless, rugged, few W to about 100 W. (Key point: AC versus DC servo)

## Related in Electric Machines II

- [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: DC servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-dc-servo-motor) — Electric Machines II, chapter 3

## Related in other subjects

- [Servo motor operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — Electrical Machine, chapter 7, asked 2×
- [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×
- [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