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

# Short note: DC servo motor

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2080 Bhadra, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-dc-servo-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Write a short note on DC servo motor.

## Short answer

A DC servo motor is a low-inertia DC motor for closed-loop position/speed control. It is either armature-controlled (constant field) or field-controlled, providing linear torque-speed, fast response, and high torque-to-inertia ratio. Used in robotics, CNC, and antenna positioning.

## Answer

A **DC servo motor** is a small DC motor designed for fast, accurate control of position or speed in a closed-loop (feedback) control system. It is the "actuator" that turns the error signal of a servo system into controlled motion.

#### Construction
- Similar to an ordinary DC motor, but with a **long, small-diameter armature** to keep inertia ($J$) low.
- Field is either a **permanent magnet** or a separately excited winding.
- Low-inertia types: slotless (smooth) armature, **disc (printed-circuit) armature** and **shell/cup armature**, which give very small electrical and mechanical time constants.

#### Types and working
1. **Armature-controlled**: field current is kept constant and the control voltage is applied to the armature. Torque $T = K_t I_a$, so torque and speed vary linearly with armature voltage. This is the most common type (larger power, good damping from back emf).
2. **Field-controlled**: armature current is kept constant and the control signal is applied to the field winding. Torque $T \propto \phi \propto I_f$. Used for small powers; the small field current is easy to control, but response is slower (large field inductance) and there is no back-emf damping.

```text
 error   +-----------+  Va   +-------+   shaft
 ------->| Amplifier |------>|  DC   |----------> load
   ^     +-----------+       | servo |     |
   |                         +-------+     |
   +------- feedback (pot / encoder) <-----+
```

#### Features
- Linear torque–speed and torque–voltage characteristics
- High starting torque, quick reversal, wide speed range
- High torque-to-inertia ratio, fast response

#### Applications
Robotics, CNC machine tools, X–Y plotters and printers, disk drives, tracking antennas, aircraft control surfaces and position control systems.

**Drawback:** brushes and commutator need maintenance and produce sparking, so brushless servo drives are replacing them in many uses.

## Check yourself

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

1. In an armature-controlled DC servo motor:
   A. Field current is constant and control voltage is applied to the armature
   B. Armature current is constant
   C. The field alone is varied
   D. No voltage is applied
   Answer: A. Torque T = Kt Ia is linear with armature voltage, with back-emf damping. (Key point: Armature control)

2. The drawback of field-controlled DC servo motors is:
   A. Slow response due to large field inductance and no back-emf damping
   B. Very high power only
   C. Brushless operation
   D. No torque
   Answer: A. They are used at small powers only. (Key point: Field control)

3. A DC servo armature is long and small in diameter to:
   A. Keep inertia low
   B. Increase inertia
   C. Increase sparking
   D. Reduce the field
   Answer: A. Low J gives fast response. (Key point: Construction)

4. The main drawback of the DC servo motor is:
   A. Brush and commutator maintenance and sparking
   B. No torque
   C. Non-linear characteristics
   D. Cannot reverse
   Answer: A. Brushless drives replace them in many uses. (Key point: Drawback)

## 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: Servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-servo-motor) — Electric Machines II, chapter 3
- [BLDC motor construction and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Abldc-construction-applications) — Electric Machines II, chapter 3

## Related in other subjects

- [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 operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — 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