---
title: "Short note: Universal motor"
subject: "Electrical Machine"
chapter: 7
key: "semester_4/electrical_machine:short-note-universal-motor"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/short-note-universal-motor
site: BCT Engineering
---

# Short note: Universal motor

Electrical Machine · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2071 Magh, 4 marks
- Video: [How DC motors and universal motors work](https://www.youtube.com/watch?v=-qS85aIvleQ) — MatthiasWandel
- 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-universal-motor
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7

## Question

Write a short note on universal motor.

## Short answer

A universal motor is a series-wound commutator motor that works on both AC and DC. Torque direction is unchanged because field and armature currents reverse together. Speed varies widely with load and can be dangerously high at no load. Used in portable tools and appliances.

## Answer

A universal motor is a series-wound commutator motor that runs on both a.c. and d.c. supplies.

- **Construction:** the field winding is in series with the armature. The stator core and armature are laminated, to reduce eddy-current losses on a.c. The motor has a commutator with carbon brushes.
- **Principle:** the torque ∝ φ · I_a. As the field and armature currents are the same current, both reverse together on a.c., so the direction of the torque stays the same. The torque pulsates at twice the supply frequency, but rotor inertia keeps the speed steady.
- **Characteristics:** high starting torque (T ∝ I²), and a speed that varies widely with load. Under no load, the speed can be dangerously high. The speed on a.c. is lower than on d.c. because of the reactive voltage drop, and the power factor is lower on a.c.
- **Speed control:** changing the applied voltage with a thyristor or a tapped field winding.
- **Uses:** vacuum cleaners, mixers, food blenders, portable drills, sewing machines and hair dryers.
- **Why "universal":** because it operates on both a.c. and d.c.

## Check yourself

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

1. Why are the stator and armature of a universal motor laminated?
   A. To reduce eddy-current loss because the flux alternates on a.c.
   B. To increase the air gap
   C. To reduce the weight only
   D. To raise the power factor on d.c.
   Answer: A. On a.c. the flux alternates, so laminations cut eddy-current losses. (Key point: Laminated construction)

2. On a.c., how does the torque of a universal motor behave?
   A. It reverses every half cycle
   B. It keeps one direction but pulsates at twice the supply frequency; inertia keeps speed steady
   C. It is exactly constant
   D. It is zero on a.c.
   Answer: B. Torque is proportional to phi I_a, which does not change sign, but pulsates at twice the supply frequency. (Key point: Pulsating torque)

3. Which statement is right?
   A. Low starting torque, constant speed, runs safely at no load
   B. High starting torque (proportional to I^2), speed varies widely with load, dangerous speed at no load; speed controlled by voltage using a thyristor or tapped field
   C. Speed is lower on d.c. than on a.c. because of reactance
   D. Used only for large industrial drives
   Answer: B. It behaves like a series motor, and the speed is controlled by changing the applied voltage or tapping the field. (Key point: Characteristics and control)

## Related in Electrical Machine

- [Universal motor operation and applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Auniversal-motor-operation-applications) — Electrical Machines, chapter 7, asked 3×
- [Universal motor and why so called](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Auniversal-motor-construction-operation) — Electrical Machine, chapter 7