---
title: "Universal motor operation and applications"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:universal-motor-operation-applications"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/universal-motor-operation-applications
site: BCT Engineering
---

# Universal motor operation and applications

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2079 Jestha, 8 marks; 2078 Chaitra, 4 marks; 2071 Magh, 4 marks (asked 3 times)
- Video: [universal motor | universal motor working principle | high efficiency motor for EVs & machines](https://www.youtube.com/watch?v=_tvMPIn59nQ) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/universal-motor-operation-applications
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

What is universal motor? Explain its operation and applications.

## Short answer

A universal motor is a series-wound commutator motor that operates on both AC and DC. Torque is unidirectional because current and flux reverse together; speed varies inversely with load. Used in vacuum cleaners, drills, mixers, and portable tools.

## Answer

#### Universal motor
A universal motor is a **series-wound** commutator motor designed to run on both ac and dc supplies, with similar performance on both. It is a small motor, up to about 750 W, and runs at high speed (3000 to 20000 rpm).

#### Construction
- Stator: a laminated (to reduce eddy currents on ac) salient-pole field, with the field winding connected in **series with the armature**.
- Armature: a laminated, slotted rotor with its winding connected to a commutator, and carbon brushes.
- Larger motors may have a compensating winding to reduce armature reaction and commutation problems on ac.

```text
 Supply ---[Field]---[Armature]--- Supply
              (series connection)
```

#### Operation
1. **On dc:** the field flux $\phi \propto I$, and the torque $T \propto \phi I_a \propto I^2$, as in a dc series motor.
2. **On ac:** the current and the flux reverse at the same time, so the product $\phi I_a$ remains unidirectional. The torque ($\propto I^2$) always acts in the same direction but pulsates at twice the supply frequency, so the motor rotates continuously.
3. The speed varies strongly with load: high speed at light load (it can reach a dangerous speed at no load) and low speed at heavy load. The speed can be controlled by changing the voltage, using a regulator or a tapped field.
4. Performance on ac is poorer than on dc: the reactance of the windings causes a voltage drop, the power factor is lower and the losses are higher. At the same voltage, the speed on ac is lower than on dc.

#### Applications
Because of its high speed, high starting torque and compact size, it is used in vacuum cleaners, mixer-grinders, hair dryers, portable drills, sewing machines, food mixers, electric shavers, blenders and other power tools.

## Check yourself

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

1. What is a universal motor?
   A. A shunt motor for dc only
   B. A series-wound commutator motor that works on both ac and dc
   C. An induction motor with a capacitor
   D. A synchronous motor with brushes
   Answer: B. It is a series motor, with field in series with the armature, designed for ac and dc. (Key point: Definition)

2. Why does the universal motor rotate continuously on ac?
   A. The flux and armature current reverse together, so torque (proportional to I^2) stays in one direction
   B. A capacitor keeps the current unidirectional
   C. The supply is rectified internally
   D. The field is permanent magnet
   Answer: A. Flux and armature current both reverse each half-cycle so their product stays unidirectional, pulsating at twice the supply frequency. (Key point: Torque on ac)

3. Why is the stator laminated?
   A. To reduce eddy currents under ac
   B. To raise the speed
   C. To increase the dc resistance
   D. To hold the carbon brushes
   Answer: A. Alternating flux would cause large eddy-current loss in a solid field, so laminations are used. (Key point: Stator construction)

4. What happens to speed with load, and what is the danger?
   A. Speed is constant at all loads
   B. Speed is high at no load and may reach dangerous values; it falls with load
   C. Speed rises with load
   D. Speed is the same on ac and dc
   Answer: B. As a series machine its speed varies strongly with load and can become dangerously high at no load. (Key point: Speed characteristic)

## Related in Electrical Machines

- [Hunting; loaded operation of synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-hunting-loaded-operation) — Electrical Machines, chapter 6
- [Series motor for high torque; compound motor characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aseries-motor-high-torque-compound-characteristics) — Electrical Machines I, chapter 4
- [Working of 3-point DC motor starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Athree-point-starter-working) — Electrical Machines, chapter 4, asked 2×
- [T-S characteristics; why linear portion operation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ats-linear-portion-operation) — Electrical Machines, chapter 5
- [Suitability of DC motors for applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-suitability-applications) — Electrical Machines, chapter 4

## Related in other subjects

- [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
- [Short note: Universal motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashort-note-universal-motor) — Electrical Machine, chapter 7
- [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×
- [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