---
title: "Hysteresis motor construction and principle"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:hysteresis-motor"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/hysteresis-motor
site: BCT Engineering
---

# Hysteresis motor construction and principle

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2079 Bhadra, 3.5 marks; 2076 Chaitra, 3 marks (asked 2 times)
- Video: [Hysteresis Motor (Construction, Working & Applications) | TECH ELECTRIC |](https://www.youtube.com/watch?v=4MB4e9ai3ms) — TECH ELECTRIC
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/hysteresis-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Describe the construction and working principle of hysteresis motor. (Write a short note on hysteresis motor.)

## Short answer

A hysteresis motor has a smooth hard-steel rotor with high retentivity. The rotating field magnetizes it, and hysteresis lag produces constant torque from standstill to synchronism; at synchronous speed it locks as a permanent-magnet rotor. It runs silently and is used in clocks, tape recorders, and timing devices.

## Answer

A **hysteresis motor** is a single-phase synchronous motor whose rotor is a smooth cylinder of hard magnetic material; it develops torque due to **hysteresis** in the rotor, and runs at synchronous speed silently.

#### Construction

- **Stator:** split-phase, capacitor or shaded-pole winding that produces a rotating field.
- **Rotor:** a smooth cylinder of **high-retentivity hard steel** (e.g. cobalt or chrome steel) with large hysteresis loop, mounted on a non-magnetic arbour. No teeth, no windings.

```text
   stator (rotating field)
   _____________________
  |  ________________   |
  | | hard steel ring|  |
  | |  (no slots)    |  |
  | |_____arbour_____|  |
  |_____________________|
```

#### Working principle

1. The rotating stator field magnetises the rotor. Because of **hysteresis**, rotor magnetisation lags the stator field by a hysteresis angle $\alpha$; this lag produces a torque (hysteresis torque) that is **constant from standstill to synchronous speed**.
2. Eddy currents in the rotor also add torque below synchronous speed.
3. At synchronous speed, the rotor becomes permanently magnetised and runs locked to the field as a permanent-magnet synchronous motor.

#### Features and applications

- Smooth, quiet, vibration-free; constant torque during run-up; can synchronise any load it can accelerate.
- Used in electric clocks, tape recorders, record players and timing devices.

## Check yourself

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

1. The rotor of a hysteresis motor is:
   A. A smooth cylinder of high-retentivity hard steel with no teeth or windings
   B. A slotted squirrel cage
   C. A wound rotor with slip rings
   D. A toothed soft-iron rotor
   Answer: A. A smooth ring of hard magnetic material (cobalt or chrome steel) with a large hysteresis loop is used. (Key point: Rotor construction)

2. The torque of a hysteresis motor arises because:
   A. The rotor magnetisation lags the stator field by the hysteresis angle
   B. The rotor has a DC winding
   C. The rotor has salient poles
   D. The brushes commute the current
   Answer: A. The lag of rotor magnetisation behind the rotating stator field produces hysteresis torque. (Key point: Source of torque)

3. The hysteresis torque from standstill to synchronous speed is:
   A. Zero
   B. Constant
   C. Proportional to speed
   D. Maximum only at synchronous speed
   Answer: B. Hysteresis torque is constant, so the motor can synchronise any load it can accelerate. (Key point: Torque during run-up)

4. At synchronous speed the hysteresis motor behaves like:
   A. A permanent-magnet synchronous motor, with the rotor permanently magnetised
   B. A DC series motor
   C. An induction motor with slip
   D. A universal motor
   Answer: A. The rotor becomes permanently magnetised and locks to the field. (Key point: Running at synchronous speed)

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