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

# Switched reluctance motor construction and principle

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2083 Baisakh (new course), 3 marks
- Video: [reluctance motor | switched reluctance motor | Industrial and automation applications | machine](https://www.youtube.com/watch?v=zob0SjIEA4s) — Mind of Engineer 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/switched-reluctance-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Discuss the construction and working principle of switched reluctance motor.

## Short answer

The motor has a salient-pole laminated stator with concentrated coils and a simple salient-pole steel rotor without windings or magnets. Torque arises from rotor aligning to minimise reluctance when a phase is energised. Unipolar currents are switched sequentially by an electronic converter using rotor position feedback.

## Answer

A **switched reluctance motor (SRM)** is a doubly salient motor in which torque is produced by the rotor's tendency to move to the position of **minimum reluctance**. The phases are switched on in sequence by an electronic converter using rotor-position feedback.

#### Construction
- **Stator:** laminated, salient poles with **concentrated coils**; diametrically opposite coils form one phase.
- **Rotor:** laminated steel with salient poles, **no winding, no magnet, no brushes**.
- Stator and rotor pole numbers differ, e.g. 6/4 (3-phase) or 8/6 (4-phase).
- A **position sensor** and an asymmetric half-bridge converter feed each phase.

```text
   6/4 SRM          phase A on:
     A               rotor poles
   /   \             pulled into line
  C'     B'          with A-A'
  |  [R]  |          then B on -> next step
  B       C
   \   /
     A'
```

#### Working principle
1. Phase A is energised when a pair of rotor poles is approaching the A poles (unaligned position).
2. The rotor turns to the aligned (minimum reluctance, maximum inductance) position.
3. Phase A is switched off and phase B switched on; the rotor keeps moving. Repeating the sequence gives continuous rotation.

$$
T = \frac{1}{2}i^2\frac{dL}{d\theta}
$$

Torque does not depend on current direction, so unipolar current is enough; reversing the phase sequence reverses rotation.

**Features/uses:** simple, rugged, cheap, high speed and fault tolerant, but noisy with torque ripple. Used in fans, washing machines, vacuum cleaners, pumps, electric vehicles and aerospace drives.

## Check yourself

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

1. What is special about the SRM rotor?
   A. It has a cage winding
   B. It has permanent magnets
   C. It is laminated salient steel with no winding, magnet or brushes
   D. It carries a DC field winding
   Answer: C. The rotor is just laminated salient-pole steel, which makes the machine simple and rugged. (Key point: Rotor construction)

2. How does an SRM produce torque?
   A. Rotor tends to move to the position of minimum reluctance
   B. Induced rotor currents interact with the stator field
   C. Commutator reverses armature current
   D. Permanent magnets align with the stator field
   Answer: A. The energised phase pulls the rotor poles into alignment (minimum reluctance, maximum inductance). (Key point: Torque principle)

3. With T = (1/2) i^2 dL/dθ, why is unipolar current enough?
   A. Torque depends on i^2, not on current direction
   B. Torque depends on i linearly
   C. The rotor has magnets
   D. Inductance is constant
   Answer: A. Because torque goes with i^2, reversing the current does not reverse torque; direction is reversed by changing the phase sequence. (Key point: Torque equation and current direction)

4. When should phase A be energised, and what happens next?
   A. At the aligned position, then the rotor stops
   B. As rotor poles approach it (unaligned); rotor aligns, then A is switched off and B on
   C. At all times together with B
   D. Only at standstill
   Answer: B. Each phase is switched on when rotor poles approach and switched off after alignment, and the next phase continues the motion. (Key point: Switching sequence)

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