---
title: "Stepper motor construction and working"
subject: "Electrical Machines"
chapter: 7
key: "semester_4/electrical_machines:stepper-motor-construction-principle"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/stepper-motor-construction-principle
site: BCT Engineering
---

# Stepper motor construction and working

Electrical Machines · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2078 Chaitra, 4 marks; 2076 Baisakh, 8 marks; 2069 Bhadra (asked 3 times)
- Video: [How Stepper Motors Work - Electric motor](https://www.youtube.com/watch?v=09Mpkjcr0bo) — The Engineering Mindset
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/stepper-motor-construction-principle
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7

## Question

Explain the construction and operating principle of a stepper motor.

## Short answer

A stepper motor uses a toothed stator and rotor; sequential phase energization aligns teeth, producing precise angular steps. Step angle depends on pole and tooth counts; used in printers, robotics, and CNC machines.

## Answer

A stepper motor is a brushless motor that converts **electrical pulses into discrete angular steps** of shaft rotation. Each pulse moves the shaft through a fixed step angle, so position can be controlled without feedback (open loop).

#### Types and construction
1. **Variable reluctance (VR) stepper:** the stator has several salient poles with windings (phases), and the rotor is a soft-iron toothed cylinder with no winding or magnet.
2. **Permanent magnet (PM) stepper:** the rotor is a permanent magnet.
3. **Hybrid stepper:** combines both; the rotor has toothed cups with a permanent magnet between them (step angles 0.9° to 3.6°).

#### Operating principle (variable reluctance type)
Take a 3-phase VR motor with 6 stator poles (phases A, B, C on pole pairs) and a 4-tooth rotor.

```text
        A
     C     B'
   B   [ R ]   C'      stator poles with phases
     A'    ...         rotor: 4 teeth
```
1. When phase A is energised, a magnetic flux is set up and the rotor turns so that its nearest pair of teeth aligns with the A poles (minimum reluctance position).
2. Phase A is switched off and phase B is energised: the nearest rotor teeth now align with the B poles, so the rotor steps through the step angle.
3. Then C is energised, and the sequence A-B-C repeats for continued rotation. Reversing the sequence reverses the direction.

Step angle:
$$
\beta = \frac{N_s - N_r}{N_s N_r}\times360^\circ \quad\text{or}\quad \beta = \frac{360^\circ}{m N_r}
$$
For $N_s = 6$, $N_r = 4$, $m=3$: $\beta = \frac{360}{3\times4} = 30^\circ$.

Speed (rpm) $= \frac{\beta\times f_{pulse}\times 60}{360}$, so the speed depends on the pulse rate.

#### Features and uses
Accurate positioning, no feedback, good holding torque at standstill. Used in printers, robotics, CNC machines, disk drives, and camera positioning.

## Check yourself

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

1. What does a stepper motor do?
   A. Converts electrical pulses to fixed angular steps
   B. Runs only with feedback
   C. Rotates continuously at constant speed
   D. Needs brushes and commutator
   Answer: A. Each input pulse moves the shaft through a fixed step angle, in open loop with no feedback. (Key point: Basic function)

2. What is the rotor of a variable reluctance stepper?
   A. A permanent magnet
   B. A wound rotor with slip rings
   C. A soft-iron toothed cylinder with no winding or magnet
   D. A squirrel cage
   Answer: C. VR steppers have a soft-iron toothed rotor which aligns to the minimum reluctance position. (Key point: VR rotor)

3. What makes the VR rotor move when phase A is switched off and B is energised?
   A. The rotor teeth nearest to the B poles align to give minimum reluctance
   B. The rotor is repelled from phase A
   C. The brushes change contact
   D. The supply frequency is reduced
   Answer: A. The rotor always turns to align its nearest teeth with the energised poles, a step at each switching. (Key point: Stepping principle)

4. What is the step angle of a 3-phase VR stepper with 4 rotor teeth?
   A. 90 degrees
   B. 45 degrees
   C. 15 degrees
   D. 30 degrees
   Answer: D. beta = 360/(m Nr) = 360/(3 x 4) = 30 degrees. (Key point: Step angle)

## Related in Electrical Machines

- [Stepper motor and servo motor principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Astepper-and-servo-motor-principle) — Electrical Machines, chapter 7
- [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5
- [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5
- [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4×
- [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×
- [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-working-torque-equation) — Electrical Machines, chapter 4, asked 2×

## Related in other subjects

- [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3
- [Single stack stepper motor principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-stack-stepper-motor) — Electric Machines II, chapter 3, asked 2×
- [Stepper motor principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-motor-principle-applications) — Electric Machines II, chapter 3
- [Single stack stepper motor principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Astepper-motor-single-stack) — Electrical Machine, chapter 7