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

# Stepper motor construction, principle and applications

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2073 Shrawan, 6 marks
- Video: [How Stepper Motors Work - Electric motor](https://www.youtube.com/watch?v=09Mpkjcr0bo) — The Engineering Mindset
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/stepper-construction-applications
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Explain the construction and working principle of stepper motors. Also give some of its applications.

## Short answer

A stepper motor has a toothed stator and rotor; phase windings are energised sequentially, causing the rotor to align with the minimum reluctance position in discrete steps. Types: variable reluctance, permanent magnet, hybrid. Typical applications: printers, CNC machines, 3D printers, quartz clocks.

## Answer

A **stepper (stepping) motor** is a brushless, synchronous-type motor that moves in fixed angular steps, one step for each input pulse. It is used for accurate open-loop position control.

#### Construction
Three common types:

| Part | Variable reluctance | Permanent magnet | Hybrid |
|---|---|---|---|
| Stator | Salient poles with phase windings | Salient poles with windings | Toothed poles with windings |
| Rotor | Toothed soft iron, no winding | Cylindrical permanent magnet | Axial magnet between two toothed iron cups |
| Step angle | 7.5°–30° | 30°–90° | 0.9°–5° (often 1.8°) |
| Detent torque | No | Yes | Yes |

```text
  VR motor: 6 stator poles, 4 rotor teeth
          A
        __|__
   C' /  ___  \ B'
     |  | R |  |     A-A', B-B', C-C' = phases
   B  \ |___| / C    rotor teeth pull into line
        --|--        with excited phase
          A'
```

#### Working principle
- Stator phases are energised one at a time (or two at a time) in sequence by a driver circuit controlled by digital pulses.
- **VR type:** the rotor turns to the position of **minimum reluctance**, aligning its nearest teeth with the excited poles.
- **PM / hybrid type:** rotor magnet poles align with the opposite stator poles.
- When the next phase is excited, the field axis shifts by one step, and the rotor follows.

Step angle:

$$
\beta = \frac{N_s - N_r}{N_s N_r}\times 360^\circ
$$

For $N_s = 6$, $N_r = 4$: $\beta = 30^\circ$, so 12 steps per revolution.

**Excitation modes:** full step (one phase on), two-phase on (higher torque), half step (step angle halved), and microstepping.

**Key relations:** shaft angle = $\beta \times$ number of pulses; speed $n = \dfrac{\beta f}{360^\circ}\times 60$ rpm for pulse rate $f$.

#### Applications
- Printers, plotters, scanners, disk drives
- CNC machine tools, 3D printers, robots, X–Y tables
- Quartz clocks and watches, cameras
- Medical and scientific instruments, valve control
- Satellite and antenna positioning

## Check yourself

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

1. The rotor of a variable reluctance stepper is:
   A. A cylindrical permanent magnet
   B. Toothed soft iron with no winding
   C. A wound rotor with slip rings
   D. A squirrel cage
   Answer: B. VR steppers use a toothed soft-iron rotor and have no detent torque. (Key point: VR rotor)

2. Which type gives the smallest step angles (about 1.8 degrees)?
   A. Variable reluctance
   B. Permanent magnet
   C. Hybrid
   D. All the same
   Answer: C. Hybrid motors combine a magnet and toothed rotors, giving 0.9-5 degree steps. (Key point: Hybrid step angle)

3. In a VR stepper the rotor turns to the position of:
   A. Maximum reluctance
   B. Minimum reluctance
   C. Zero flux
   D. Maximum slip
   Answer: B. The rotor aligns teeth with the excited poles to minimise reluctance. (Key point: Rotor motion)

4. Shaft speed of a stepper depends on:
   A. Step angle and pulse rate: n = (beta f / 360) x 60 rpm
   B. Supply voltage only
   C. Load only
   D. Number of brushes
   Answer: A. Speed is proportional to pulse rate, and the shaft angle equals beta times the number of pulses. (Key point: Speed relation)

## Related in Electric Machines II

- [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_5%2Felectric_machines_ii%3Asingle-stack-stepper-motor) — Electric Machines II, chapter 3, asked 2×
- [Schrage motor construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-construction) — Electric Machines II, chapter 3
- [Hysteresis motor construction and principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ahysteresis-motor) — Electric Machines II, chapter 3, asked 2×
- [Switched reluctance motor construction and principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aswitched-reluctance-motor) — Electric Machines II, chapter 3
- [Reluctance motor construction and principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Areluctance-motor) — Electric Machines II, chapter 3, asked 3×

## Related in other subjects

- [Stepper motor construction and working](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Astepper-motor-construction-principle) — Electrical Machines, chapter 7, asked 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
- [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
- [LVDT construction, working and applications](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Alvdt-construction-working-principle) — Instrumentation and Measurement, chapter 5