---
title: "Short note: Stepper motor"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:short-note-stepper-motor"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/short-note-stepper-motor
site: BCT Engineering
---

# Short note: Stepper motor

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2082 Baisakh, 4 marks; 2072 Chaitra, 3 marks (asked 2 times)
- 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/short-note-stepper-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Write a short note on stepper motor.

## Short answer

A stepper motor rotates in discrete steps for each input pulse, enabling open-loop positioning. Types include variable reluctance, permanent magnet, and hybrid. Step angle is 360°/(m*Nr), and it offers precise control, no cumulative error, and digital compatibility, used in printers, CNC machines, and disk drives.

## Answer

A **stepper (stepping) motor** is a brushless motor whose shaft rotates in discrete angular steps, one step for each input pulse, so position is controlled without feedback (open loop).

#### Construction and types

- Stator has salient poles with windings grouped into phases, energised in sequence by a digital driver.
- Rotor types: **variable reluctance** (soft iron, toothed), **permanent magnet**, and **hybrid** (PM + toothed).

#### Working

- When a phase is energised, the rotor turns to the position of minimum reluctance (or aligns its magnet) with that phase.
- Switching the next phase moves the rotor by one **step angle**:

$$
\beta = \frac{360^\circ}{m N_r} \quad (m = \text{phases},\ N_r = \text{rotor teeth})
$$

- Speed is proportional to pulse rate; direction depends on the switching sequence.

```text
 pulses --> [Logic/driver] --> phases A,B,C --> motor
             A -> B -> C -> A  : clockwise steps
```

#### Advantages and applications

- Precise positioning, no cumulative error, digital compatibility, holding torque at rest.
- Used in printers, plotters, disk drives, CNC machines, robots and camera lenses.

## Check yourself

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

1. A stepper motor rotates:
   A. Continuously at constant speed
   B. In discrete angular steps, one per input pulse, in open loop
   C. Only with feedback from an encoder
   D. Only at synchronous speed
   Answer: B. Each pulse moves the shaft by one step angle, allowing position control without feedback. (Key point: Definition)

2. The step angle of a stepper motor with m phases and Nr rotor teeth is:
   A. 360/(m + Nr)
   B. 360/(m Nr)
   C. 360 m/Nr
   D. 360 Nr/m
   Answer: B. beta = 360 degrees /(m Nr). (Key point: Step angle)

3. In a stepper motor, speed and direction depend on:
   A. Pulse rate and switching sequence
   B. Supply voltage only
   C. Load only
   D. Number of brushes
   Answer: A. Speed is proportional to pulse rate and the phase switching sequence decides direction. (Key point: Speed and direction)

4. Which is a stepper rotor type?
   A. Variable reluctance
   B. Wound rotor
   C. Squirrel cage
   D. Drag cup
   Answer: A. The types are variable reluctance, permanent magnet and hybrid. (Key point: Rotor types)

## Related in Electric Machines II

- [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
- [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

## 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