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

# Stepper motor principle and applications

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2075 Asoj, 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-motor-principle-applications
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Explain the operating principle of stepper motor and list their application.

## Short answer

A stepper motor converts electrical pulses into discrete angular steps. Stator phases are energized in a sequence, moving the field axis; the rotor aligns to minimum reluctance or magnetic poles. Step angle β = 360°/(m·N_r). Applications include printers, CNC machines, robotics, quartz watches, and 3D printers.

## Answer

A **stepper motor** is a brushless motor that converts electrical **pulses** into discrete angular movements called **steps**. Each pulse turns the shaft by a fixed angle, so position is controlled without feedback (open loop).

#### Operating principle
- The stator has several phase windings on salient poles. A drive circuit energises the phases one after another in a fixed sequence.
- The rotor (soft iron, permanent magnet or hybrid) always moves to the position of **minimum reluctance** (or aligns its magnet poles with the stator poles).
- When the next phase is energised, the field axis jumps by one step and the rotor follows it.

**Step angle:**

$$
\beta = \frac{360^\circ}{m N_r} \quad\text{or}\quad \beta = \frac{N_s - N_r}{N_s N_r}\times 360^\circ
$$

where $m$ = number of phases, $N_r$ = rotor teeth, $N_s$ = stator poles.

Example: a 3-phase VR motor with $N_s = 6$, $N_r = 4$ gives $\beta = \frac{6-4}{6\times4}\times360^\circ = 30^\circ$; 12 pulses make one revolution. Speed (rpm) $= \frac{\beta \times f}{6}$ where $f$ = pulses per second.

```text
 pulses  +---------+  A  +--------------+
 ------->|  logic  |---->|   stator     |
 (f pps) | sequenc |  B  |   phases     |--> rotor
 dir --->| + driver|---->|  A,B,C ...   |    steps
         +---------+  C  +--------------+
```

#### Types
1. **Variable reluctance (VR):** soft-iron toothed rotor, works on minimum reluctance.
2. **Permanent magnet (PM):** magnet rotor, larger step angles, detent torque.
3. **Hybrid:** magnet plus toothed rotor, very small step angles (e.g. $1.8^\circ$), high torque.

#### Modes of drive
One-phase-on (full step), two-phase-on (more torque), and half-step (alternating, halves the step angle); microstepping gives even finer steps.

#### Features
- Rotation angle proportional to number of pulses; speed proportional to pulse rate
- Holding torque at standstill; easy reversal by changing sequence
- No brushes; errors do not accumulate

#### Applications
- Computer printers, plotters, floppy/CD drives, scanners
- CNC machines, 3D printers, robotics
- Quartz watches, cameras (lens focus), medical equipment
- Valve and process control, X–Y tables, satellite antenna positioning

## Check yourself

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

1. What does each input pulse do in a stepper motor?
   A. Turns the shaft continuously
   B. Turns the shaft by a fixed angle (one step)
   C. Changes the supply frequency
   D. Reverses the rotor
   Answer: B. Each pulse produces a fixed angular step, so position is controlled open loop. (Key point: Stepper definition)

2. Why does the rotor follow when the next phase is energised?
   A. It moves to the position of minimum reluctance or aligns its magnet with the new field axis
   B. Brushes switch current to it
   C. It is pulled by induction currents
   D. Gravity returns it
   Answer: A. The field axis jumps one step and the rotor aligns with it (minimum reluctance or magnet alignment). (Key point: Operating principle)

3. A 3-phase VR stepper has N_s = 6 and N_r = 4. The step angle is:
   A. 15 degrees
   B. 60 degrees
   C. 30 degrees
   D. 90 degrees
   Answer: C. beta = (6-4)/(6x4) x 360 = 30 degrees, so 12 steps per revolution. (Key point: Step angle)

4. What does half-step mode do?
   A. Doubles the step angle
   B. Halves the step angle
   C. Removes holding torque
   D. Needs feedback sensors
   Answer: B. Alternating one-phase and two-phase excitation halves the step angle. (Key point: Half stepping)

## Related in Electric Machines II

- [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×
- [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2×
- [Shaded pole motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashaded-pole-motor) — Electric Machines II, chapter 3
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- [Schrage motor diagram and application](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-application) — Electric Machines II, chapter 3

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