---
title: "Single stack stepper motor principle"
subject: "Electrical Machine"
chapter: 7
key: "semester_4/electrical_machine:stepper-motor-single-stack"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/stepper-motor-single-stack
site: BCT Engineering
---

# Single stack stepper motor principle

Electrical Machine · Chapter 7: Fractional Kilowatt Motors

- Asked in: 2069 Bhadra
- Video: [Variable Reluctance stepper Motor - Stepper Motor - Electrical Machines 1](https://www.youtube.com/watch?v=uMRfUF7pCfM) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/stepper-motor-single-stack
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7

## Question

Write about the working principle of a single stack stepper motor with neat diagram.

## Short answer

A single-stack VR stepper motor has a toothed soft-iron rotor. Sequential stator phase excitation causes the rotor to step to minimum reluctance positions. Step angle = 360°/(m·Nr). For 3 phases and 4 rotor teeth, step angle is 30°. Reversing sequence reverses direction.

## Answer

A stepper motor converts electrical pulses into discrete angular steps of the shaft. The single-stack variable-reluctance (VR) stepper motor has one stator stack and one toothed soft-iron rotor stack, and has no permanent magnet or winding on the rotor.

#### Construction (example: 3-phase, 6 stator poles, 4 rotor teeth)
```text
 Top view (stator poles in 3 pairs, rotor with 4 teeth)

           A1
      C2  [ ]  B1
       [ ]  +--+  [ ]       Rotor teeth: 4 (soft iron)
           | R |            A: poles A1-A2
       [ ]  +--+  [ ]      B: poles B1-B2
      B2  [ ]  C1          C: poles C1-C2
           A2
```
- Stator: laminated, with 6 poles. Opposite poles are wound in series to make a phase (A, B, C).
- Rotor: toothed laminated soft iron, 4 teeth, without winding.

#### Working principle
When a stator phase is energised, the rotor turns to the position of minimum reluctance, in which its teeth align with the energised stator poles. Phases are then excited in sequence by a controller (driver):

| Pulse | Phase on | Rotor aligned with |
|---|---|---|
| 1 | A | poles A1-A2, teeth 1-3 |
| 2 | B | poles B1-B2; rotor moves 30° |
| 3 | C | poles C1-C2; rotor moves another 30° |
| 4 | A | next tooth aligns; total 90° (one tooth pitch) |

Each pulse moves the rotor through one step. Reversing the phase sequence (A-C-B) reverses the direction.

**Step angle** β = 360° / (m × N_r), where m = number of phases and N_r = number of rotor teeth. Here β = 360 / (3 × 4) = **30°**.

**Speed:** N = β × (pulse rate in pulses/s) / 360 revolutions per second. Speed is proportional to the pulse frequency.

**Features:** open-loop position control (no feedback), good accuracy, holding torque when the coil is energised, low rotor inertia, but resonance can occur and the torque falls with speed.

**Uses:** printers, plotters, disk drives, robotics, CNC machines, clocks, and process control valves.

## Check yourself

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

1. Why does the rotor move when a stator phase is energised?
   A. It turns to the position of minimum reluctance, aligning its teeth with the energised poles
   B. It is attracted by a permanent magnet on the rotor
   C. Eddy currents push it
   D. The brushes switch the winding
   Answer: A. The VR stepper rotor has no magnet or winding and aligns to minimise reluctance. (Key point: Minimum reluctance principle)

2. What is the step angle of a 3-phase VR stepper with 4 rotor teeth?
   A. 90°
   B. 12°
   C. 45°
   D. 30°
   Answer: D. beta = 360/(m x N_r) = 360/(3 x 4) = 30°. (Key point: Step angle)

3. How is the direction of rotation reversed?
   A. By reversing the phase excitation sequence (A-C-B)
   B. By increasing the pulse rate
   C. By removing the rotor teeth
   D. By grounding one phase
   Answer: A. Exciting the phases in the reverse order moves the rotor the other way. (Key point: Direction reversal)

4. Which statement is correct about a stepper motor?
   A. Speed is proportional to pulse frequency and it works in open loop without feedback
   B. Speed is independent of the pulse rate and needs feedback
   C. It always needs a tachogenerator
   D. It cannot hold position
   Answer: A. N = beta x pulse rate/360 rev/s, and position control is open loop. (Key point: Speed and control)

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## Related in other subjects

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