--- title: "Single stack stepper motor principle" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:single-stack-stepper-motor" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-stack-stepper-motor site: BCT Engineering --- # Single stack stepper motor principle Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2073 Chaitra, 6 marks; 2069 Chaitra, 6 marks (asked 2 times) - Video: [Variable Reluctance stepper Motor - Stepper Motor - Electrical Machines 1](https://www.youtube.com/watch?v=uMRfUF7pCfM) — Ekeeda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-stack-stepper-motor - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain operating principle of single stack stepper motor. ## Short answer A single-stack VR stepper motor has a salient-pole stator with phase windings and a toothed soft-iron rotor. Energizing phases sequentially pulls the rotor to minimum reluctance positions. For a 6-stator-pole, 4-rotor-tooth motor, step angle is 30°, computed as (Ns-Nr)/(Ns*Nr)*360°. It runs by A-B-C sequence. ## Answer A **single-stack variable-reluctance (VR) stepper motor** has one stator and one rotor stack; the rotor moves in fixed steps because it always aligns itself to the position of **minimum reluctance** with the energised stator phase. #### Construction - **Stator:** laminated, with salient poles; each pair of opposite poles carries a phase winding (e.g. 3 phases A, B, C on 6 poles). - **Rotor:** laminated soft iron with teeth, **no winding**. The number of rotor teeth $N_r$ differs from stator poles $N_s$ (e.g. 6 stator poles, 4 rotor teeth). ```text A ___|___ C' / __ \ B | [rotor] | 6 stator poles (A,B,C, B \___ ___/ C' A',B',C'), 4 rotor teeth | A' ``` #### Operating principle (3-phase, 6/4 motor) 1. **Phase A energised:** flux flows A-A'. The rotor turns so two of its teeth align with poles A and A' (minimum reluctance). Rotor is held there. 2. **Phase A off, B on:** the nearest rotor teeth are pulled into line with B-B'. The rotor turns by one step. 3. **B off, C on:** another step in the same direction. 4. Repeating A-B-C-A gives continuous stepping clockwise; the sequence A-C-B-A gives anticlockwise rotation. Step angle: $$ \beta = \frac{N_s - N_r}{N_s N_r} \times 360^\circ = \frac{6 - 4}{6 \times 4} \times 360^\circ = 30^\circ $$ (equivalently $\beta = 360^\circ/(mN_r) = 360^\circ/(3 \times 4) = 30^\circ$). | Step | Phase on | Rotor position | |---|---|---| | 1 | A | $0^\circ$ | | 2 | B | $30^\circ$ | | 3 | C | $60^\circ$ | | 4 | A | $90^\circ$ | #### Modes and features - **Half-stepping** (A, AB, B, BC, ...) halves the step angle to $15^\circ$. - Speed $= \dfrac{\beta \times f}{360^\circ} \times 60$ rpm, where $f$ = pulses per second. - No permanent magnet, so there is **no detent torque** when unenergised; rotor inertia is low, so response is fast. #### Applications Printers, plotters, X-Y tables, machine tools, floppy disk drives and robotics. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A single-stack VR stepper moves in steps because the rotor: A. Aligns with the energised stator phase at minimum reluctance B. Is pulled by a permanent magnet in the stator C. Slips behind a rotating field D. Is driven by brushes Answer: A. Energising a phase pulls the nearest rotor teeth into alignment with it, the minimum-reluctance position. (Key point: Principle) 2. For a 3-phase 6/4 VR stepper, the step angle is: A. 15 degrees B. 30 degrees C. 60 degrees D. 90 degrees Answer: B. beta = 360/(3 x 4) = 30 degrees, also (6-4)/(6x4) x 360. (Key point: Step angle calculation) 3. How is the rotation direction reversed? A. Reverse the phase sequence from A-B-C to A-C-B B. Reduce the pulse rate C. Reverse the supply polarity of all phases D. Remove one tooth Answer: A. The switching sequence determines direction. (Key point: Direction) 4. Half-stepping (A, AB, B, BC ...) in this motor gives a step angle of: A. 60 degrees B. 15 degrees C. 7.5 degrees D. 30 degrees Answer: B. It halves the 30 degree step to 15 degrees. 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