Single stack stepper motor principle

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).

          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 $\beta = 360^\circ/(mN_r) = 360^\circ/(3 \times 4) = 30^\circ$).

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.