Stepper motor construction and working

A stepper motor is a brushless motor that converts electrical pulses into discrete angular steps of shaft rotation. Each pulse moves the shaft through a fixed step angle, so position can be controlled without feedback (open loop).

Types and construction

1. Variable reluctance (VR) stepper: the stator has several salient poles with windings (phases), and the rotor is a soft-iron toothed cylinder with no winding or magnet.
2. Permanent magnet (PM) stepper: the rotor is a permanent magnet.
3. Hybrid stepper: combines both; the rotor has toothed cups with a permanent magnet between them (step angles 0.9° to 3.6°).

Operating principle (variable reluctance type)

Take a 3-phase VR motor with 6 stator poles (phases A, B, C on pole pairs) and a 4-tooth rotor.

        A
     C     B'
   B   [ R ]   C'      stator poles with phases
     A'    ...         rotor: 4 teeth

1. When phase A is energised, a magnetic flux is set up and the rotor turns so that its nearest pair of teeth aligns with the A poles (minimum reluctance position).
2. Phase A is switched off and phase B is energised: the nearest rotor teeth now align with the B poles, so the rotor steps through the step angle.
3. Then C is energised, and the sequence A-B-C repeats for continued rotation. Reversing the sequence reverses the direction.

Step angle:

$$\beta = \frac{N_s - N_r}{N_s N_r}\times360^\circ \quad\text{or}\quad \beta = \frac{360^\circ}{m N_r}$$

For $N_s = 6$, $N_r = 4$, $m=3$: $\beta = \frac{360}{3\times4} = 30^\circ$.

Speed (rpm) $= \frac{\beta\times f_{pulse}\times 60}{360}$, so the speed depends on the pulse rate.

Features and uses

Accurate positioning, no feedback, good holding torque at standstill. Used in printers, robotics, CNC machines, disk drives, and camera positioning.