Stepper motor principle and applications

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.

 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