Stepper motor construction, principle and applications

A stepper (stepping) motor is a brushless, synchronous-type motor that moves in fixed angular steps, one step for each input pulse. It is used for accurate open-loop position control.

Construction

Three common types:

  VR motor: 6 stator poles, 4 rotor teeth
          A
        __|__
   C' /  ___  \ B'
     |  | R |  |     A-A', B-B', C-C' = phases
   B  \ |___| / C    rotor teeth pull into line
        --|--        with excited phase
          A'

Working principle

• Stator phases are energised one at a time (or two at a time) in sequence by a driver circuit controlled by digital pulses.
• VR type: the rotor turns to the position of minimum reluctance, aligning its nearest teeth with the excited poles.
• PM / hybrid type: rotor magnet poles align with the opposite stator poles.
• When the next phase is excited, the field axis shifts by one step, and the rotor follows.

Step angle:

$$\beta = \frac{N_s - N_r}{N_s N_r}\times 360^\circ$$

For $N_s = 6$, $N_r = 4$: $\beta = 30^\circ$, so 12 steps per revolution.

Excitation modes: full step (one phase on), two-phase on (higher torque), half step (step angle halved), and microstepping.

Key relations: shaft angle = $\beta \times$ number of pulses; speed $n = \dfrac{\beta f}{360^\circ}\times 60$ rpm for pulse rate $f$.

Applications

• Printers, plotters, scanners, disk drives
• CNC machine tools, 3D printers, robots, X–Y tables
• Quartz clocks and watches, cameras
• Medical and scientific instruments, valve control
• Satellite and antenna positioning