Single stack stepper motor principle

A stepper motor converts electrical pulses into discrete angular steps of the shaft. The single-stack variable-reluctance (VR) stepper motor has one stator stack and one toothed soft-iron rotor stack, and has no permanent magnet or winding on the rotor.

Construction (example: 3-phase, 6 stator poles, 4 rotor teeth)

 Top view (stator poles in 3 pairs, rotor with 4 teeth)

           A1
      C2  [ ]  B1
       [ ]  +--+  [ ]       Rotor teeth: 4 (soft iron)
           | R |            A: poles A1-A2
       [ ]  +--+  [ ]      B: poles B1-B2
      B2  [ ]  C1          C: poles C1-C2
           A2

• Stator: laminated, with 6 poles. Opposite poles are wound in series to make a phase (A, B, C).
• Rotor: toothed laminated soft iron, 4 teeth, without winding.

Working principle

When a stator phase is energised, the rotor turns to the position of minimum reluctance, in which its teeth align with the energised stator poles. Phases are then excited in sequence by a controller (driver):

Each pulse moves the rotor through one step. Reversing the phase sequence (A-C-B) reverses the direction.

Step angle β = 360° / (m × N_r), where m = number of phases and N_r = number of rotor teeth. Here β = 360 / (3 × 4) = 30°.

Speed: N = β × (pulse rate in pulses/s) / 360 revolutions per second. Speed is proportional to the pulse frequency.

Features: open-loop position control (no feedback), good accuracy, holding torque when the coil is energised, low rotor inertia, but resonance can occur and the torque falls with speed.

Uses: printers, plotters, disk drives, robotics, CNC machines, clocks, and process control valves.