How does a stepper motor move in commanded steps?

A stepper motor uses an ordered set of magnetic field states to establish successive rotor positions. The stationary windings form the stator, while the rotor turns. Energizing a winding makes a preferred magnetic alignment, toward which the rotor experiences torque.

The simplified illustration has four alignment directions produced by two orthogonal windings. Labels A+ and A− mean opposite current directions in winding A; B+ and B− do the same for winding B. They are state labels, not four unrelated power connections.

The one-phase-at-a-time sequence A+, B+, A−, B− advances the preferred alignment counterclockwise by 90 degrees each time in this model. From A+, one advance selects B+ at 90 degrees; two select A− at 180 degrees. Reversing the order commands clockwise travel instead.

This discrete field sequence differs from the brushed DC motor's mechanically switched coil. A stepper driver selects winding states electrically, and holding a state can provide a holding tendency. Position inferred from command count is valid only while the rotor successfully follows those commands.

Real motors usually obtain different, often much smaller, step angles through their magnetic construction and drive pattern. Load, acceleration and available torque can cause missed steps, so command count is not universal proof of actual position. The illustration assumes ideal following and distinguishes cumulative travel from the orientation repeated every full revolution.