The most basic stepper motor consists of four coils (see image). Current applied to the two phases causes magnetic poles to form on the coils, directing the rotor magnet to a stable position between the two phases. This position is called a "full-step position". If the current direction is reversed at one of the phases, the rotor magnet rotates by 45° to the next full-step position, making a "step".
After four steps, one complete period is finished. In the simple 2-phase stepping motor of the example, a four-step period corresponds to one full revolution. In practice, high-pole stepping motors with up to 400 steps-per-revolution are used. The electrical conditions in every fourth step are the same. A high-pole stepping motor has several stable positions in one revolution.
The larger the step width, the higher the over- and undershoot. The stepper motor’s torque is generated by twisting the position of the rotor and the electromagnetic field of the stator up to a full step. The system torque is dependent on the load angle and rotor position.
Deviation from desired positions leads to restrictions in desired torque. As a result, large transients occur, particularly in the area of resonant frequency, leading to considerable torque derating. If the rotor exceeds a distance of more than two steps from the actual target position, it jumps to the next stable position (step loss).
Microstepping attempts to drive motors with a sine/cosine-like current waveform. Regardless of the waveform used, as microsteps become smaller, motor operations become smoother, greatly reducing resonance in both the motor and parts to which the motor is connected.