Unipolar stepper motors have two windings per phase. The common wiring configuration for unipolar motors is four wires connected to the motor windings (A+, A-, B+ and B-) and a center tap on each phase. The center taps can be electrically isolated; or they can be shorted electrically. The center taps are generally connected to the motor voltage supply, VM.

Figure 2-1 Unipolar Stepper Motor

Current flows from VM supply to only one half of a winding at any given time. The center tap allows the magnetic field to be reversed without having to reverse the direction of current in a winding. The tradeoff is that because only half of each winding is used at a given time, unipolar steppers result in lower torque and efficiency.

However, as current flows only in a single direction, the driver electronics for unipolar stepper motors can be simpler compared to bipolar stepper motors. Only four low-side MOSFETs and circulating diodes are sufficient to drive a unipolar stepper motor.