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Stepper motors come in many different varieties, but there are two main winding configurations for permanent magnet and hybrid stepper motors: unipolar and bipolar. Bipolar motors are currently more widely used, and some legacy systems are slowly phasing out unipolar stepper motors in favor of bipolar steppers. Nevertheless, unipolar stepper motors are still used in several applications such as textile machines, printers, Pachinko, and slot machines.
The DRV8xxx stepper drivers from Texas Instruments are primarily meant to drive bipolar stepper motors. But some of them can drive unipolar stepper motors as well. The following sections describe how various DRV8xxx drivers can be adapted to drive unipolar stepper motors.
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.
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.