Count stator and rotor teeth, not phases
Randy Frank, Contributiing Editor — Design News
Before choosing between a two-phase or five-phase step motor, take a closer look at the application’s requirements and the motor’s capabilities. More is not always better, but more phases certainly cost more.
To compare these two mechanically different step motors, Lin Engineering ran both motors at a step resolution that provided a microstep equivalent of 0.018 degree. For a two-phase, 0.9-degree motor, this meant driving at 50 microsteps and for a five-phase, 0.72-degree motor it required 40 microsteps. Surprisingly, the two-phase motor produced four to five times less step error than the five-phase motor.
This situation occurs because the accuracy of a step motor depends upon the number of stator and rotor teeth. Although most two-phase, 0.9-degree step-motor designs use eight or 16 stator poles, typical five-phase motors use 10 stator poles. Lin’s 12-stator pole design used a two-phase stepper for this evaluation. With more stator poles, there are more stator and rotor teeth within the motor. As the test data shows, this enables the two-phase motor to produce higher accuracy than the five-phase motor.
“One of the main reasons why someone would be using a five-phase system is to obtain reduced torque ripple,” notes Mindy Lin, an application engineer at Lin Engineering. “When people compare two-phase to five-phase, they are usually comparing at full stepping.” Although the five-phase motor has lower torque ripple than the two-phase, users typically would be microstepping and not full stepping in an application. The demonstrated reduction in error for the two-phase stepper in the microstepping mode certainly warrants a closer evaluation of the two-phase stepper for critical applications. D_More Info
Recent stepper-driver techniques have reduced the torque ripple in two-phase motors, allowing their use in applications that previously would have dictated the use of a five-phase stepper.
One of the inherent advantages of the two-phase stepper motor is lower heat dissipation. A two-phase driver uses eight power transistors to switch the phases instead of the 10 transistors the five-phase driver requires. If heat is an issue in the application, the two-phase driver is slightly better, because it uses fewer transistors and requires less heat-sinking.
So, unless the application is extremely sensitive to torque ripple, which would require the five-phase stepper, the two-phase motor, with the right driver, may step up to meet accuracy, thermal and cost requirements.