Stepper motor Controllers:
Uni-polar vs Bipolar
PWM or chopper stepper motor controllers come in two varieties, uni-polar and bipolar. Although uni-polar drivers are becoming less popular, there are still several uni-polar driver options available. For example, the CNC stepper motor driver kits by
HobbyCNC.com.
The stepper motor itself can be defined as uni-polar or bipolar, but that is a discussion for the
Stepper motor section.
Uni-polar vs Bipolar drivers
Uni-polar and bipolar refers to the winding arrangements for the most popular type of stepper motor in the CNC arena, the two-phase stepper motor. A discussion on stepper motors and phase arrangements again may be found in the stepper motor section. Uni-polar stepper motors have two coils/windings per phase. This is why they are sometime referred to as four-phase motors. The two windings per phase are for the polarity of the magnetic fields per phase. Okay, so what does that mean for the driver?
Because of this dual winding arrangement per phase, the driver does not have to switch the direction of current to reverse the magnetic pole. Of course a change in the magnetic pole is what causes any motor to rotate. Thus the circuit for the uni-polar stepper motor driver is relatively simple and can be made cheaper, making it popular among the hobby CNC router maker. The downside of uni-polar drivers is inefficiency. Because uni-polar drivers only energize one phase at a time, much of the supplied power is lost and becomes heat. The potential motor power output for a uni-polar driver is much less than that of a bipolar driver given the same inputs.
Bipolar two-phase stepper motors have one winding per phase. A bipolar driver must rapidly reverse the current through the windings to change polarity. As you can imagine this adds complexity to the driver. This is why bipolar stepper motor controllers are usually more expensive. However, bipolar drivers are increasing in popularity and availability driving the price down. Bipolar drivers are much more efficient and can provide greater toque output of a motor. An 8 wire NEMA 23 stepper motor driven on a uni-polar driver may produces 305 oz-in. of torque, the same motor on a bipolar driver will produce about 425 oz-in. On average, a bipolar driver can produce 30 to 40 percent more torque out of the same motor, than that of a uni-polar driver. In the next sections, the modes of operation and the features to look for when buying stepper motor controllers will be discussed.
Continue : Stepper motor Controllers- Modes of Operation
Return to: CNC Controllers
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