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From the Q&A

With TJ Byers


Stepper Motor Overview

Question:

Stepper motors are very confusing, or at least the stepper motor specs. I keep reading stuff like "hybrid" (as opposed to unipolar or bipolar). And then there is the famous statement: "Of course, this bipolar stepper motor can be driven as a unipolar motor, if desired." Well, I'm the first to admit that the "of course" business ain't the least bit obvious to me! Sheesh. Any light you can shed on this would be most appreciated.

E. Nicholas Cupery
via Internet


Answer:

This is a very large topic to take on in the space of this column, but I think I can make enough sense of it for your question. To begin with, hybrid is a type of stepper motor construction, not how the stepper operates. Basically, there are three different ways to build a stepper motor: variable reluctance, permanent magnet, and hybrid. The chart below highlights their differences.

Stepper Motor Type Rotor Stator Coil Type
Variable reluctance Iron vanes Electromagnetic coils Unipolar
Permanent Magnet Magnets Electromagnetic coils Unipolar/Bipolar
Hybrid Magnets Magnets/coils Unipolar/Bipolar


The terms unipolar and bipolar have to do with the coils associated with the stepper motors in the chart above. Bipolar stepper motors have just two coils with four wires. To operate the motor, the polarity of each coil must be changed in the same way you'd reverse the direction of a DC motor using a DPDT toggle switch, relay, or H-bridge (see "H-Bridge Defined" above). A unipolar motor also has two coils, but this time the coils are center-tapped, which results in six wires instead of four. The reason it's called unipolar is because you can tie the center tap high and alternately ground the ends of the coil to create the same effect as reversing the polarity across a bipolar (untapped) coil. Bipolar is two coils untapped; unipolar is two coils center-tapped. That's it!

Now that you know that "hybrid" refers to motor construction, and has nothing to do with unipolar or bipolar, let's up the ante! You can mix and match construction types with coil types — to an extent. For example, you can have a hybrid motor with either a bipolar or a unipolar coil arrangement. Same with the permanent magnet stepper. Variable reluctance steppers, on the other hand, are unipolar only. To answer your final question, "Of course, this bipolar stepper motor can be driven as a unipolar motor  ..." This refers to using a tapped coil as an untapped coil. That is, any tapped coil can be used in the bipolar mode by ignoring the tap. If the bipolar stepper has an unused tap, then it can be connected and operated in the unipolar mode. Of course, the unipolar stepper and the bipolar stepper require different controllers. Unipolar can work with half-bridge drivers while bipolar requires a full-bridge controller. Clear as mud, huh? Hope this helps.


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