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What Is A Unipolar Step Motor Drive?

Oct
28
by Jeff Kordik

What is a unipolar step motor driver?  How does a unipolar driver compare to a bipolar step motor driver? When step motors first became popular as a simple, inexpensive means to control position and speed, the transistors required to drive them were very expensive.  What, transistors expensive?  Don’t they put, like, a billion of them on a chip?

transistor  

Sure, now they do, but go back, way back, before Friends and Seinfeld, before Charlie’s Angels and Fantasy Island, even before WKRP in Cincinnati. (Did you wonder how I was going to work a Herb Tarlek reference into this?)  In the 1960’s and 1970’s power transistors were large and expensive.  Using four transistors to drive a motor instead of eight constituted a huge savings.  Today, most motors are driven with eight transistors in a bipolar driver, so called because each motor wire can have current flowing into or out of it.  This is an animation of a typical modern bipolar step motor driver moving the motor at a slow speed.

bipolar step motor driver

The number of transistors required can be cut in half if you use a unipolar driver.  To accomplish this, the coils of most early step motors were wound with parallel strands of magnet wire in what came to be known as the “bifilar” winding.   The end of one bifilar pair is connected to the opposite end of another, creating a “center tap” that gets connected to a DC power supply.  The remaining winding ends get switched to power supply ground by transistors as required to position the motor.

unipolar step motor driver

Because of this legacy, many motors now come with bifilar windings with all eight wires brought out so that the motor can be driven unipolar, or bipolar.  But wait, there’s more: with eight wires, you can connect the individual windings to your driver in parallel or series.  Parallel connections reduce inductance and back emf, so the motor can run faster than a series connected motor from the same voltage.  But a parallel connection requires twice the winding current, so sometimes series connections are preferred, especially if the maximum speed requirement is low.  Most eight wire motors include specifications for unipolar, bipolar series and bipolar parallel operation.

step motor specifications

Note that the unipolar torque rating is lower than bipolar.  That’s because the unipolar connection uses only half of each winding at any given time, which decreases torque. The chart below compares the speed torque characteristics of a NEMA 23 step motor driven all three ways.  The performance of the unipolar scheme is simply inferior to bipolar operation.  So if you’re shopping for a step motor driver, with the best performance look for a bipolar drive.

step motor torque curve

   
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