Re: [CAD_CAM_EDM_DRO] unipolar or bipolar?

> what is the difference? what are the benefits
> and, or drawbacks of either
> thanks kevin

This is a big topic. The following is the simple version. I urge you to
search the web for more details. Here's a fair link on steppers:
http://www.cs.uiowa.edu/~jones/step/

The simple answer is torque. Given the same stepper motor with the same
voltage and currrent, you'll get higher torque when driven bipolar. Around
40% more if memory serves.

You'll also waste less power as heat.

Unipolar drives are easier to build. They only require 4 switching
transistors per motor.

Bipolar requires an H bridge for each of the two motor windings. So that's 8
switching transistors per motor. Now since you are in the H bridge world,
you have a new set of problems to deal with. If you use bipolar ( there's
that word again ) transistors, you'll waste a lot of energy as heat due to
the higher on resistances of this style transistor. When you move to FET
type switching transistors, you have the problem of the high side drive if
you use all N type fets or negative gate voltage if you use P type fets for
the high side. There's lots of other issues like shoot through, dead band
and all the goodies that makes H bridge design not quite as simple as a
unipolar drive.

Typically Unipolar drives will also cut corners on things like current
limiting. To get good responce from a stepper motor, you want to supply it a
lot more voltage than the motor is rated for and limit the current. Bipolar
drives usually do this with a current sensing resistor or coil and feed that
to a comparator to shut down ( aka chop ) the drive current. Most unipolar
drives just hang a big current limiting resistor in series with the motor
supply and that resistor will get HOT!

So, why waste all that mechanical energy your stepper is able to supply?
Just get a bipolar drive and be done with it. But you won't wire up a
bipolar drive on your kitchen table in 20 minutes from scratch. That you can
do with a unipolar drive.

later,
Larry E.