Re: Why Bipolar?

Chris,

First we have to do some "assuming". We'll assume the unipolar is l/r
and the bipolar is chopper current control, since these are the
standard uses of these drive types. Keep in mind that it is possible
to "chop" unipolar also; it just isn't done much. For hi-speed;
unipolar chopped, with single phase on "wave" drive (full step) has
much to recommend it. But it is not so good as the speed goes down,
which is why it remains used in special circumstances.

Back to your question:

Now, one reason (most often cited) is economy of power usage. We
don't have the dropping resistors in the bipolar chopper drive, so it
is more efficient.

Second is speed. Most bipolar drives provide a "better" path for the
current "exiting" the coil. This means the current being "shoved" in
can get into the coil more quickly, which allows for faster cycles of
coil on-coil off. So we get better overall speed from the motor.

Third relates to the first. As the motors get bigger, and the power
supply voltage goes up, we pay a MUCH higher price (in wasted
electricity) for the unipolar setup than the bipolar chopper. the
watts rating needed for the dropping resistors gets quite high, and
now we have a little heater to go with our system. No charge :-) The
size enclosure needed to safely encase these hot resistors provides
another reason choppers have gained ground in recent years.

Hope this helps.

Ballendo

P.S. Dan Hudgins has a circuit in his "full kit" version of
Dancad/cam which uses capacitors to speed up a unipolar drive.

--- In CAD_CAM_EDM_DRO@y..., Chris Stratton <stratton@m...> wrote:
>
> If one is going to run a stepper half-coil as has often been
> recommended here in order to control inductance, what is the
> advantage of having a bipolar driver compared to unipolar?
<snip>