Re: Running Unipolar steppers with a Bi-polar driver?

Hello Tim,

True, I was thinking of bipolar series when I mention TWICE the
resistance and FOUR times the inductance (when compared to a unipolar
drive) (bipolar parallel inductance will be the SAME as unipolar
inductance, while bipolar parallel resistance will be HALF that of
unipolar resistance). Although the inductance is 1/4 in bipolar
parallel as opposed to bipolar series, the INDUCTIVE TIME CONSTANT
remains the same for both bipolar series & parallel, while unipolar
is about half of that. Thus, you still suffer at higher speeds for
both bipolar series & bipolar parallel with respect to unipolar,
while you gain the 1.414 power advantage at lower speeds.
Bipolar drives are usually more expensive than unipolar drives, so
why anyone would use a bipolar drive and run the motor in unipolar
mode(only use 1/2 the copper) is beyond me. Nevertheless, it could
happen so I guess I should have taken that into consideration. Lesson
learned.
Finally, looking at a chart for bipolar parallel vs. bipolar
series, you can see they start out with essentailly the same amount
of torque (slow metal cutting speeds), while at higher speeds
(rapids, engraving & perhaps wood router speeds), the power rating of
bipolar series starts to taper off sooner. This becomes especially
evident at lower voltages, thus a bipolar drive with a higher voltage
capability can give better top end performance (while costing quite a
bit more too!). Probably the biggest concern between running in
bipolar series as opposed to bipolar parallel is that there seems to
be few chipset that can handle the higher current required to run in
bipolar parallel (often > 4 Amps). To get the higher current (like
when trying to get the higher voltage), you need to go to discrete
components. This really make the drive more complex, and the price
of the drive shoots up again.

Does all of this sound OK now?

Jeff
www.xylotex.com

--- In CAD_CAM_EDM_DRO@y..., "Tim Goldstein" <timg@k...> wrote:
> Jeff,
>
> Nice to see you on the CAD_CAM group.
>
> I have to dissagree with you a little bit on this. You are correct
that
> the inducance is 4X and the resistance double only if you connect
the
> motor as bipolar series. If you instead connect it as half coil your
> inductance and resistance is identical to a unipolar (you are
running
> through the same amount of coil) and if you connect it as parallel
your
> resistance is 1/2 and one of the electronic gurus (Help Mariss!!)
will
> have to quote on the inductance, but it is not 4X the half coil
> (unipolar) rating.
>
> So, depending how you connect it your running torque can vary. Just
take
> a look at the torque curves for parallel and series connetion that
many
> of the motor makes have and you will quickly see the difference.
>
> Tim
> [Denver, CO]
> Sherline products at Deep Discounts www.KTMarketing.com/Sherline
>
>
> > -----Original Message-----
> >
> > Hello Peter,
> > When you run stepper motors in bipolar mode you will be driving
> > twice the amount of copper (compared to unipolar). This turns
into
> > TWO times the resistance, and FOUR times the inductance the drive
has
> > to overcome, thus the time to fully energize the coils will
DOUBLE.
> > As you start getting faster and faster step rates, your drive
doesn't
> > have time to fully charge/discharge the current in the coils to
the
> > proper level. Thus at HIGHER step rates bipolar can end up
having
> > less torque than unipolar (given equivalent voltages for both
> > drives).
> > The good news is that the holding torque, or torque at slower
step
> > rates will be about 1.414 times MORE than a unipolar drive. This
is
> > where most of the power is really needed; when cutting metal at
the
> > slower, non-rapid feed rates.
> > If you can find datasheets for your motor, they might show
torque
> > curves for both unipolar and bipolar. You can go from there when
> > deciding on whether to choose a bipolar or unipolar drive.
> >
> > Jeff
> > www.xylotex.com
> > The XS-3525/8S-3 is a bipolar 3 Axis 2.5Amp 35Volt Stepper Driver
> > board