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

Jeff,

Right data, wrong conclusion; the "inductive time constant" has
nothing to do with it.

Rather what matters is V/L. V is the voltage applied across
inductance L and is expressed in amps/second.

Say you have a 2A/phase (parallel connection) motor with 2 mH of
inductance and you are using a 40VDC supply. V/L is 20,000 amps/sec
and it will take 100 uS to go from 0 to 2A in the winding.

Re-connect the motor in series. It is now a 1A/phase, 8 mH motor. The
V/L is now 5,000 amps/sec and it will take 200 uS to go from 0 to 1A
now.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "jeffalanp" <xylotex@h...> wrote:
> 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