Re: Stepper questions

The two step motor phase windings don't share a common flux path and
thus don't interact. Proof: try using a 4-wire motor as a
transformer. Put a function generator on one winding (the "primary"),
a scope on the other winding (the "secondary"). You will see nothing
on the scope.

------------------------------

Unipolar drives have some severe drawbacks in high performance
designs. It is because a single center-tapped winding DOES act as a
transformer and the consequences are all bad:

1) The center-tap goes to +V supply. When one end-wire is taken to
GND by a transistor (the "on" one), the other end-wire goes to twice
the +V supply. This voltage is seen by the "off" transistor.

Transistors in an 80V bipolar drive see 80V stresses, transistors in
an 80V unipolar drive see at least 160V stresses.

2) Inductive energy recovery. The "off" center-tap to end-wire
winding carries (L*I^2)/2 energy when turning off. There is no easy
means of returning it to the supply, so it must be dissipated
(wasted). Usually zener clamps do this dirty work.

A bipolar drive easily returns this energy back to the power supply
for re-use.

3) Circuit topology. A high-performance (switching type) unipolar
drive requires 3 transistors and 3 power diodes per winding to
efficiently handle energy return. However the transistors still
see "twice supply voltage" stresses.

At this point the unipolar drive requires 12 power devices vs. a
bipolar drive's 8, the unipolar drive transistors see twice the
voltage of those in a bipolar drive and 6 motor wire connections are
required vs. 4 for a bipolar drive. All that to get the same
performance as a bipolar drive.

4) Failure modes. A failed transistor in a high performance unipolar
drive cooks the motor. A failed transistor in a bipolar drive shorts
the supply, blows a fuse and spares the motor.

Most designers ask "All that for what?" Then they discard the
unipolar drive topology and go for the bipolar one.

--------------------

If anyone is interested, I can try to dig up a high performance
unipolar drive power section design and post it. Something I designed
and we sold about 30 years ago. I may still have it around on some
old 10MB hard-drive.:-)

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Alan Marconett" <KM6VV@...>
wrote:

>
> HI Dave,
>
> That's an interesting point! I don't recall reading anything like

that, I

> could be wrong. Basically the windings do not interact as far as I

know.

> Actually, one would think that "turning around" a winding, as in a

bipolar

> drive would take longer. The field would have to collapse before

it could

> build up in the opposite direction. Therefore wouldn't a unipolar

drive,

> with its four windings have the advantage? And I'd think that the

unipolar

> stepper would have the further advantage of having two windings

available

> for microstepping. Half-stepping of a unipolar stepper is

relatively easy!

> Just alternate one winding on and two windings on! Ever see a 5

phase

> stepper? I still haven't figured out the stepping sequence for

that, but

> they do 400 steps!
>
> I'd like to see the "dual" specs for such a stepper. I'm guessing

that they

> rely on a difference in current, and possibly some dissipation

differences

> between the two modes. Same current * turns = same torque!
>
> I have the opinion that it's better to use ALL of the windings

available.

> But maybe it doesn't matter.
>
> Alan KM6VV
>
>
>
> > > A bipolar drive will provide more torque to a given motor than
> > > unipolar. This is why we see the "same" size 23 double stack

3amp, 1

> > > ohm motor nowadays labeled with either 280 oz.in. (bipolar) or

200

> > > oz.in. (unipolar).
> > >
> > >
> > I was under the assumption that on a uni-polar, one coil with a
> > decaying charge prevents the other coil from attaining the same

power

> > as fast as a bi-polar drive ?
> >
> > Dave
> >
>