Re: Stepper questions

Hello,

"Compared to what" is often a good question...

Early Cam-Tech sign routers 4x4 and 4x8 use(d) L/R unipolar drives
and still go 240IPM with a 3HP router head. Sure they waste
electical energy and heat up, but 240 is plenty fast to make signs
and cut sheet stock...

Not to mention that in th eevent of failure--though the only failure
I've seen with these is corroded wiring from salt air!--these low-
tech drives are EASY AND INEXPENSIVE to repair.

So, "compared to what" is needed to do the work required, these
drives are an excelelnt fit.

"Compared to what" is possible with newer drives, these fall far
short.

Ballendo

P.S. BTW, these machines use old single stack 34 steppers. 150 oz.in.
Many hundreds were sold worldwide and they still are giving good
service in hundreds of shops.


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

>
> Hi Mariss,
>
> Thanks for the comments. And I'd like to see the early high

performance

> unipolar design!
>
> All of the circuits we used (dark ages) were decidedly LOW

performance. A

> few years ago I attempted to use one of the old drivers (L/R) with

an old

> SS50 NEMA #43 stepper to rotate a beam antenna (RDF), and I

couldn't get

> anywhere with it! I changed to a new design (CMD?) driver and

stepper, and

> it really spun!
>
> >
> > 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.
>
> That's a lot of reasons to use a bipolar drive. Several not so

obvious!

>
>
> > --------------------
> >
> > 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
> >
> >
>
> Best regards,
>
> Alan KM6VV
>