RE: [CAD_CAM_EDM_DRO] Re: Stepper questions

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