Re: [CAD_CAM_EDM_DRO] Re: Xylotex or Gecko step driver for my Mill/Drill?

I've tested our new unipolar chopper and a Xylotex board on a PK266-02B (2A
6 wire) stepper and found no difference in performance at all 28VDC. Our
board did a bit better at 40VDC at the max step rate.

Regards,
Dave Rigotti
HobbyCNC.com

----- Original Message -----
From: "treadlemill" <ackland@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, May 16, 2004 10:06 PM
Subject: [CAD_CAM_EDM_DRO] Re: Xylotex or Gecko step driver for my
Mill/Drill?


> ---
> I tried to run a small lathe with a very popular, at the time, 5amp
> bipolar board with very dissappointing results. I used 6 wire
> (center to end tapped) surplus motors recommended and purchased from
> the supplier of the drive kit.
> I gave up with it for several years than changed to an allegro
> SLA7044 based unipolar board and the difference was astounding in
> both smoothness and speed.
> I cannot understand why some of the well intentioned advice I have
> read goes against what I have actually experienced. Could it be that
> many of the prejudices are based on the crude boards that were the
> only ones affordable for the hobbyist at that time?
>
> CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> >
> >
> > Roy J. Tellason wrote:
> >
> > >On Saturday 15 May 2004 11:05 pm, Jon Elson wrote:
> > >
> > >
> > >
> > >>>but I ruled that one out because it's a unipolar only board.
> > >>>
> > >>>
> > >
> > >
> > >
> > >>Good move, as most unipolar drives are very poor performers at
> higher
> > >>speeds
> > >>
> > >>
> > >
> > >Why is that?
> > >
> > >Would it have something to do with a bipolar setup energizing
> coils in the
> > >reverse direction rather than just turning them off?
> > >
> > >
> > >
> > Yup, that is basically the problem. With the typical unipolar
> drive,
> > the power
> > source is connected to the center tap, and transistors ground one
> end or the
> > other of the winding. When you ground one end, there is still
> circulating
> > current in the other half, fighting the field you are trying to
> build in the
> > newly powered winding. This energy has to go somewhere, and it
> ends
> > up being burned off in some diode-resistor arrangement. It is a
> hard
> > topology to harvest this energy constructively. If there is no
> resistor,
> > the current in the shut-off winding dies very slowly. If there is
> a
> > resistor,
> > then it gets quite hot at high step rates.
> >
> > With a bipolar drive, the power source is applied across the whole
> winding,
> > and is reversed to reverse the field. The field already in the
> winding
> > opposes the power source at the moment the transistors are
> switched,
> > and the reversed voltage across the winding collapses the existing
> field
> > and quickly builds it again in the opposite direction. The energy
> mostly
> > ends up back in the power supply, so there are no big resistors to
> burn
> > off the left-over field.
> >
> > There are ways around this, but they are generally more
> complicated than
> > the bipolar drive scheme. The only advantage to unipolar is it
> saves a
> > 4 transistors per motor.
> >
> > Jon
>
>
>
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