Re: [CAD_CAM_EDM_DRO] RE: Stepper questions

<snip>
The most common technique is to place diodes to return the current to the
power supply, but this basically leaves the coil short-circuited.
<snip>

From all I have read and all the schematics I have looked at, the porpose of
the diodes is to protect the power supply from harmful reverse current cased
by inductive kick-back as the field collapses. Shorting out the coil is
precisely the objective. Otherwise the switching transistors can become
reverse-biased and burn out.
--Phil M.
http://www.ohmikron.com/

----- Original Message -----
From: Jon Elson <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, May 03, 2006 10:05 AM
Subject: Re: [CAD_CAM_EDM_DRO] RE: Stepper questions


> Alan Marconett 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.
> >
> >
> The two halves of a stepper phase coil (on 6- and 8-wire motors) DO
> interact.
> They are usually wound on the same coil bobbin, and act as two windings of
> a transformer. If you have twice the turns, then the inductance is
squared.
> Because you only need half the current in full winding mode, it only
> takes twice
> the voltage to turn the current around in the same time. But, at the
> same voltage,
> it takes 4 times longer to turn the current around. That's why the
> half-winding
> scheme is often used on a 6-wire motor with a bipolar drive.
>
> >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?
> >
> No, because the current in the winding that is shut off has to decay,
> and that takes
> time, or infinite voltage standoff of the transistors. Second, the
> decaying of the
> "old" winding interferes with the filling up of the "new" winding, so it
> is the SAME
> problem as the bipolar faces, really.
>
> > 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!
> >
> >
> >
> You can do this with a bipolar drive, too. Just turn one winding
> completely off inbetween
> + and - polarities. Of course, you lose 33% of your torque that way.
>
> >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!
> >
> >
> >
> For holding torque, this is absolutely correct. For running torque,
> EVERYTHING is
> up in the air. It depends entirely on how the drive manages the
> collapse of the
> field in the winding that is being turned off. Providing an effective
> scheme to
> dump the inductive energy quickly can greatly improve the running
> performance.
> But, that adds a lot of parts. The most common technique is to place
> diodes to
> return the current to the power supply, but this basically leaves the
> coil short-
> circuited. Putting some resistance in series with those diodes will
> increase the
> voltage on the transistors, but will make the field collapse faster.
> But, then the
> resistors will get hot. Especially in a chopper-type drive, they will
> be carrying the
> circulating current nearly all the time while the motor is holding
> position, and get
> REAL hot. So, you need more transistors to shunt the resistors except
> when moving
> above some speed. Eventually, it becomes obvious why a bipolar drive is
the
> easiest way to get a simple, high performance drive. And, as for
> microstepping,
> if it doesn't do microstepping, it isn't worth having! (My opinion, of
> course! But,
> if you've worked with a Gecko G201 drive, not much else even comes
> close! And,
> a full-step, or even half-step unipolar, L/R drive is so FAR from a
> Gecko that it is just
> a really bad joke.)
>
> Jon
>
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@...,
timg@...
> Moderator: pentam@... indigo_red@... davemucha@...
[Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
>
> Yahoo! Groups Links
>
>
>
>
>
>