Re: Stepper questions

"Shorting the coil out" does all that but it also severely limits
motor performance. Current decays at a V/L rate, a diode is 0.7V so
current decays at a very leisurely rate.

The idea is to both rapidly inject and remove current in the winding.
If a 70V supply were used, current would decay at 1/100th the rate it
built up at (0.7V / 70V). This slow decay would bind the motor to a
tiny percentage of its potential.

The motor-winding energy is still wasted as heat, albeit slowly. It
didn't get recycled and it is substantial a high speeds (motor back-
EMF is also "shorted out").

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
<mattison20@...> wrote:

>
> <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
> >
> >
> >
> >
> >
> >
>