RE: [CAD_CAM_EDM_DRO] RE: Stepper questions

HI Jon,

I was thinking of windings on different bobbins, not a "center tapped"
winding on the same core (5-wire). You're certainly correct if they share
the bobbin. Do they ALL share the same bobbin for a 5-wire stepper? I
didn't think they did (could). 4 phases, right?

For bipolar drives, they appear to use two bobbins. I guess a "transformer"
test would show who is coupled to whom.

>
> 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.

So we shoot down the old unipolar steppers again. And my question got
answered, the winding can be turned around much faster if it's being driven
then if it's left to it's own decay. But ya' gotta love them for low-end,
low speed SIMPLICITY! Maybe they're still good for teaching! I've retired
all of mine!

> > 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.
>

Maybe why nobody does it!

> >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
>

I really have to agree, the G201 Gecko drives are the top! But after all,
the old L/R stuff is really old (30+ years for mine)! They DID get me
interested in motion control, and I had a lot of fun with them.

Thanks for the comments (and corrections) Jon!

Alan KM6VV