RE: [CAD_CAM_EDM_DRO] Re: Tawagawa Seiki steppers

If you read my response to Ward you will see that I have also decided this
argument to be useless, but for other reasons than you state below.

It is generally a bad thing to cause a transistor to conduct backwards, that
is why we protect them with reverse biased diodes.

And if you factor in your earlier statement about 5 phase steppers in a
pentagon configuration providing a pull up for the end I was concerned about
going negative I have two things (the diode and the other coil) working to
solve what i was considering a problem.

See, no magic at all! ;)

Truce?

-----Original Message-----
From: Alan King [mailto:alan@...]
Sent: Saturday, August 11, 2001 4:24 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Tawagawa Seiki steppers



Yeah, but we're not saying it like we don't get your point, we're
saying it like your point is in error, which anyone who's actually done
braking mode with an h-bridge knows. Like I said in the other post,
take two transistors and any motor coil you like, switch them both on to
ground, and spin the shaft.

First off you don't assume transistors work like diodes 100%, and
always check what's going on. The CE drop is often only .3V which by
itself is a strong indicator to not think of it as a diode, with two
junctions that have less drop than 1 normally. They are also
symmetrical, and some will conduct with the C and E reversed etc.
You've got to check your work before assuming they will work like you
think they will in differing situations.

BUT, you won't even get far enough to check if negative voltage won't
work. You're somehow magically assuming that you can disconnect at
least that side of the coil, let it go negative, and the transistor
won't conduct. But you have no reason to prefer 'high side is grounded,
other side goes negative' over 'low side grounds, and high side is
dumped to ground too'. And there is no 'option' on which happens. Both
ends of the coil will be at their respective Vce voltage, say .3V. You
turn the shaft, moving a few electrons. Say around .01V difference if
the ends were floating. Well guess what? Both ends are still around
.3V and very much above ground. The low end doesn't just shut off
because you go .01V down on it's knee voltage. And since the side
that's trying to go lower is still ground referenced, that other side is
dumping all your energy to get back down to it's .3V. You never get to
build enough potential to shut the low side off. That current's flowing
almost instantly so you never even get close. If the conductors and
magnets were perfect, you wouldn't even be able to move the shaft at all
in brake mode, as it is you just experience braking just like shorting
the coil. It's a faulty assumption that both sides are at true ground
and one side gets to go negative.

Even this explaination may not be 100% precisely correct, but it is
the right general idea. I clearly stated that it DOES work in the below
quote, not sure why you didn't get that. At any rate DO it as I
suggested before, and you'll see for yourself that the point you are
arguing is useless, regardless of the exact reasons why.

Alan

> Two ends of a coil switched to ground brakes, just shorting
> #)the wires you'll feel it, and it also works with two semiconductors
> #)switching to ground.

"Scott M. Thomas" wrote:

>
> I am not arguing that point. If they are connected to directly to ground
> you will have braking.
>
> Now, if it is connected to ground through semiconductors which will only
> pass current in one direction and they are both set up to sink current to
> ground you will have no path for the induced current to take. In other
> words: one end of your coil will take on a positive potential which will
> flow to ground through the now forward biased semiconductor (saturated NPN
> transistor), however, the other end now has a negative potential which

will

> reverse bias the semiconductor. How can you have braking with no place

for

> this current to flow?
>
> -----Original Message-----
> From: Ward M. [mailto:wardmerk@...]
> Sent: Friday, August 10, 2001 10:41 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] Re: Tawagawa Seiki steppers
>
> I disagree, if both ends of a coil are connected to ground, there will be

a

> braking action. Remember, braking is resistance to motion. With zero
> applied force and zero RPM, there will be no current in the winding. With
> any rotor motion, a current will be induced in the shorted coil, causing a
> resistance to motion, i.e. braking.
>
> Thanks,
>
> Ward M.
>
> wardmerk@... <mailto:wardmerk@...>
>
> #)-----Original Message-----
> #)From: Scott M. Thomas [mailto:scott@...]
> #)Sent: Friday, August 10, 2001 6:29 PM
> #)To: 'CAD_CAM_EDM_DRO@yahoogroups.com'
> #)Subject: RE: [CAD_CAM_EDM_DRO] Re: Tawagawa Seiki steppers
> #)
> #)
> #)I wasn't making any distinction between 2 or 5 phase and I agree that

you

> #)need to consider the other windings.
> #)
> #)I also agree that if you short the windings together or to ground that

it

> #)will cause braking.
> #)
> #)However, if I connect the coil to ground on both ends with an NPN
> #)transistor
> #)at each end (and they have sufficient base current to be in saturation)

I

> #)will not have current flow, therefore no braking. Now if I have
> #)enough back
> #)EMF (if the RPM is high enough) you would experience one of the adjacent
> #)windings losing effectiveness because the current flow would decrease.
> #)
> #)I am curious what you mean by braking mode on a normal stepper.
> #)
> #)-----Original Message-----
> #)From: Alan King [mailto:alan@...]
> #)Sent: Friday, August 10, 2001 9:13 PM
> #)To: CAD_CAM_EDM_DRO@yahoogroups.com
> #)Subject: Re: [CAD_CAM_EDM_DRO] Re: Tawagawa Seiki steppers
> #)
> #)
> #)Hey,
> #)
> #) If that were the case, then braking mode wouldn't work on a normal
> #)stepper either, there isn't anything special about a 5 phase motor for
> #)that case. Two ends of a coil switched to ground brakes, just shorting
> #)the wires you'll feel it, and it also works with two semiconductors
> #)switching to ground. Try it and you'll see, and if what you're thinking
> #)were true, braking wouldn't work for a single seperated motor coil on a
> #)normal stepper either.
> #)
> #) Also, on a 5 phase you can *NEVER* (one of the few times you can use
> #)that word and be ok!) consider one coil by itself without then taking
> #)that info, and apply it back through the other coils. Your end going
> #)negative would still not be high impedance, it'd be being pulled up
> #)through that other coil that has it's other side connected to +V. But
> #)it doesn't get the chance to go negative, braking a coil works even with
> #)a seperate coil, so 5 phase or 2 phase doesn't make a difference..
> #)
> #)
> #)
> #)"Scott M. Thomas" wrote:
> #)>
> #)> If you think about it, with 4 coils powered you really don't have one

in

> #)> brake mode. Even though both ends of the coils are at the same
> #)potential
> #)> (either + or -) the semiconductors on either end can only pass
> #)current in
> #)> one direction, therefore which ever direction the back EMF generated

on

> #)that
> #)> coil was flowing, one end would be always be in a high
> #)impedance state so
> #)no
> #)> braking would occur.
> #)>
> #)
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