Re: Electronics gurus please help

Scott,

The reason for the transistor in the center-tap circuit is increase
the rate of current removal from the winding at the end of a step.
Without it, current decays at a very lesiurely rate and severely
limits the maximum speed of the motor.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Scott M. Thomas" <scott@d...> wrote:
> Hmmm. I suppose you could use 6 if you want to. I built one using
a PIC
> and chopped the input to the 4 "end" transistors, worked very
well. I also
> used a common current sense resistor, so this simplified it even
further.
>
> I'll have to check out the voltage doubling effect, hadn't thought
of that.
> But this shouldn't have a huge effect on things depending upon the
power
> supply voltage.
>
> Thanks
>
> Scott
>
>
> -----Original Message-----
> From: mariss92705 [mailto:mariss92705@y...]
> Sent: Wednesday, September 11, 2002 3:38 PM
> To: CAD_CAM_EDM_DRO@y...
> Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
>
>
> Scott,
>
> If it were an R/L drive you would be correct. The center-tap of the
> motor coil would go to the + supply thru a large current limit
> resistor and the winding ends would have the transistors.
>
> Since a unipolar chopping drive was mentioned, 6 transistors are
> required; the current limit resistor is replaced with a transistor
to
> which pulse width modulation (chopping) is applied. The end
> transistors still would behave the same.
>
> A center-tapped (6-wire motor) winding is a 2:1 step-up
transformer.
> because the center tap goes to the + supply, pulling one end wire
to
> ground causes 2 times the supply voltage to appear on the other end.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., "Scott M. Thomas" <scott@d...> wrote:
> > I count 4 mosfets not 6 for a unipolar drive, did I miss
> something? And,
> > because there are half the parts and no high side drivers
involved,
> I would
> > say that a unipolar drive is much simpler. I also don't
understand
> why the
> > voltages would be different.
> >
> > Scott
> >
> > -----Original Message-----
> > From: mariss92705 [mailto:mariss92705@y...]
> > Sent: Wednesday, September 11, 2002 12:59 PM
> > To: CAD_CAM_EDM_DRO@y...
> > Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
> >
> >
> > Hi,
> >
> > It takes 6 power transistors and 6 high speed rectifiers to build
a
> > unipolar "chopper", it takes 8 power transistors and 8 high speed
> > rectifiers to make a bipolar (full bridge) one.
> >
> > The power rectifiers are not across the power transistors in the
> > unipolar drive. If MOSFETs are used in a bipolar drive, the
> > rectifiers are included in the transistors.
> >
> > The power transistors "see" twice the power supply voltage in a
> > unipolar drive. In a bipolar drive, the maximum voltage "seen" by
> the
> > power transistors is equal to the supply voltage.
> >
> > The performance of a unipolar drive is about equal to a bipolar
> drive
> > running a motor in a half-winding connection.
> >
> > A bipolar drive is prefered because it is simpler (when MOSFETs
are
> > used) and the circuit voltages are lower.
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@y..., Vajk Fekete <vajk.fekete@o...> wrote:
> > >
> > > question1: is a chopper unipolar driver much worse than a
chopper
> > bipolar drive?
> > >
> >
> >
> >
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> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>
> 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 as
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........
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> Post messages: CAD_CAM_EDM_DRO@y...
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