RE: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help

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@...]
Sent: Wednesday, September 11, 2002 3:38 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
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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http://www.metalworking.com/news_servers.html

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