RE: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help

Agreed. I am not sure about my motors, but I can follow along with the
proposed numbers. Thanks for the rule of thumb of 10%, I will have to check
that out.

Scott

-----Original Message-----
From: Jon Elson [mailto:elson@...]
Sent: Thursday, September 12, 2002 1:29 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help


"Scott M. Thomas" wrote:

> I would be interested in seeing your output topology.
>
> I am having no problems with the MOSFETs even getting warm (they are not
> attached to a heatsink), perhaps its a fluke, but no problems there.
>
> I would be also be interested in how you would prevent the sawtooth.

Since

> you state that the current would drop to zero in 40uS, are you saying that
> my 25uS delay is too long?

Yes, at least for the example of the 2.4 mH motor winding. I don't know if
that
is a reasonable approximation of the inductance of YOUR motors, though.

Normally, you don't want the Delta-I in the windings to exceed 10% of the
average current. So, for a 2 Amp nominal current, you would not want it
to drop below 1.8 A during the chopping cycle. There is a constant energy
loss due to hysteriesis of the Iron laminations every time you charge and
discharge the flux in the magnetic circuit. The part of this due to
stepping the
motor is unavoidable. But, the part due to the chopping cycle should be
minimized to reduce motor heating.

So, if the current did fluctuate more than 10%, you would want to reduce the
delays and so increase the chopping frequency. This will reduce the
variation in current.

Jon


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