Ahh! So the PWM frequency should be high enough so that the current
through the motor windings is nearly constant (DC + ripple).  You don't
want to stop the current at all, just give it a place to go without
loosing too much energy in diode losses.  That makes much more sense to
me than my previous ramblings.

Sorry if I threw mud in the water.

Sherpa Doug

> -----Original Message-----
> From: Spehro Pefhany [mailto:speff@INTERLOG.COM]
> Sent: Tuesday, October 09, 2001 2:31 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Mosfet speed control of 12V DC motor - Partially
> Solved
>
>
> At 12:14 PM 10/9/01 -0400, you wrote:
> >         OK, I was thinking the energy that was going back
> into the motor
> >was restricting the motor from turning.  I guess this isn't
> the case.  That
> >energy is kind of keeping the motor turning?
>
> Yes, it does. As long as the PWM frequency is high enough
> that the ripple
> through the motor inductance can be ignored it's like running
> the motor
> from a voltage source of Vin * ton/(ton + toff). The motor
> current flows
> through the MOSFET during ton and through the diode during
> toff, and is
> essentially constant (though the voltage at the motor terminal wags
> around). Ignore the comments that imply there's something
> wrong with this,
> it's how PWM motor control works.
>
> Best regards,
> Spehro Pefhany --"it's the network..."            "The
> Journey is the reward"
> speff@interlog.com             Info for manufacturers:
> http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:
http://www.speff.com
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