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[DPRG] PWM vs. voltage motor control

Subject: [DPRG] PWM vs. voltage motor control
From: Kipton Moravec kip at kdream.com
Date: Wed Jun 13 22:18:05 CDT 2001

I was thinking around the same thing, but for a slightly different reason.

At the lower PWM percentages you need to get the current flowing.

I have been wondering if some of the problem is the battery.  The chemical
reaction for producing electricity is not instantaneous.  Perhaps if he had
a large capacitor to help provide the current for the short "on" times, it
might fix the problem.

This is another reason to have very low resistance devices for your
H-bridge.

Kip

----- Original Message -----
>From: "Dean Hall" <dwhall256 at yahoo.com>
To: <dprglist at dprg.org>
Sent: Wednesday, June 13, 2001 1:51 PM
Subject: RE: [DPRG] PWM vs. voltage motor control


> Here's a little explanation into what Clay was seeing:
>
> Motor torque is proportional to the input current, not
> the voltage.  A PWM signal connected to a motor-driver
> chip will drive current for the "high-time" of the PWM
> signal.  When the PWM signal has a short "high-time"
> the motor experiences very little current to drive it.
>
> In order for a motor driver chip to drive a motor at
> low RPMs, it needs to be able to push a significant
> amount of current in a small amount of time.
> "significant amount of current" is purely a matter of
> scale.  A LM293 or SN754410 can drive a motor that's
> the size of a soupcan (but not very well at the low
> RPMs).  However, those same drivers can drive at low
> RPM a motor that's the size of an egg just fine.
>
> Another major reason that motors stall at low RPMs is
> due to friction.  So don't try to solve everything
> electrically; make sure your mechanical system spins
> freely before plopping in a 24V/5A motor driver.
> (I'm saying this in general to all aspiring robot
> builders, not directly to Clay;  I'm pretty sure he
> knows all this <smiley>)
>
> !!Dean
>
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