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[DPRG] PWM vs. voltage motor control

Subject: [DPRG] PWM vs. voltage motor control
From: Elliot Johnson elliotjo at yahoo.com
Date: Thu Jun 14 22:34:17 CDT 2001

> I think in voltage and not as much in current. Which is probably where my
> problem is.  But from what I think, if we add a capacitor across the motor,
> it will provide a smoother on/off and will smooth out the voltage change
> (low pass filter).  This will provide a more steady current through the
> motor (because the voltage is more steady).  In this case the faster the PWM
> frequency is the better for the capacitor, because it can be smaller.
>

    I would expect the capacitor to cause problems with low duty cycles.   When
the PWM signal first turns on, current would rather flow into the capacitor
instead of facing the initial resistance in the motor's coils.   Since the
signal would be high for such a brief period of time, the point where current
would rather flow into the motor instead of the capacitor might never be reached
so all the current is stored in the capactor.   Then when the PWM signal is
turned off, the current would flow out of the resistor and through the motors,
but at a much lower voltage.  That's just a guess though, I've never tried it.
    The motor inductor's natural resistance to a change in current should help
smooth out the current from the PWM signal, but that effect might be more
dependent on the relationship between frequency, inductance, and the duty cycle

Elliot


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