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[DPRG] PWM Frequency and torque

Subject: [DPRG] PWM Frequency and torque
From: Vimal Shankar vimalsshankar at yahoo.com
Date: Tue Feb 27 00:25:24 CST 2007

> I have driven both 18 volt Panasonic gear motors and
> small Mabuchis at 2000 Hz without problems.

Yes... I have a few Mabuchi too and they perform
nicely at 2Khz.. but these unbranded motors I got give
a workable torque at 250Hz.. is tha really unusual ?

> They
> tended to stall out around 30% duty cycle. Note that
> the stall point exhibits hysteresis.

Could you explain this part a little ?

> Everything I've read advocates much higher PWM
> frequencies than 25 Hz!

My point exactly - that's why I fired this quesiotn..

> My guess is that one or both of the duty cycle or
> voltage is decreasing with PWM frequency. That could
> be a firmware bug from the microcontroller or a
> limitation of the H-bridge circuit. I've always used
> large discretes for driving motors. So the
> transistors have a comfortable safety margin over
> what the motors draw.

It's just a L298N with a fn gen connected to Enable. I
think the function gen gives a 50% duty cycle sq wave.

> If you have an oscilloscope, I'd use that to see
> what the PWM signal really looks like. Check the
> microcontroller output. Then check the voltage
> across the motor terminals. If the motor is so big
> that it scares you to connect the scope probes to it
> (flyback voltage), then get a small Mabuchi or
> similar tiny motor and drive this. It should be
> obvious what is going on.

Don't have a scope at home.. but will get the work
done on a off day.

Could someone answer this question too :

> >PS : Locked antiphase is something I would not
> worry
> >about now. It requires good HBridges (instantaneous
> >swithicng is important). Plus, the software has to
> >keep track of the direction and adjust the phase
> >accordingly. If I used signed magnitude, I get
> double
> >the resolution, simple driving logic...
> >BTW - what is "sign-magnitude, synchronous
> >rectification". I googled. I got phd papers :(



 
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