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[DPRG] Motor Filter Capacitors
Subject: [DPRG] Motor Filter Capacitors
From: Zac Wheeler
zac at poor-robot.com
Date: Wed Mar 21 19:07:14 CDT 2007
>
>
> Now if you are instead talking about putting caps across the
> motor, you don't want much. Since the motor is being PWM'd at
> some fixed frequency, the larger the capacitance you put on the
> motor, the more of the PWM bleeds straight through the cap and
> makes waste heat.
Sorry, come again? I have used caps in motor applications a bit and
never had an issue with power dissipation. Caps are reactive. The only
power dissipation you'll see is the surge currents through the ESR. You
can put any cap with the right voltage rating and low ESR (i.e. any 25V
ceramic cap assuming a 12V motor) across the motor and it won't heat up
from "conducting" AC current. The motor may not work as well because
you're altering its frequency response, but that is a completely
separate issue.
Counter to what seems to be popular opinion, you also aren't really
removing a lot of noise by picking a "standard" capacitor value and
dropping it across the terminals - you're simply steering the noise
around the motor. If you want to remove noise, add capacitors from the
motor terminals to ground, and choose them for the noise frequency
you're trying to eliminate. AVX offers a nice program that shows the
frequency response of each of their caps:
http://www.avxcorp.com/SpiApps/default.asp
Eric, it sounds like your problem is you processor is crashing due to
the voltage dipping during hard loads. As Randy said you could use
capacitors across the H-bridge power supply lines, but you are going to
need a lot of capacitance at a high voltage for reliable operation. A
better solution is to add bulk (100uF+) capacitance where it will work
harder to keep the processor running - at the voltage supply for your
servo controller, and preferably after the processor's voltage regulator
on the board. This allows you to use cheaper capacitors and fewer of
them. If this doesn't work, a second good option is to use separate
battery packs for your motors and controllers to sidestep the problem
altogether.
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