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[DPRG] Motor Filter Capacitors
Subject: [DPRG] Motor Filter Capacitors
From: Randy M. Dumse
rmd at newmicros.com
Date: Wed Mar 21 22:53:09 CDT 2007
> 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.
So are you driving Sign Magnitude? Not allowing for LAP?
Consider this. If you put a push pull audio amplifier, and
isolate it from the speakers with a 100uF to 1000uF cap, will
any power get to the speakers? Or course, that's a normal design
feature to block the DC from the speakers. But it passes many
watts of AC signal.
If you're driving LAP, you're putting large amounts of AC,
across a cap which is large enough appears as a direct short,
what sorts of currents are you going to draw? For instance,
10,000uF at 100,000Hz measures 160 ohms. If your Hbridge supply
is 40 volts, you've got 80VAC across 160 ohms. That's going to
be some current ~.5 amp, and significant power loss, ~20W. (I
may be off by a factor of 2 up or down. I'm not looking at this
deeply, just looking for basic principles.) Hence heat.
On the other hand, being so much smaller, a .1uF amounts to very
little loss, but does bypass much of the higher harmonics that
would be in the RF region and represent significant EMI.
Randy
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