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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 23:38:23 CDT 2007
> 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.
>
Passes current, yes.
> 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.
In sustained high-current high-voltage AC/RF situations yes, you are
going to get some power dissipation and heat. But the current and
voltage are 90 degrees out of phase, so it's not as bad a picture as
you've painted here. It's not a resistor.
I didn't mean to imply he should put some large electrolytic cap across
the motor, and I specifically stated ceramic. Even up to 10 uF he is not
going to see any significant power dissipation. You stated that anything
over 0.1uF would dissipate excessive power.
And (just to be petty) in your example he'd have to use two electrolytic
caps to be non-polar, bringing the impedance to 320 ohms (by now the
real power dissipation is getting low enough that the caps might be able
to handle it) and requiring two $30 10,000uF 200V (they're effectively
100V once placed in series) caps per motor. How about a more practical
hypothetical?
Zac
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