ECN Forum Forums Technical Discussion Electrical Theory and Applications Capacitor in motors

Small DC motors use caps to decrease EMI and AC harmonics; the capacitor is placed across the DC terminals so that any AC current caused from arcing at the commutators is shorted through the caps. It's especially common in R/C, as the EMI can seriously mess with the radio control. I'm sure anything trying to pass the FCC would have to have these caps, too.

[This message has been edited by SteveFehr (edited 12-04-2005).]

Deang

I stumbled onto this site while researching the use of motor run capacitors in loudspeaker filters (crossovers). Yeah, I'm a long way from home.

I guess my question would be this: Is the harmonic information found in audio signals, and passing through a filter comprised of motor run capacitors -- harmful to these capacitors?

I would just like to add that the cap is just a cap when you think about it starting a motor. It has alot more going for it (or against it) that we need to think about if we use it in another application. That cap has equiv. series resistance and inductance that lump in with its more dominant capacitance. They might not come into play within the design tolerances of your network. There will be a frequency though, where that cap will be a tuned cicuit, with its own L & C, and Q factor determined by its own series resistance. This point is likely above the audio band.
Temperature stability might even be an issue with automotive applications. You wouldn't want your midrange center f or 3dB badwidth shifting on you from winter to summer would you?
I'm not saying that these things would definately cause problems, just that I wouldn't rule out the possibility.
Joe

[This message has been edited by JoeTestingEngr (edited 12-04-2005).]

Quote:
"Can anyone explain how a capacitor provides a "second phase" in an electric motor."

Everyone knows that motors run on smoke. When the smoke escapes from the motor, it will no longer run.
Capacitors are also called condensors because they condense the smoke into the motor, giving it more torque (or oomph to use more technical terms).

Scott35's explanation is good too.