Re: Power Factor Capacitor -- Was -- RE: schematics...Adam Smith's
At 08:03 PM 02/25/2000 -0600, you wrote:
>Actually, I've been told that Aluminum Electrolytic will work for a 120VAC
>PFC cap. These are inexpensive and very common. And to head off criticism
>that Aluminum electrolytic caps shouldn't be used in an AC circuit, I was
>told by an Electrical Engineer (Technician?) who runs all the electrical
>labs here at school. He said that electrolytics can take the AC, just not a
>constant negative DC bias.
While I was looking up info on caps for the MMC thing, I ran across a lot
of motor start capacitor papers and such.
You should NEVER us a standard electrolytic capacitor for AC line
applications for the following reasons:
If the cap shorts or fails, it will draw hundreds of amps off the AC line.
"Real" AC cap are designed to safely handle this failure mode. A standard
cap will blow apart with great violence.
AC capacitors use oil/poly or other very low loss materials. They are
designed to handle considerable RMS currents at 50/60Hz. Their loss and
value as been specially tailored and tested for this at the needed
temperatures. A lot of thought goes into each type and their price
reflects a much more care in their manufacture.
AC caps are "true" AC caps that are not polarized.
Since PFC caps need to be fairly high value (10+uF) and will take many amps
of current continuously, they have to be very specially made. They are far
far different that their electrolytic cousins...
Fortunately, AC motor run capacitors can be found surplus easily or they
can be purchased from many sources as needed.
I use Panasonic JSU18X206AQD from DigiKey (# P9404-ND) 20uF at $5 each.
They are the small, easily mounted, dry type which can be paralleled to get
any value I want. They are UL, CSA, JIS listed as are most AC caps these days.