On Sat, 29 Mar 2008 22:30:21 -0700 (PDT), willray
wrote:


Greetings all,

Haven't had time to stop in much recently, but I'm finally getting my
new shop close to ready to power up, and in wiring up the phase
converter for the Rivett, I've hit a parts question for which I
can't seem to find a ready answer:

How much AC current would be reasonable to switch, using
a 40A DC-rated contactor (not a DC coil - magnetically
extinguished DC-rated contacts).

Magic 8-Ball Sez: "Answer Hazy - Call Manufacturer..."

My intuition is to believe that it can safely carry (break) a heck of
a lot more AC current than DC current. I believe that I saw,
somewhere, once upon a time, a derating rule of thumb, for using
AC-rated contacts with DC. Even if my memory is playing tricks
on me, it stands to reason that if it can break and extinguish a
40A DC arc safely, then the self-extinguishing zero crossings of AC
should make it good for a bit more than that.

Ahh, but the zero-crossing is accompanied by the polarity switch of
the waveform every half-cycle, and the fact that it will build up
magnetic forces differently.

AC rated breakers and contactors have specially built arc chutes
inside them, with horseshoe shaped steel stampings mounted in an
insulator (so they are all electrically floating) that magnetically
pull the arc away from the contacts and cool it enough to break the
plasma connection between the contacts. If you don't have arc chutes,
you have to pull the contacts WAY FAR apart to break the plasma.

DC contactors don't have or need arc chutes, they just get the
contacts far enough away from each other to physically break the arc.

You can use an AC contactor for DC, but you often have to wire two
or three sets of contacts in series to get enough raw distance between
the contacts to break the arc. You can't go the other way and run a
DC unit on AC without the arc chutes.

The question comes up because the spare contactor from the parts box
that fits the right hole in the converter box, for the start-cap
relay, happens to be a Josyln Clark "definite purpose" 40A DC
contactor. I'm hoping that split between its two NO contacts, I can
switch enough start capacitance for my 15Hp idler.

Switching a capacitive load will REALLY pull an arc across the
contacts. Don't guess whether a part will work for that application,
find out for sure, or you'll have things burning up and/or blowing up
rather spectacularly...

Unless you enjoy "Here, hold my beer and Watch This!" moments, in
which case feel free to press on with your original plan. And be sure
to roll tape for "America's Funniest Home Videos"... ;-)

-- Bruce --