DC contactor carrying AC - rerated value?

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

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.

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"... ;-)

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Sorry buddy, but you have it backwards. DC rated contactors DEFININITELY require either arc-chutes or magnetic arc deflectors. DC contacts arc WAY MORE than AC

My experience is opposite to that.

An AC capacitive load is different than a DC capacitive load. I'd switch an AC capacitive load without too much trepidation, while switching a DCcapacitive load would give me pause.

For start cap - run cap, and balancing information see some good articles in Metal Web News.

Bob Swinney

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).

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.

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.

I suppose that raises another question - should I split the start-cap bank into two banks, and run one off of each NO contact, or connect them all up as a single bank, and just treat the pair of contacts as parallel wiring?

If I parallel just the contacts, the trailing contact will be effectively breaking the entire starting current by itself - this seems to argue for splitting it into two banks, one on each contact?

Here's hoping this beast doesn't let the smoke out when fired up! Will Ray

Splitting the capacitors into two banks will force the contactor contacts to properly share the load. Paralleling the contacts works but does not prevent load-hogging by one set of contacts. My sense is that the contactor will be adequate for the purpose.

Don Young