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Old 09-04-2014, 15:20   #31
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Re: 12V Relay Amp Ratings?

If the issue is turning off an inductive load, like a motor, causing the contacts to degrade from arcing, then adding a snubber diode on the contacts is a common solution to protect the contacts.
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Old 10-04-2014, 06:36   #32
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Re: 12V Relay Amp Ratings?

"Snubber" diodes are an option with DC loads. You just need to get the polarity correct & size them properly. Another device exists that goes by the trade name of "quench-arc" It is basically a capacitor in series with a resistor, all in a single package with just 2 wires coming out of it. This works with AC or DC loads.

Inductive DC loads tend to arc worse than inductive AC loads. For this reason, it is common practice in some industries to switch inductive DC loads with 2 sets of contacts in series, to distribute the arc effect.
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Old 10-04-2014, 11:16   #33
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Re: 12V Relay Amp Ratings?

Jim-
"For this reason, it is common practice in some industries to switch inductive DC loads with 2 sets of contacts in series, to distribute the arc effect. "
Interesting concept but I'd expect that to be the same fallacious logic that called for 4 engines instead of 2 on aircraft (proven to be wrong, double the number of engines and you quadruple the number of engine failures per aircraft) and dual belts on alternators. Proven inferior to ribbed belts for the same reason that I'd expect dual relay arms to fail. One always winds up taking the brunt of the load, one relay arm will always close a fraction sooner, and then weld shut or wear out and cause the whole system to fail, sooner than one properly sized one would.

Relays: diodes, caps, snubbbers, all well and good and better than "just" a relay, but I had wanted to work with 'just' the basics, i.e. first start with sizing the relay properly. (And yes, eventually I found one that should be adequate enough.)

Again, thanks to everyone.
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Old 10-04-2014, 12:46   #34
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Re: 12V Relay Amp Ratings?

Quote:
Originally Posted by hellosailor View Post
Jim-
"For this reason, it is common practice in some industries to switch inductive DC loads with 2 sets of contacts in series, to distribute the arc effect. "
Interesting concept but I'd expect that to be the same fallacious logic that called for 4 engines instead of 2 on aircraft (proven to be wrong, double the number of engines and you quadruple the number of engine failures per aircraft)
Perhaps the logic is "fallacious", as you say. I didn't do any of the research on that subject myself, so I'm not going to be the one to argue that point. I was just reporting what I had seen done as common practice in industry. If you would like to research the factual basis of that concept, you might choose to contact the engineers in the motor control divisions of Asea Brown Boveri, Siemens or Sprecher & Schuh. When I worked with that type of equipment on a daily basis, those engineers were my best sources of information.

The next time I get access to a double series contact rig in the field, I'll try to take a look at the arc chutes associated with each contact pair to see if the soot accumulation appears even or not, in an attempt to learn something for myself.

The concept of doubling the number of engines on an aircraft causing quadruple the number of failures is new news to me. Could you please point me towards some published background data on that subject? I'd like to learn more about it.

Thanks,
Jim
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Old 10-04-2014, 12:56   #35
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Re: 12V Relay Amp Ratings?

Jim, I don't have any sources on aircraft engines. Heard it first from someone who is an aviation consultant, he was saying that's why the transatlantics dropped from 4 to 2. Every engine has a maintenance schedule and sooner or later...a failure rate. If any one fails, you still have to abort the flight and having more engines means more failures. At the time we were talking about some of the commercial aircraft no longer in use and IIRC I mentioned that I liked seeing propellers out there working, and lots of them, and he said "lots" was what you didn't want to see because fewer was actually better.

Apparently that plagued the B29 when they were new, read that from a B29 pilot's memoirs. Unreliable new engines and too many of them (4) versus later designs with fewer engines, when they could get away with it. (As opposed to simply needing more engines and not being able to get enough power from less of them.)
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