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Old 03-18-2020, 11:13 PM
Brice Turnbull Brice Turnbull is offline
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SSR's and Amps...

Hello all,
I hope everyone is in a relatively peaceful place at the moment.

I run a wire melter with an Orton ceramic-kiln keypad controller, and a custom bank of 3 dual 40 amp SSR's. Been working well in this format for 6 or 7 years.

My element configuration (recommended by the electrical engineer who designed my SSR box, but who is no longer available) yeilds 7.8 Ohms. 244 volts (my measured incoming volts at my main service panel) 7.8 amps = 31.3 Amps. The SSR box has a 30 amp breaker for each SSR. In the past I've run the power wires from 2 of the SSR's to the ends of my element array and it has worked well. I can't explain the physics of that.

Just rebuilt my furnace body and replaced all the elements, turned it on and one of the two SSR's I was using blew fireworks. It had been running for a few hours, popped the 30 amp breakers on both SSR's, and when I reset them, ka bloom. I unplugged it. Switched out the whole SSR box for my back up SSR box built exactly the same way. Turned it on. First shot of electricity directed from the control pad threw no onboard 30 amp breakers, but did throw both the 50 amp breaker at the main service panel, and the 200 amp master breaker for the whole service panel.

I unplugged it, disconnected both those SSR's power wires, hooked up the one remaining SSR's power wires, turned it back on. It ran about 8 hours and made the SSR hot (I know they typically run hot, so I'm not sure if that is meaningful - I can hold my finger on it, but it is uncomfortable, so ?150 degrees?), got the furnace up to about 300 degrees, and then popped the onboard 30 amp breaker.

I reconfigured the elements to draw 11.4 Ohms. 244 volts 11.4 Ohms = 21.4 Amps. It's running now, but the elements are getting power for much too much % of time at only 600 degrees to believe it will reach working temp.

Questions -
- why did the power wires from 2 SSR's throw the main 200 amp circuit breaker this time but not in the past? I checked, and I had the + leads joined correctly to one pigtail end, and the - leads joined together to the other pigtail end, so it wasn't that simple.

- 21.4 Amps is not throwing the 30 amp breaker, but is it going to damage the coil elements?

- lastly - I have one spare SSR I can switch in for one of the two fried ones (I'm thinking they are fried, as even if the current controller is turned off and no computer brain activity, when I plug it in, power is sent through both of those SSR's as indicated by external current indicator lights, and by the hum I hear if I hook either up to the elements as the elements first get power). What kind of paste/gel could I buy locally (I have access only to an Ace Hardware store and a wholesale plumbing supplier and a mom & pop lumber yard) to put behind the SSR on the heat sink?

- what am I not thinking of correctly in general? (ha ha)
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Old 03-19-2020, 09:03 AM
Shawn Everette Shawn Everette is offline
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As with most things, could be several things that have gone wrong.

Overall it sounds like you are drawing too much power, and the most likely culprit is the elements. Unless you've got a short, it sounds like that is the only thing you've really changed. Relays can always fry, but it sounds like you've swapped those and are still experiencing problems. Incorrect wiring could also be a possibility.

As far as the heat sink goes, Dudley's got solid ones that may match your unit, if you're set on paste check out newegg.
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Old 03-19-2020, 12:18 PM
Gary Grebus Gary Grebus is offline
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It sure sounds like you have a short circuit somewhere, one that only shows up when the furnace gets hot. Could the elements or wiring be moving as they heat up, shorting out the element string or maybe contacting the metal shell of the furnace?

The fact that it ran for a few hours before the first failure suggests it's not an obvious mis-wiring. When you swapped in the backup SSR box, was the furnace still hot? That would tend to confirm the theory that there is an intermittent short.

The single SSR configuration popping the breaker after getting to 300 deg. could just be a 30 amp breaker running too long at 100% capacity.

re: heat sink compound, Google turns up a wild variety of supposed alternatives. Automotive anti-sieze compound seems like one of the most plausible.
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Old 03-19-2020, 12:36 PM
Shawn Everette Shawn Everette is offline
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Since it is SCR, what kind of control system are you operating?

If the program isn't trying to draw full power to a certain point, that might explain the intermittent failure. Shorts to me are fairly consistent, and leave a mark.
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Old 03-19-2020, 02:00 PM
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Pete VanderLaan Pete VanderLaan is offline
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I thought it to be SSR, not SCR. Would that not elict a different circumstance?
Brice, you say you just rebuilt the furnace body. Is it metal sheathed?
You also say you replaced all the elements.
It ran for a while. Warmed up, expanded and blew.

My supposition would be that as it heats up, it expands. In expansion something is grounding. It's either the element pigtails on a nice new exterior, or the exterior grounding to the frame. I would not expect an SSR to fail just because you reinstalled it in a new environment. I would immediately be suspect of my insulation of the elements coming through the walls.
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Old 03-19-2020, 07:03 PM
Brice Turnbull Brice Turnbull is offline
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Thanks for the thoughts everyone.
I am running through the whole system looking for possibilities again.

Hmmm - that's an interesting thought Pete - I had not re-tested it when at temp to see if the element pigtails had made contact with the furnace skin. Had only checked it before hand.
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Old 03-20-2020, 12:45 AM
Brice Turnbull Brice Turnbull is offline
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OK - thanks for helping me think that through everyone, I think I've got it solved.

For the curious.....

It was 2 separate problems that appeared seemingly in sequence, so at least I don't feel foolish that I couldn't decipher it sooner.

The SSR's are arranged 3 per box. It only takes 2 to operate the furnace, and so I have been wiring two of the SSR's as a working pair to the elements, and the 3rd is just in place with it's lead wires covered with wire nuts as a reserve in case one of the other SSR's fails (which hadn't happened in the 7 years since these boxes were put in service).

Problem 1a - one of the original pair of SSR's failed

1b - but the warning light on the SSR array box also failed, so it wasn't apparent to me, and since it was paired with a second SSR the elements were still getting power (just not enough to reach working temp) so I didn't realize that SSR (which failed open, and not sending power through) was bad because power was still flowing through one half of that pair.

1c - when I rewired things the first time, the second original SSR (which must have been overheated following the other SSR failure) had a melt down. Since I didn't understand the first SSR had failed earlier, I thought they both failed at this point and was confused by the event I saw at that moment.

Problem 2a - When I switched in the entire back up SSR array box, I figured it had been assembled correctly (because the other two - original for my furnace, and original for my annealers, were assembled correctly). I was wrong - the incoming mains were backwards on one of the SSR's in the back up box.

Problem 2b - When I hooked up 2 of those 3 SSR's to the furnace elements it fried them both in the closed position because of this, but I only looked at the wiring that I had just done when hooking up the back up SSR box which under double inspection was correct, and I should have kept looking for the culprit deeper in the wiring. I guess I was still confused with what had happened with the original SSR box, so I wasn't trusting myself to puzzle it out accurately. I just felt it was a mystery SSR failure of some weird variety.

2c - When I switched to the 3rd SSR in the back up box, it worked, but didn't feed enough power to the elements. So then when I replaced one of the fried SSR's and wired it in with the one good one, the mains were still backwards on the newly installed SSR, so when I directed power to it it had a melt down, even though I had just tested it in a solo arrangement and it had worked... both those SSR's would work one at a time, but when I hooked them together and the neutral of one met the hot of the other - whizzzz bloom.

I've got it all together now - it's on it's way up in temp with the last two SSR's I could scratch up in the shop, and I will get new ones to replace everything again in a couple weeks, so hopefully these two used ones I had from the past will hold out until then.

And... I'm going to bed
Cheers all - I do appreciate the support!
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