Thermolyne Heat Treat Oven Issues

[walterwoj]

I recently acquired this Thermoline/Temco heat treat oven.



I decided to rebuild the control system to ensure it was up to snuff. I installed a Inkbird PID (With ramp/soak functions), dual 25 amp SSRs, K-Type thermocouple and all new wiring from scratch. The oven cost me $300 and I have another $100 in parts in it so not a bad deal! It took 2-3 hours to wire everything up and I was ready to fire it up. Then came the sparks! On first run it INSTANTLY burned out BOTH 25 amp SSRs, causing them to short to ground and blew a double 15 amp breaker, NOT COOL! (No people were harmed in the making of this disaster)

I have been through the wiring and have not found any miss-steps, so I started checking the elements (the only real load). I ohmed them out and got 3.8 Ohms combined (or 1.8 and 1.9 ohms each) as they are in series. That didn't seem bad until I did the math volts/ohms = amps or 220/3.8 = 57.89 AMPS !?!?!? The data plate says 17 amps! Have my elements gone bad (how?) or am I making some dumb mistake?

 

[ErichKeane]

That definitely sounds like a problem with the elements. They should be about 13 Ohm total to get 17 amps. Do you know the history of this thing? Did it work when you got it, or did someone try to replace the elements, realize they messed up, then sell it instead of dealing with it?

If the elements ohm similarly, I find myself thinking perhaps that is the case! I can't imagine them degrading THAT similarly, and not blowing up breakers way before this point. Unless someone introduced a consistent short by trying to 're-attach' it to the wall in some way? Probably near the beginning?

What did the original controller look like, could it have some sort of voltage converter and just be stepping down teh volts a bunch?

 

[walterwoj]

I bought it in pieces, the guy who had it didn't work on it (he got it as an estate) but someone was definitely in it ahead of me, I was wondering if maybe they tried to convert it to 110v before I got it (that would make it 30 amps right?). The elements are very old and show signs of a lot of use so I don't think they were replaced recently...

Sent from my Pixel 6 Pro using Tapatalk

 

[ErichKeane]

Yeah, at 110, it would be about 30A, but I don't know why you would convert it to 30A at that point, that is an even tougher amount of power to find

If you can, use the multimeter to try to find a short somewhere perhaps in the elements? A part of me wonders if someone else had their elements break for some reason, and just 'stretched' them after cutting them to be the 'full-size'?

OR, lost 1 while disassembling, then re-assembled by cutting the other one in parts and trying to stretch it? It would be someone who doesn't really understand how this all works, but thats my only guess.

 

[Reddinr]

Is there continuity from elements to ground (enclosure)? Maybe a short. BTW. I think I have a couple of SSRs laying around if you want them. Just will cost the shipping. (Once the ice melts off the roads here.)

Can you make a quick sketch of your power wiring?

 

[JimDawson]

It could be the inrush current that killed the SCRs. The resistance of the coils goes up dramatically once they heat up. Maybe some 100 amp SCRs would do the job.

A 15 amp breaker might be a bit light also, since the oven is rated at 17 amps. I would use a 50 amp breaker and 8 Ga copper wire to the oven.

 

[RJSakowski]

Most metals increase in resistivity with tempoerature so the measured resistance sounds about right. SCR's normally can take a fairly large short term current overload. The spec sheet should give a value. There is a parameter called i^2t, current squared x time, which is a measure of this. The idea is that the heater coils will increase in resistance quickly enough to avoid damage to other components.

One solution is to use an inrush current protector. It is a negative resistance component that decreases in resistance as it heats up. It will limit the the starting current to a safe value but after heating up, its resistance drops to practically nothing. Here is one which may work for you. It has a 10 ohm cold resistance and a working current of 18 amps.

https://www.mouser.com/ProductDetail/Ametherm/MS35-10018?qs=PVyXUKBeRH3sUEikFWbR5w%3D%3D

 

[Reddinr]

The resistance change of nichrome wire between 0 - 2000F is about 25% maximum, typically 15%, depending on the nichrome wire type. So, the SSRs have to be able to handle that. I usually use 2X for SSR / SCR sizing if it is for a one-off application. Also, heatsinking for the SSRs is important. For higher currents you can't just mount them to sheet metal.

So, I bet there is something else going on here.

 

[Winegrower]

I have purchased nichrome wire and wound heating elements to suit on the lathe. It’s easy to control the nominal resistance by choosing the wire length. I used glass rods to support the elements.

 

[markba633csi]

I definitely would have ohm'd it out first. Gotta assume it's wrong until proven otherwise
Somebody's been in there and put in wrong elements, sounds like