Electric oven element trouble

[dan pfanenstiel]

Anyone know why my elements would stop working all of a sudden?

 

Got the oven up and running this weekend and got a couple of blades heat treated with it. I first ran the oven up a little at a time, until full heat achieved and then started playing with steel in it. Runs up to 1500 deg with no problem (240v version) in about 15 minutes.

 

On the last time, it was heating up and at about 1200 degrees, the elements started to cool off even though the controller was still telling the relays to keep going. I let the unit cool down a bit then put a tester on the wiring. I'm showing power all the way to the elements the way it should be, but the elements won't heat up now. Can't find a break or burn in em, and like said, got power all around. Weird.

 

Dan

[kb0fhp]

Are the relays working properly? Did you have a broken thermocouple on your excess temperature instrument causing the instrument to trip? Could you have a crack in one of the elements that opens up when heated? Sounds like a system reset somehow - or a relay that is causing problems...Could you have an open at the element?

 

Do you have a circuit diagram that I could look at to help you troubleshoot?

 

Scott

[dan pfanenstiel]

Tested everything with a volt meter, Scott. I'm showing 120v on each leg of the 240v circuit. Tested the legs after the relays (SSR's) and they were each putting out 120v. Tested the elements with the relays on and they each tested at 120v. I don't understand, if there's current in the elements why they won't heat up now.

 

Here's the set-up.

 

 

Dan

[Steve R]

How much current does this setup draw? This may be an obvious answer, but...if your meter can read current and can handle the amps, put it in the circuit and turn the system on. If it's not drawing current you have an open somewhere.

 

cheers,

/steve

[Howard Clark]

Dan if you have voltage everywhere, then the element wire is either broken (open circuit), or there is a very high resitance connection where the copper conductor wire joins the element wire end(s). The NiCr wire does not just stop conducting, there is another cause.

 

Shut the power off, disconnect both ends of the element wire at the relays, then check the resistance with your ohm meter from there, all the way through the coil circuit and see what you get. That should give youa definitive answer about what, but maybe not where, exactly.

 

Good luck.

Edited October 29, 2007 by Howard Clark

[Ed H]

I agree with Howard, I am an industrial electrician and when I check out a heating zone on an injection press I use a clamp on ammeter to see if there is current flow. If no current flow, I look for a blown fuse since that's the easiest to TS and if it's good, I disconnect the leads for the heaters themselves (power off and locked out). Resistance depending on the size should generally be no more than 25 ohms, but you can calculate what it should be. P=I*E where P is power in watts, I is current, and e is voltage. Solve for I and plug it into E=I*R and solve for Resistance. If the resistance checks out and I am guessing it won't, move on the the Thermal Couple, but I am guessing you won't get that far. In my experience the Temperature controller won't turn on an output if it doesn't think the TC is connected as a safety feature. If it didn't have this feature, with a TC break, the controller wouldn't see any change in temperature so it would continue to heat until melt down. There isn't anything special about the junction for the thermal couple (point where temperature is taken) if you find that it came appart, just twist the two wires together and crimp on solderless connector of your choice without plastic obvoisly. One more point is if you have the leads reversed for the TC, the controller won't like that either.

[dan pfanenstiel]

O.k. after getting advice from the HVAC guy at work, and you guys, here's where I'm at.

 

-Tested the resistance in each element. I got a reading of just under 10 ohms for each separate element. Dunno about the math there, Ed, but at least I know the elements aren't broken and they read about even. Hopefully rules that out.

 

-I don't think it's a thermocouple issue, the controller is calling to switch the SSR's on, and there's an indicator light on the SSR's showing them to be on.

 

-Now the fun part. I got everything hooked back up all proper and tested across the two legs of the 240v supply, read 240v.. Tested across both legs of the wires coming out of the SSR's to the elements and read nothing. I can test the current coming out of one SSR (there's two) and ground and read 120v, as well as on the other, but can't read anything when testing for 240v on both. Wondering if the SSR's are blown? But if so, I shouldn't be able to read anything on the downside of the SSR's, right?

 

We get this figured out and I'm e-mailing a beer to everyone!

 

Dan

[Ed H]

At first glance I didn't quite understand your last point but I think now I inderstand, what your meter reads is difference in potential energy (voltage) If you place your two leads of your meter at two different points on the same wire you will read close to 0 volts (wire has some resistance per 100). If your SSR is conducting, that is passing voltage from the input side to the output side there should not be much difference in potential (same wire deal). Now if the SSR is off and not conducting, there will be the full potential of 120. On European machines, they do things a little differently where as they on a 220 circuit will only break one of the two hot wires to control the temperature for cost savings measure. They also feed power to the bottom of the Circuit breaker (found out....the hard way) If you think that just one SSR is bad, I would for testing purposes connect the good SSR up as normal and wire the second leg of the 220 directly into your supply (your diagram doesn't show fuses but I think it would be prudent to add them if they are in fact not there). With this setup you should still have control of your temperature, but it is not as safe as it could be. Definately unplug your oven if you are going to change the elements. You could check continuity of all your wires, maybe something burnt out with no fuses?

[dan pfanenstiel]

Hey all, got that thing figured out finally. After many hours of trying different things and testing the friggin circuits, it turned out to be the SSR's. At least that's what I'm blaming. Might have been a little grounding problem by someone who should remain nameless, but I'll blame the SSR's anyway.

 

Stuck in some shiney new SSR's and all's well.

 

Thanks for the input, all that did.

 

Dan

[Howard Clark]

Glad you are back up and running. Non functional equipment is a PITA that really bugs me a lot.