oven build frustration

Just a guess here. The coil spacing looks pretty tight. You may have introduced a higher inductance.

Someone with electric furnace experience would have to decide if the spacing looks tight in comparison.
 
Lenz/ Faraday type stuff. By creating a "coil" you are producing another force aside from the "resistance" you want to use.
 
I am only aware that the tight spacing may have led to a short. I have never heard of inductance being an issue.
 
And I'm only suggesting you compare your coil spacing to other ovens that work. Since you using a common design it would make it easy to rule out. And sometimes pictures deceive the eye.

You did not measure a "short"? 8-11 ohms...
 
Alternating current thru a coil creates additional inpedence via induction.
Essentially, the alternating magnetic field of the coiled conductor (heating element) fights against the electric current changing polarity.
It actually creates a little bit more heat to the coils without added current.
 
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Inductance would be a non-issue even if we looked at the coils as an inductive circuit. As such, the coils resistance is considered to be in series with the 'ideal' coil. The inductive reactance combines with the resistance by taking the square root of R squared plus XL squared where R is the resistance and XL is the reactance at 60HZ. So even if we were to take into consideration the inductance of the coils we'd wind up with an overall impedance greater than the resistance alone.

I concur with others that have addressed the issue. There is most likely a short to ground somewhere in the circuit as evidenced by the way in which the glass fuse blew apart.
 
Did you measure the coils after you put them into the oven? Depending, how long your staples ( holding the coils in place) it might be possible, that two staples are touching in your firebrick and creating a short.

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Thank you Lieblad and TpnyRV2. My grasp of the theory is pretty rusty.

I agree with what you've said. I was just trying to compare the current differences between a purely resistive load (outlined by the use of Ohm's law, and the spreadsheet to anticipate the current in the given furnace) and the actual (combined?) Circuit I-R, that is in use.

Thanks for explaining inductive impedance so well, I had a good reason to pull a reference off the shelf this afternoon and read some of it.
 
My best guess is coils is grounded somehow. IF your fuse blowing main blowing problem is only when the coil section is hooked up problem is with the coil. Seriously doubt it's a INSTANT blown fuse unless the coils are grounded. But that's just a guess at this stage.
 
So I checked the continuity between the coils and the case, and there is nothing. The coil measures 17.5 ohms resistance. Any thoughts?
 
I think you should actually measure the current. The fuse blowing at its rating isn't very revealing.
 
kdogmcg said:
So I checked the continuity between the coils and the case, and there is nothing. The coil measures 17.5 ohms resistance. Any thoughts?
Click to expand...

Assuming your line voltage is 220VAC, the coils will draw 12.6 amps at 17.5 ohms.





I am only asking to be sure, but have you closely checked the fuses to assure they are 20 amps rated? Have you tried several ones to assure it isn't a fluke. Did the fuse blow violently every time? I would stick in fuses from another box of fuses and see if that makes any difference.
Also, have you tested the unit with the control box cover with the SSRs not closed up. Something may be pressing against another component or a ground when the covers are installed. If that is the case, everything will test fine until you close it all up, and then it won't work.

Some ideas for testing ( test with the cover off the control box):
Disconnect the coils at the SSRs and turn the power on only ( the PID off) - did the fuse blow. If it blew on this first test with nothing on but the main power switch, you have the switch wired wrong ( or a bad switch) ... which is what I am starting to suspect.
If it did not blow, turn on the PID, but don't run the heat program - did the fuse blow?
If not, run the program- did the fuse blow?
If not, then the coils are the culprit. At this point you have run everything in the circuit with only the coils removed.
To verify that the coils are truly the problem, connect a 100 watt light across one SSR and turn the system on - did it light and the fuse not blow? If so, change it to the other SSR. Did it still light and things are OK. If both those tests are good, there has to be a short or other fault in the coils.
If the fuse blew at any stage of these tests, the problem lies between that test and the last one.
 
The fuses were 10 amp fuses. I thought that each leg would draw about 7 amps so 10 would be good. Is that not the case?
 
Total circuit current is what it is. There is no such thing as half on one side and half on the other for a total of "x" amps.

By the way, your original post claimed you were using 20amp slow blo fuses. Accuracy helps with troubleshooting.
 
No, Kevin, that is not the case. Look at it as sort of a 'push-pull' arrangement. When one side pushes 14 amps, the other side pulls 14 amps, and then the situation reverses, at the rate of 60 times per second...60 Hertz. For this reason, each side needs to be rated at the appropriate level. As Stacy stated previously, you'll need 20 amp fuses on each leg. This gives you a little head room and keeps you below 80% capacity which is the norm on electrical circuits. Glad you found the problem...now on to heat treating issues!! ;)
 
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