PID control on old electric kiln.

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brian923

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Jul 20, 2015
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Hey guys, been doing a lot of thinking about converting an old kiln i have into a heat treat oven. It's an old top open pottery kiln, I know not the best, but it's what I have. I'm going to replace the mechanical timers with a ramp and soak PID, use a high temp k-style TC. My question to you is, should I set it up for 1 or 2 SSR's?

The info I have is:
Power: 220 single phase.
4 sets of elements. (See pic)
Each element ohms out at close to 40 ohms. From top to bottom I have 38.7, 39.5, 39.0, and 33.6 measured ohms.

I figure I could either run them all in parallel with a total of 5176.5 watts @ 23.52 amps. Or split it up into 2 circuits of 2468.5 watts - 11.22 amps and 2710.42 watts-12.32 amps. Total amps of the single circuit rated at 125% for resistive heating loads makes it 29.4 amps.

What would you guys do? Is there anything gained by running two SSR'S over a single SSR?

 
It is up to your power supply to decide about the amperage desired. If you have heavy enough lines in the shop to rum a 40 amp 220 line, then the higher wattage will be OK. IF a lesser draw line is needed, use the lower wattage.

As to the SSRs, use two - one per leg of the 220VAC.

Why do you have different readings of ohms written by the same coil in the photo? Way that two different tests?
 
Yes, I forgot to explain that. The ones on the left are the elements by themselves. The ones on the right were how it was hooked up when I took the old control box off. The middle two elements were hooked up together in parallel.

And your saying it's best to control both hot legs of the 220 circuit? That's what was confusing me. I keep seeing all the 220 v pid circuit's with one ssr controlling only one of the legs. Makes sense to control both along with a 220v disconnect to isolate the entire 220v circuit.
 
Many of the 220 volt PID diagrams online are from Europe where the 220 is straight phase, like our 110.

Using two SSRs to control the circuit assures there is no potential on the coils. One SSR would turn it on and off, but the coils would still have 110 volts potential to ground. This could kill you if the blade touched the coils and you were grounded. Other problems could happen, too. This is enough of a concern to me to suggest the use of two SSRs, which will completely de-energize the coils.
 
I have been doing a lot of research on this as I am an electrician by trade but most of my training is in distribution systems. I have some training in controll and am learning a lot more. I have always thought that control systems were super hard and complex, and they can be, but most are just simple circuit's and once you figure out the mechanics behind the components, you can build simple systems that work pretty amazingly. Thanks for the help, I really appreciate it!
 
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