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I fried my SSR........ HELP!!!

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WattsThat:
You don’t need a neutral but the elements do need to be wired correctly.

You have (4) elements that are 8 ohms each. In order to have 7200 watts, you’ll need to:

1) Wire two elements in series, making two 16 ohm elements.

2) Wire the two 16 ohm elements in parallel so that you end up with an 8 ohm element.

That will be a 30 amp load at 240 volts, no neutral wire required. Changing out the 3 wire plug for a 4 wire only provides for a proper 120 volt control voltage which your modified kiln does not use (or need).

Personally, I would use mechanical relays and increase the cycle time of the controller to 30 seconds. The operating temps required versus the watt density means that turning on an element for a 30 seconds doesn’t raise the temperature by a degree, it takes several minutes to do that. Solid state relays almost always fail shorted and that results in a run away, not good with a kiln. That’s why they use mechanical controls. You could use two 20 amp contactors wired in parallel. If you’re going to stick with solid state, use four contactors, two for each 16 ohm pair to lower the amps and add some safety margin.

Proper high temperature wire is fiberglass braid over Teflon with the copper wire and all lugs being nickel plated. Anything else just oxidizes and fails at those temperatures. The right stuff is pricey and not available from the usual sources, the kiln manufacturers are probably the easiest place to source it.

My wife’s L&L kiln is controlled by their own custom PID unit that operates two mechanical contractors, one element on is low, both on is high. When the PID is within the proportional band, it use low heat, high when out, typically at startup.

james_s:
That's a really good point, a lot of those PID controllers can have the cycle time turned way down and for a kiln there's really no reason to have the precision you can get by rapidly cycling. At the very least you'd want a separate mechanical contactor with an over-temp cutout or a circuit to monitor the SSR to ensure that the output is not live when the control has it shut off.

jwilson:

--- Quote from: WattsThat on January 27, 2020, 05:30:43 am ---You don’t need a neutral but the elements do need to be wired correctly.

You have (4) elements that are 8 ohms each. In order to have 7200 watts, you’ll need to:

1) Wire two elements in series, making two 16 ohm elements.

2) Wire the two 16 ohm elements in parallel so that you end up with an 8 ohm element.

That will be a 30 amp load at 240 volts, no neutral wire required. Changing out the 3 wire plug for a 4 wire only provides for a proper 120 volt control voltage which your modified kiln does not use (or need).

Personally, I would use mechanical relays and increase the cycle time of the controller to 30 seconds. The operating temps required versus the watt density means that turning on an element for a 30 seconds doesn’t raise the temperature by a degree, it takes several minutes to do that. Solid state relays almost always fail shorted and that results in a run away, not good with a kiln. That’s why they use mechanical controls. You could use two 20 amp contactors wired in parallel. If you’re going to stick with solid state, use four contactors, two for each 16 ohm pair to lower the amps and add some safety margin.

Proper high temperature wire is fiberglass braid over Teflon with the copper wire and all lugs being nickel plated. Anything else just oxidizes and fails at those temperatures. The right stuff is pricey and not available from the usual sources, the kiln manufacturers are probably the easiest place to source it.

My wife’s L&L kiln is controlled by their own custom PID unit that operates two mechanical contractors, one element on is low, both on is high. When the PID is within the proportional band, it use low heat, high when out, typically at startup.

--- End quote ---

IT WORKS!!!!!!!!!!! I did what you said and wired the elements in series and then in parallel and disconnected the ground and it works!!!!!!!!!!!

YOU SIR ARE A GENIUS!!!

THANK YOU! :clap: :clap: :clap:

WattsThat:
I’m no genius but I understand that words can sometimes work when pictures don’t.

Now, the fun begins - tuning the PID. Hopefully the controllers auto-tuning algorithm works well enough for your application.

At the risk of repeating myself, I’ll mention again that you should increase the controller cycle time as the 2 second default value is extremely short and is likely to result in the inability to maintain a stable final temperature. 10 to 20 seconds is more realistic with such a large thermal mass like a kiln.

You’ll need to test the range of final temperatures you’ll be using to insure there is little to no overshoot of the target (setpoint) value, that’s what the tuning is all about - reaching setpoint as fast as possible without overshooting.

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