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Heatsinking requirements for a 3 phase SSR. (converted a kiln to PID)

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Hey all, so I finally finished converting an old kiln to PID with a custom designed control box, for use as a burnout oven, and it's working great so far, with some caveats.

I'm not too sure if it's the proximity to the outside, or the SSR heat output itself, but when I'm at the higher end/the relay has been on for an extended period, it gets pretty damn toasty.

This is a fairly generic/cheap relay, so finding a datasheet isn't proving super easy but I'm hoping there's a sort of general rule of thumb guide when it comes to heatsinking these larger relays based on the power they are switching. It's usually about 34 amps/4000w when going at full tilt.

I'm going to give it a shot with a ramp and soak program, since if it's not running at 100% duty cycle it should hopefully stay cooler, but when the kiln is up near 700 degrees the outside does get fairly toasty. I'm going to look into designing some cooking fans with a duct, and hopefully find a heatsink in the right form factor to pull heat from the back panel of the enclosure, but that's going to take some time.

If the RNS program doesn't make much difference then I'll unmount the box and try it further away from the outside, perhaps adding some glass blanket insulation between the kiln wall and the box, though even at 750c the outside isn't as hot as the actual back of the control box.

You can check it out here if it's of any interest. https://youtu.be/UcpD4H6Yyy4

I had been able to find some charts for single phase relays that basically showed the temperature the rear plate should be at based on the load they're switching, but no such luck for 3 phase so far, though I was fairly sleep deprived when looking.

So, I slapped an old amd heatsink on top with some thermal paste, and an 80mm fan, and that helped a fair bit with temps. It was only about 18c outside, but the enclosure dropped from 72 down to 64, and the other day it had reached over 95.

I designed a duct that will fit two 40mmx15mm delta fans in front of the heatsink and direct the air out one of the vets, then I'll mount it up inside the case. Should be enough room to fit, and since I ordered some delta fans with speed control I should be able to fine tune the volume vs temperature or set up a fan curve with an arduino.

Still interested in any rule of thumb or general calculations for heat output on 3 phase relays.

AC only SSRs use a TRIAC as the load switch.   A typical TRIAC will drop between 1.4V and approx. 2V over most of its rated current range, so if you figure on 2W per Amp, you'll be 'in the ballpark'.

Often the SSR manufactures will have properly sized heat sinks available.   'Proper' being what ever environmental conditions they designed for.   You also need proper clearances for those heat sinks to work.   In any event I always mount the SSR's in a way to dissipate heat, that means using heat sink compound even if the SSR is directly panel mounted.   Generally some sort of heat sinking is a good idea.

Any electrical box that has more than a few components in it should be ventilated usually with a fan.   It should be obvious that everything producing heat in the panel box needs a way to get rid of that heat.   You can quickly exceed a sealed panel boxes ability to dissipate that heat.

"Toasty" is a meaningless term.   SSR's like many components produce heat and are expected to have a temperature rise.   Until you actually measure the temperature you really don't know if the components are getting too hot.   This is the same problem I have with guys that say their stepper motors are getting too hot.


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