Heatsinking requirements for a 3 phase SSR. (converted a kiln to PID) - Page 1
Forum section: electronics / projects

author: XOIIO
author_rank: Super Contributor
timestamp: May 09, 2022, 04:18:05 pm
content: 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

author: XOIIO
author_rank: Super Contributor
timestamp: May 09, 2022, 04:33:36 pm
content: 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.

author: XOIIO
author_rank: Super Contributor
timestamp: May 10, 2022, 04:46:04 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2022, 05:34:23 am
content: 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'.

author: wizard69
author_rank: Super Contributor
timestamp: May 11, 2022, 12:08:21 am
content: 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.

author: XOIIO
author_rank: Super Contributor
timestamp: May 11, 2022, 02:39:42 am
content: Thought I had mentioned it in the comment but apparently not, must have been thinking of the video.Those temperatures are off of a probe on the back of the enclosure, right next to the solid state relay. I have it heatsunk with some MX-2 thermal compound to the back, so it's effectively the temp that the rear of the SSR is going to be at. The 2w per amp estimate is pretty useful though, had I thought of this in advance I would have probably designed it a little different but I do feel optimistic about the solution I hopefully found. Fans should be here tomorrow I think, and I have the shroud printing right now, though I'll have to check clearances in the actual box, since it's not 100% accurate to my cad setup.There are cutouts for ventilation on the sides, so it's not sealed, but at 70 watts from the SSR, convection cooling isn't going to be enough for sure, though you can definitely feel some airflow out the vents when it was at the high end.at .12 ish cubic feet for the enclosure, not taking into account the stuff in it, and assuming I get the full spec out of the fans which I won't because of resistance in the duct and whatnot, I'd be able to pass the entire volume of the box through the heatsink 230+ times per minute. Pretty sure I'll be able to dial the speed down a fair bit lol

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2022, 02:47:49 am
content: It doesn't really matter whether it's single or 3 phase, the dissipation is going to correlate pretty well to the total current through the relay(s). Single phase it will all be through one, three phase it will be divided across three, the total current is the same either way.Be wary of cheap generic SSRs, the vast majority of these are fake and are incapable of handling anywhere near the spec'd ratings. I would be very nervous about using them to control a kiln, I really hope you have some type of secondary cutout that will kill the power using a mechanical contactor if the temperature reaches excessive levels. Either open the circuit or crowbar it to blow a fuse or pop the breaker.

author: XOIIO
author_rank: Super Contributor
timestamp: May 17, 2022, 10:52:35 pm
content: Yeah, I'm not super confident about the relay either, but I went with a Novus PID controller, and have an audible alarm I can configure for over temperature, and the whole thing will be outside to be powered since there's nowhere indoors that I can use it.It also has a twist lock cable, so easy to disconnect quickly.

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2022, 12:34:40 am
content: Does the PID controller have an alarm output? I would encourage you to use that to power a contactor that will open the circuit to the heater, even if it's outside, safety matters and this is fairly easy to do.

author: XOIIO
author_rank: Super Contributor
timestamp: May 18, 2022, 03:45:59 am
content: it's got four configurable outputs, though the size of the box is extremely cramped and now with the addition of the heatsink it's packed. I might look into adding one at some point since SSR's do tend to fail closed, but the kiln is rated up to 1500 ish c, and I honestly kind of doubt it can go that high. I haven't left it on long enough to test but the elements are from the 70's, so they probably are a bit worn.It's the sort of thing I'll use on the weekend when I'm home all day anyhow, it will probably take like, 8 hours to do the full cycle, so I'll prep molds the day before, put em in first thing in the morning and then pour in the evening.