Okay, I see what is going on here now. That makes sense. That’s definitely good info to know, thank you!

Yeah, its a bit hard to explain because of all of the dwell periods it goes through. It pulses 80ms every 2 seconds for 4 times (~130Apk), then dwells like 5 seconds, does another set of 4 pulses, dwells for 10 seconds, turns on constantly for about 30s (so only pulling 8Arms at this time), dwells 20s again, turns on for 30s again (again 8Arms), dwells 10s, and restarts the process.

So this circuit is 400VAC 60Hz 3Ph… dV/dt seems to work out to 0.137v/us which is much less than 500V/us. di/dt works out to 0.031A/us which is also much less than 50A/us. Off state voltage is 400Vrms, so ~565Vpk which is also less than 1600Vpk rating. I think we should be good there. Only question is, can the SSR handle reverse voltage applied to the outputs? So we have a pair of contactors per motor, one for forward and one with two leads swapped to be in reverse so when the first relay is not active but the second one in the pair is active, the first one is seeing a reverse voltage. That would still be less than 1600Vpk though. Will this be okay?

I am thinking this will not be terribly important in this scenario since the current mechanical contactors take a certain amount of time to physically move and close the contacts, though I don’t know what that time is I would assume a few milliseconds. Also, the nature of this quick pulse is just to bump the motors in a direction, so the timing is not critical. We can adjust the pulse width if necessary to account for this though if needed.

Again not a problem, small leakage current is acceptable and we have separate safety disconnects on this machine.

If I did math right, The average rms amperage over the duration of a sample pulse we measured is 59Arms for a duration of 0.06s. Plugging that into your equation gives 59A*(1V+0.002Ω*59A)=66W, or 3.96J for 0.06s duration. So average power until the next pulse would be roughly 2W, and using our best case heat sink with a value of 0.55C/W we get 1.1C temp rise which is great. Even considering absolute worst case here of 66W continuous, that gives a temp rise of 36.3C + 30Cambient = 66.3C which is still within allowable limits. So we should be perfectly fine using this SSR, sound correct-ish?

So one thing I read in the link to the thermal calculations you attached earlier was the breakdown and degradation of the thermal junctions, increasing thermal resistance. Is that something we will need to worry about with this type of device also? Of is that specific to the type of device referenced in your attachment?