Just a quick update on some issues that came up in the final stages of assembly.
SSRs…
First off the Inkbird solid state relays I was using, bought from amazon, failed. As I’ve mentioned before these are semi-dangerous because they fail closed causing runaway heating. Unfortunately due to the contactor not working as expected the thermal runaway failsafe doesn’t work however, I was still in testing phase so I was closely monitoring the system. It’s also worth noting these were rated for significantly higher current than what was running through them. I know they weren’t over-capacity because this whole thing is on a 50 amp Siemens breaker which would have tripped if we got anywhere near the rating of the SSR.
So I did the only thing I could, which I had been trying to avoid all along. I got on mouser.com and dropped about $80 to replace the $12 amazon SSR with a real name brand, industrial grade relay. This is rated for 75 amps so it should stay nice and cool as I don’t plan to draw more than 30. An interesting side note is this thing is significantly smaller than the Inkbird relays but weighs at least twice as much as one of them. I intend to take the Inkbird apart and get the part number from the MOSFET (that’s all these things really are) inside of it so we can see what the actual rating was. I’d be surprised if it was even 20 amps.
These coils are getting hot
So on to the next problem. The heating coils I’m trying to use keep burning out. They really aren’t rated for the current I need to pass through them to reach the temps we need. Ideally we would need to add more coils to increase the current rather than shorting the coil to reduce the resistance which increases the current which in turn increases the heat output. Pretty much what is happening is they are melting and failing apart.
Unfortunately there just isn’t room to increase the number of coils. These things, when properly stretched out, are at least 10 feet long and they need room between each segment so when they reach temperature and expand (drastically, probably 50% increase in length) they don’t touch the coil below them shorting the whole thing out. With the chamber being 9 inches by 9 inches by maybe 18 inches high we really don’t have a lot of room for these. One 10 ft string covers probably 80% of the internal surface area.
The latest coil winding I tried took 2 hours to get to 600 degrees (F) and pretty well leveled off there. I then cut the coil in half which got up to 1200 degrees in about 10 minutes but shortly after the relays blew and eventually the coil melted apart.
So I ordered some 17 awg nichrome wire, or should I say allegedly nichrome. I did some experimenting with a 20 awg piece of nichrome which measured about 16 ohms for 20 feet or so, which I then turned into a coil with the lathe resulting in about 8-10 feet with a pretty big radius per coil, maybe 3/4 of an inch. It’s currently in the smelter and may just end up being the ticket. Now the 17awg you would expect to have lower resistance, which it does, but I wouldn’t expect it to have drastically lower resistance. This stuff measures 0.5 ohms per 2 feet, or 5 ohms per 20 feet. Granted that would get some power into the smelter. Unfortunately I’m pretty sure putting 50 amps into it would just cause it to melt. I’m not convinced it actually is nichrome, the resistance seems too low but I haven’t actually done the math because it doesn’t really matter. The other problem is the piece I was able to get is only 10 feet long, so you know, about 100 amps.
So I’m kind of stuck right now. Seems like I need somewhere in the neighborhood of 18.5 awg nichrome which, as far as I can tell, is not a thing. The next test is going to be with a second coil of the 20awg nichrome. That will give us two which should get us to around 30 amps which will give us approximately a 7200 watt heater. The trick is to find just the right balance before I run out of money.
More to come, not much shop time this week, maybe next week. Subscribe to be notified and look out for my next upcoming series, DIY 3D printer and the upcoming-er series Propane Iron Foundry!! Don’t worry every series is unending and runs concurrently… life is pain