Induction Heater Tank Capacitor

My earlist induction testing used a smattering of stock capacitors of unknown ratings. As I drove up the power, I purchased some pulse-rated capacitors suitable for a few kVA. This was the first bank proper:

Capacitor Bank

(Those are some strips of titanium getting about 1800°F.) Power was a few hundred watts at this time. Quick calculations showed these would not be anywhere near suitable for the 100kVA tank capacity I suspect I need. Therefore, another browse through DigiKey. I figured I would have to settle for less dense capacitors, in the 0.1μF range. Which means a hundred or two for what I want. Sigh...


...And here are all two hundred of them, stacked inside the large work coil (a bit over 3" height and diameter, around 4uH). That's 20μF at 630VDC (250VAC), 2.3 amperes maximum capacity per, 40 peak amps per, or 460A rms continuous, 8 kiloamperes(!) peak capacity.


Here's how they're going together: twenty per 12AWG pair, 10 sub-banks total. Use a heavy solding iron -- 40W at least -- to get these together. I used a 30W in one hand and a 140/100W soldering gun in the other to put these together. The 140W would've been enough except that it's rated for hardly 25% duty.


A little work every day and these turned up. I made the mistake of leaving only one end long, as I wanted to mount these from both ends on the tubing. More length wouldn't be a bad idea either, as, short on heat, I used a propane torch (uh...not on the carpet...I uh..swear....yeah...), VERY carefully, as the polypropylene capacitors melt easily.


Here's the finished unit, sideways. Four pieces of tubing snake around three segments in an "S" shape, with one end bent down to connect to the first half loop. This reduces inductance a bit (which is already pretty low by wiring each pole per side, look closely!).

Almost There...

The tubes are spaced pretty close (between 1/2 and 1 1/2", on centers) and always run in pairs, giving low inductance.


At first, I had the tubes joined in a custom angled tee, which didn't solder quite right so some gaps leaked, forcing me to use silicone goop on it... Later I decided to do it Right, once and for all. I got out some 5/16 and 3/8" tubing and silver solder and built my own tees. Not as elegant as the narrow angle tee I started with (that is, if it weren't a mess), but looks a whole lot cleaner and works fine. The four tee pieces were put together and soldered to the ends of the "S" pieces. Oh, and 3/8" flare joints, on the left for the coil and on the right for connecting to 3/8" hose for the water system. Another annoying story there, as Ace Hardware doesn't sell 3/8" flare to hose barb fittings. What I did was split a mating connector of each and solder them together. I also learned that it's impossible to braze brass, at least with the flux and filler I have. I got a much tighter and stronger joint using electrical solder!

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