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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi everyone,
After a long, long time I finally did some more runs on this onster, all successful with hgtg30N60 igbt's still in place. I now installed an overcurrent protection circuit, replaced the troublesome primary former and filmed for whatever is going to come out of the coil.
From the action of the curent limiter I could now estimate the values of the primary current, and they are indeed batshit insane. For the run in the video I set it to over 60amps, while these igbt's are rated for only like 30amps at 100kHz. I put the faith into the fact that the devices are only switching a portion of this current, so perhaps less than 30A in "hard switching", as well as into shear ruggedness of these devices.
The coil stops in the video once the current limiter acts, I set it with a quite large time constant, so I can turn the variac down a bit when this occurs. Had the coil break out once from the side of it's toroid when it restarted, will need a bigger toroid soon!
Only one big problem remains - my fans seem to slow down when the coil is run (this can actually be heard in the video) apparently due to very close proximity to live heatsinks (yet I didn't see this happen with other models of fans). So I'm most likely going to have to shield or replace the fans, the first is a lot of work and the second is costly. But for short runs this seems ok and heatsinks are dead cold all the time.
I may try to run this until destruction just for the heck of it tomorrow... as it certainly looks like I'm not going to get much more power with this bridge in this topology. I measured the DC bus voltage and single phase current with moving coil meters - in this run I was up to about 300V and 16A; my brain hurts now because I'm unsure how to read this data, if those 16A were real RMS current per phase and power factor is close to unity this would mean over 14kW input which just can't be right... I also have a 10A breaker that didnt trip even with this overload.
Regarding the fate of this coil, I might definitely want to upgrade it to series resonant topology or something similar that won't put such extreme reactive power stress onto the bridge. Instead of buying prohibitive amounts of copper pipe I was thinking of using a bigass ferrite transformer, to step down the output from my bridge. With a core such as one of these:
The resonant cap still remains a problem though, for my current primary I'd need about 100nF cap with very high current rating. (BTW, those doorknobs I had turned out to be the useless type).
Still if I got a proper resonant cap I could even upgrade the coil into QCW one day.
Marko
PS. Can anyone point out to some usable capacitors on ebay? I'm kind of lost regarding what to look for. Would I have any use from those old mica/silver bricks? At last I thought I can use a big geek cap MMC borrowed from the DRSSTC, although that's probably rather overkill.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hey guys,
well, neither my caps nor the fans have arrived yet, but I did some work on fixing my variac startup circuit to finally stop it from tripping breakers when being plugged in... Fluorescent tube starter now adds the extra delay required. Should do some good CW spork when I finally get myupgrade parts.
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"Big Bad" the CW coil
