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[TCML] Re: Hot Primary
Dave Leddon wrote:
As I push my DRSSTC into ever higher currents (presently limited to
3000 amps) I am experiencing some significant heating in the primary
circuit. I can see that in the future, as we push the envelop with
even higher power silicon, we may have to resort to water cooling.
Even with 3/4-inch od type L soft copper water pipe (was that ever a
bear to bend) the primary becomes too hot to touch after running the
coil for one minute. I even changed the interconnecting wires from
#4 to #2 welding cable to prevent meltdown, but oddly the larger wire
gets just as hot. I wonder if there isn't some sort of rf heating
effect unrelated to voltage drop which is generating some of the
heat. A related issue which has been bugging me for some time, why
are the active primary turns progressively warmer as you move inward
toward the secondary? One would think that all turns carry the same
current.
I'm unfamiliar with the particulars of your primary drive circuit, but
it seems to me that there shouldn't be *any* noticeable heating of the
copper at all, otherwise the unloaded Q of the primary will be very low.
When I first brought up the twin prototype, I noticed the unloaded Q of
the primary circuits was horribly low... about 10. Even then, the IGBT
heatsink was the only thing that got warm to the touch. To raise the Q,
I had to beef up the copper in key areas. I discovered the weak points
by passing 100A DC continuously through the entire primary circuit, and
surveying IR drops point-to-point with a DVM. After a bit of copperwork
I got the unloaded Q above 30 and stopped there, since the loaded Q is
well below 10. The primary is a single turn of 6" Cu strap, as can be
seen near the bottom of this pic:
http://www.lightninglab.org/misc/NLL_Prototype.jpg
A flat strip about 2pi the skin depth in thickness gives the best copper
utilization. There's no noticeable rise in the primary Cu temperature.
[snip]
I'm still waiting for a window of opportunity to drag the coil over
to Greg Leyh's laboratory where I can really crank it up without fear
of irritating the neighbors.
Bring it! We have 480V 3ph 400A service now. ;>
-GL
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