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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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