Re: [TCML] Hot Primary

[Steve Ward <steve.ward@xxxxxxxxx>]

The inner turns run hotter because the apparent resistance of the material
is higher due to "current bunching", which just means that the magnetic
field produced by the coil is such that extra eddy currents are induced on
the inner-most surfaces of the coil.  The b-field is higher in the center of
the primary, so the effect is worse, making the inner turn perhaps 2-5X the
AC resistance of the outer turn (per length).  I have done extensive
finite-element modeling of the problem and learned a few things:

-crowding the turns into a smaller space makes current bunching worse, as
the field is more concentrated around where the copper is.

-going to a smaller diameter tube is not as big of an efficiency hit as you
might think BECAUSE there is less copper surface area to be impinged by the
flux lines.  For example, going from 5/8" copper to 3/8" copper on my
primary design would only incur a 16% Rac increase.

-copper strap is the worst because it gives the builder the option to really
over-crowd the primary turns.  For a constant surface area, and turn
spacing, ribbon does beat tubing just slightly.  Every case is a little
different, though.  But generally, ribbon will not be a big winner, even if
you do space it out a lot, you could space out the tubing just as much and
get the same result, except the tube is likely easier to obtain and less
likely to cause flash-overs (another reason i dont pursue it).

I suggest you go to harbor freight and buy their 10 dollar submersible pump
and get a low flow rate of water running through your primary.  If you could
re-work your primary wiring to also be tubing, you could cool it too.  Water
cooling opens up a lot of options if you really utilize it (water cooled
bus, IGBTs, capacitors, etc..).  Welding cable sucks because you dont get
much more surface area by going down in wire awg, and the thick rubber
insulation surely increases the thermal impedance!

In any case, running 3000A on your primary should be enough to make over 15'
sparks.  If you are still operating the coil in your garage, it sounds like
an awful waste of potential, and you are just beating up on the system for
no good reason.  Of course, you could just be looking for a challenge, in
which case water-cooling is pretty standard for big power electronic
systems.  If your wiring gets so hot, id really be worrying about your
capacitors (particularly the DC bus filter caps).  They have to handle just
as much RMS current as the primary (actually a little more), SURELY they are
not rated for the RMS current you are pushing them to.  It sounds like you
are grossly overloading them...  watch for failures on extended runs!

Steve

On Thu, Feb 18, 2010 at 6:11 PM, Dave Leddon <dave@xxxxxxxxxx> 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.
>
> Dave Leddon
> Pleasanton, CA
>
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