current limiting chokes was Re:Bombarder XFMR

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

One can space the layers a bit with wooden or plastic spacers to allow
cooling air to get between the windings without greatly increasing the
leakage inductance (which isn't much of an issue here anyway) or decreasing
the inductance (same).

I'd worry more about eddy current losses in your solid bar core. Scrounge
up some transformer laminations to make the core out of, or, use a bundle
of insulated (varnish is ok) iron rods (as in welding rods?).

Of course, in this application, you might not care about extra losses,
since most folks find that adding a bit of resistance helps "stabilize" a
mostly inductive ballast.

One could also just put taps on your inductor.  If you divide your big coil
up into a bunch of smaller coils, you can interconnect them in various ways
to change the inductance.  For instance, if you had 60 turns, you could
split it into a 32, a 16, a 8, and a 4, and get 16 different inductances
fairly easily (and probably more if you take into account the transformer
action between windings and work out some complex switching schemes).

For what it's worth, to limit current at 240V to 30Amps, you'd need an
inductive reactance of 4 ohms, which, at 60 Hz, is about 10 mH ( 2*pi*60 =
377 =ca.400).

An interesting design equation I have for bulk coils (air core) is:

L (uH) = 1.05 * D/m * N^2 * (D/u)^.75
where (all metric units):
D = mean diameter
u = circumference of cross section
m = mean length of winding
valid for D/u < 1 (that's a one)

Ballparking.. 200 turns on a form 10cm (4inch) in diameter and 10 cm long
(4"), with the winding thickness on the order of 1-2 cm gives 7.3 mH....

No idea if this equation is really right, though...

Someone should go through and do some measurements on bulk 100 and 500 foot
spools of things like #10 wire.  It sure would be nice to be go to Home
Depot, plunk down a few bucks and have a ready made inductor.  Anyone have
an impedance bridge?

Tesla list wrote:
> 
> Original poster: "Loudner, Godfrey by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>
> 
> Hi Matt
> 
> You can have multiple layers up to the desired inductance. Because you need
> heat sinking, not too many layers are encouraged. For example, three layers
> are just fine.
> 
> Godfrey Loudner
> 
>  > Original poster: "Matt Skidmore by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <fox-at-woozle-dot-org>
> >
> > maybe one could make such a choke? if we get the dimentions right.. wind
> > some thick gauge wire on some pvc and then use a metal bar as the slider.
> > would that produce the same results? one question i have is, do chokes
> > have to be wound say like a secondary, or can you put several layers on
> > cable on it?
> >
> > -matt
> >