Re: ballast core

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Marc,

> <MShock8073-at-aol-dot-com>
> 
> In a message dated 4/1/01 3:22:52 PM Eastern Daylight Time,
> tesla-at-pupman-dot-com writes:
> 
> << One of the better variable home made inductors I have made so far
> consists
>  of an E
>  core ( 13.5" sqr area ) with 200 turns of # 10 THHN. To make the
> adjustments, I
>  slide the windings upwards ( off of the center leg) by 3/4"-1.25".
>  This increased the current from 15A to 40A. Other methods include
>  increasing the distance between the E and the I  ( increasing the air
>  gap) for increased Amp flow. >>
> 
> I know this may sound dumb, but exactly how do you measure the area of
> the core. I usually think of cores as 3 dimensioned items and that
> indicates volume as opposed to area. But, I see references to square
> inches. Does that mean that the core is measured by height and width
> of the cross section of one leg (IE looking from the 'end')?

The effective cross-sectional area of an E/E-I core is the cross-
sectional area of the centre leg. 
      In general, for any correctly built core forming a closed 
magnetic loop, you will find that the sum of the cross-sectional 
areas of *all* mating portions divided by two = the area of the 
largest leg alone. This includes all the potcore designs I've 
encountered. Anything else makes the core prone to leakage at high 
flux densities and less than optimal linearity up to that point.

> Big question: 
> 
> How important is it to have the silicon steel from an actual
> transformer core to make a ballast? Can I make a suitable replacement
> by plasma cutting 10Ga, or 3/16, or 1/4 hot rolled plate iron (common
> sheet metal) into a suitable shape, and stacking them into a big
> enough cross section? If so, then I have the perfect situation. I can
> plug the dimensions needed into my CAD program, and use the CNC plasma
> cutter I have access to, and make my E's from scrap material that
> usually get's sold for pennies a pound to the scrap yard.  I could
> burn a strategically placed hole or two in the corners to use for
> bolting together the laminations or stagger some holes to use for plug
> welding, and have a handy replacement for something I can't seem to
> find elsewhere. Although It might take a while to collect up enough
> laminations from scrap to do the job.

You want material that doesn't have a propensity towards significant 
remnance. I've come to the conclusion (along with others) that 
significant amounts of power are being lost in ballasts and 
transformers in conventional AC systems since the cores are having to 
deal with near squarewave switching conditions at breakrates 
significantly higher than mains frequency. Such power losses might 
not be obvious on short runs as the mass of the core would take a 
while to heat. For this reason, I've been leaning towards the idea of 
SMPS chargers for a long time. Power losses are easily made far lower 
than using conventional iron. If anyone doubts this, consider the 
results that Marco Denicolai has obtained from THOR. I suspect most 
people can do better than they are currently doing before resorting 
to solid state gap systems.

     As far as your proposed idea goes, I'd say that your laminations 
would be far too thick and prone to significant eddy current loss.  

Regards,
malcolm