Re: I could cry......

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Peter Lawrence by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> I am very puzzled by this claim. A 12-120 NST actually puts out about
> 6-kv at 60-ma when in service in a neon sign, and I've disected some NSTs
> and measured their iron core cross section area and that always seems to
> agree with 6 x 60 => 360 watts for this example, not 1440 or 2880 watts.
> 
> I've never seen an NST whose core cross section would support its face-plate
> rating kV x mA (as opposed to other kinds of power transformers).
> 
> It was my understanding (possibly incorrect...) that core cross section
> determines maximum power transfer because the cross section limits the
> magnetic field energy and therefore the secondary power output.
> 
> Can anyone clarify this issue?
> 
> -Peter Lawrence.

	The 12 kV is the open-circuit voltage, and the 60 ma is the
short-circuit current.  The cross-sectional area of the core sets the
number of turns/volt required for a given flux density in the core; the
bigger the cross section the fewer volts/turn.  In principle has nothing
to do with the power handling capability of the transformer.  In
practice, because of winding resistance, more wire is required to handle
higher powers, requiring a bigger core on which to fit the windings,
etc.

	Note that the voltage of the transformer under load depends on the
nature of the load.  With a resistive load your transformer should put
out 6 kV at a load current of about 42 ma.  With a capacitive load the
current can vary over wide limits; for the "matched value" recommended
by some people the no-load voltage would rise until the insulation broke
down or the core saturated.  A transformer with a capacitive load such
as encountered in TC service can and probably will put out much more
power than the product of the nameplate voltage and current.

Ed