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Re: NST power rating con
Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
Admittedly, the "VA rating" for an NST may not have much meaning. However,
there are probably rules and electrical codes somewhere that require that
rating to be known, labeled, designed for.... So many NST manufacturers
put it on. People that buy NST's don't care. There is no sales advantage
to selling a 15/60 NST rated at 900VA vs. 950VA :o)) Once in a great while
you may see a 15/60 rated at 450 "watts". But again, no one goes by that...
"VA" is an industry standard rating used in practically all power
transformers. It is the maximum rated voltage multiplied by the maximum
rated current. In the case of a 15kV 60mA NST, that IS 900VA. This rating
has nothing to do with how much "power" the transformer can handle or
deliver, but rather serves as a warning as to the types of currents and
voltages other equipment has to be designed for. Transformers are almost
never actually run at their VA rating due to imperfect power factor.
As an example that really does apply... What size fuse should an NST have
(neon sign use). Since it can only deliver 450 watts, the current should
be 450/120 = 3.75 amps. But if you use that fuse, it will blow all the
time. NSTs easily drive dead shorts (designed for it). So you really do
have to use the VA rating of 900. 900/120 = 7.5 amps which is the right
answer for neon sign use. Coilers fuse at about 15 amps >:-))) But we run
"overload" in complex ways NST makers never dreamed of...
So there is no "con game" here. Just too many engineers, rule makers, and
electricians thinking way too hard about it ;-))
At 08:10 AM 10/2/2003, you wrote:
>In a message dated 10/2/03 9:16:02 AM Eastern Daylight Time,
>Why is this con(fidence trick) by NST manufacturers not mentioned on any
>Tesla coil design web sites (that I've seen)?
>It's almost as bad as the *peak music power* con used by cheap audio gear
> The reason for the "trick" is the shocking fact that NST
> manufacturers are not designing their units for the benefit of the TC
> market. ;-) Most of my NSTs have a primary-side VA rating, which is a
> design-load value for knowing how to fuse the circuit that the NEON SIGN
> is on. Secondly, there is a secondary peak-voltage rating which is useful
> to the SIGN MAKERS because the peak is what determines how long a tube
> can be started by the transformer. Then there is the secondary
> peak-current rating which determines how brightly the tube will be lit
> after it starts. Nowhere do they specify an output VA rating.
> Evidentially, maximum continuous output power to a resonant transformer
> circuit is NOT a major concern of NEON SIGN makers. Fortunately or
> unfortunately, They are the market for which the transformers are made
> and for whom the specs on the plate are published.
> I believe that the last person to design and build power
> transformers specifically for TC use was Nikola Tesla himself.
>Hope this gives some insight into the why of it.