Re: I could cry......

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 4/16/01 9:04:11 PM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes:

> 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.

Peter,

In neon sign service, the load is resistive, and the input voltage to
the NST is
normal.  This condition lets the shunts work normally.  In a Tesla
coil however, the primary tank cap can form a resonant circuit
depending on the capacitor size, that causes the magnetic
leakage reactance of the NST to be "cancelled out" during operation.
This makes the NST behave as a non-current limited transformer.
It is with a so called "matched size" or resonant sized capacitor
that this effect will occur (or near resonant size).

Another way the NST can draw more than its rated power is if
the input voltage is turned up higher than normal using a 0 -140
volt step-up type variac (this is often done).  The higher input
voltage changes the behaviour of the shunt, and lets more power
though.  
>  
>  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).

The NST's can all support more than their face-plate ratings.  They
may run hot, but Tesla coil use is intermittent, so we can get away
with it.  Many folks remove the shunts from their NST's causing the
NST's to deliver much much more than their rated power.
>  
>  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.

The core cross section limits the voltage but not the current.  Too
much current will increase the losses, but it will still work.

My unmodified 12/30 NST delivers 610 watts in my TC, and gives
42" sparks.

John Freau

>  
>  Can anyone clarify this issue?
>  
>  -Peter Lawrence.  
>  
>  
>  >> > What is the max I can expect for arc length with a 12/120? I got a 
>  >feeling 
>  >>  > I'm TERRIBLY inefficiant right now. 
>  >>  
>  >>  65"  SRSG would help.
>  >
>  >It is helpful to keep in mind that a NST with a SRSG (or even with
>  >a static gap), can provide double its rated power, esp if a step-up
>  >type variac is used.  For instance, a 12/120 NST, although rated at
>  >1440 watts, can actually deliver 2880 watts.  With this power, an
>  >efficient coil should give 91" sparks.
>  >
>  >John Freau
>