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Re: NSTs and The HEAT
This post never made it through, so I am re-sending it
with a few minor changes.
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> From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: NSTs and The HEAT
> Date: Friday, September 25, 1998 8:06 AM
>
> Coilers,
> The confusion comes from the multiple meanings we assign to the term
> "burn out the transformer".
>
> When you run a neon sign transformer un-loaded then you stand the
> greatest chance of destroying the transformer *DUE TO OVER-VOLTAGE
> ON THE SECONDARIES*. Recall (or learn for the first time) that neon sign
> transformers are designed to run under load and are current-limited.
> When you first turn one on, the quite high voltage of several
> kilovolts will cause the neon gas in the neon sign to ionize and
> begin conduction. Once the sign is 'on' then the voltage required to
> maintain electrical conduction is reduced *greatly*. Typically less than
> a kilovolt is necessary to *maintain* conduction, even though several
> KV are needed to *initiate* conduction.
>
> If the NST is run with no-load, then the transformer gets to
> experience the full stress of the rated secondary voltage. But the
> NST was not designed to experience this level of sustained voltage
> indefinitely. Thus, running the NST un-loaded can cause a voltage-
> related breakdown of the transformer. Rare, but it happens.
Now I know that there are going to be folks on the Tesla list who will
swear that a 15KV NST can produce 15KV all year long and not fail.
And I will simply reply that *most* of them will not fail. But any
coiler who has opened up numerous NSTs to re-pot them will tell you that
it is not uncommon to find cracks in the potting tar of older units.
Moisture can enter through the cracks, and the high voltage does the
rest. The "tar", BTW, is used not only for insulation, but also for
its ability to distribute the heat within the transformer.
>
> When the NST is destroyed in this manner you might refer to it
> as having been 'burned out' due to the HV breakdown, but this
> terminology is somewhat misleading.
>
> In a "regular" un-limited transformer you usually experience true
> "burn out" when one of the secondaries gets shorted out and the
> secondary begins to draw excessive current. A neon sign transformer
> cannot experience this form of "burn out" since it is current-
> limited.
>
> If you short out the secondary of a neon sign transformer then the
> current-limiting shunts will limit the current. The secondary has
> a resistance. The limited current will flow through the secondary
> and there will be ohmic heating of the secondary following the
> familiar P=E*I or P=I^2*R power formula. You can run the neon
> sign transformer in this shorted mode all day and it will *NOT*
> "burn out", because it was DESIGNED to run in current-limited mode.
> It *WILL* get warm/hot because of the above-mention power loss in
> the secondary windings, and it is this heating that helps get the
> tar next to the transformer loosened up after you have frozen
> a neon sign transformer prior to final disassembly.
>
> To re-cap:
> (1) An un-loaded NST will run cool but can experience voltage-
> related breakdown, since its output should normally be less than a
> kilovolt once it has ignited the neon sign tube.
>
> (2) If you short the secondary of a neon sign transformer then the
> current-limiting kicks in. The transformer will run hot, but
> it was designed to do so, and this will not damage the NST in any
> way.
>
> I hope that this explanation helps.
> Fr. Tom McGahee
>
> ----------
SNIP