Re: NST failure modes

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

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

Hi Gerry,

On 2 Jul 2003, at 2:13, Tesla list wrote:

 > Original poster: "Gerry Reynolds by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <gerryreynolds-at-earthlink-dot-net>
 >
 > Hi everyone,
 >
 > I'm new to this group and enjoy reading the posts very much.  I have an
 > electrical engineering background but only limited experience with HV
 > TCs.  I have built one coil 3.5"x40" using 30 guage.  I have an unknown
 > transformer (single pole and maybe 5KV output that charges a glass plate
 > capacitor in parallel with the xformer output.  I have a spark gap (nails)
 > in series with the 18 gauge helical primary of the TC. I was happy with the
 > results and got maybe 4-5 inch leaders and maybe a 9 inch spark when a
 > grounded wand was moved close to the 3 inch brass top end load.
 >
 > After reading some TC theory posted in this group, I want to upgrade this
 > setup to a NST to get more power and longer sparks.  Next, I want to build
 > an new coil 12"x36" using 21 guage and use that as a vehical to learn more.
 >
 > Now for my question.  I read some of the posts about NST failures and want
 > to learn more about these failure modes.  I reviewed my engineering books
 > on transformer modeling (its been a long time since I have had to deal with
 > transformers)  and think the appropriate model for this application is the
 > "ideal xformer - modeling the appropriate turns ratio.  The primary of the
 > xformer would then have an R and L in series with the primary as well as in
 > series with the secondary.  There are other shunt elements in the model
 > that I think can be neglected at least for now.
 >
 > The NST being current limited by flux leakage in both the primary and
 > secondary and by the shunts that many have talked about, is normally
 > protected against shorts.  I can see that if a capacitive load is applied
 > to the secondary, the load reactance can reduce the inductive reactance due
 > to the leakage flux in the xformer.  If a series resonance occurs,  I can
 > see how the current may increase above rating maybe damage the secondary
 > windings.  I can also see how a series resonance can amplify the voltage
 > out.  I have also read where the RF in the TC primary can affect the
 > voltage on the NST secondary.  Can someone explain what kills NSTs,
 > overcurrnet, overvoltage, resonant charging, or whatever?

In my experience, NST deaths usually result from insulation failure
due to overvolting, rarely if ever from overcurrent (this would of
course depend on the type of winding wire insulation). The spark gap
settings can easily dispense with voltage rises due to resonant
charging of the primary cap. The real killer is either/and/or
differential and common mode voltages of low energy reaching the
transformer terminals from shock-excited parasitic reactive components
between the main gap and the transformer. The two most common ways of
dealing with this are an RC filter, maybe incorporating VDRs, or
simply keeping the wiring between the transformer terminals and the
main gap as short as possible (a few inches - this is my favourite
method). It is also good practice to place the main gap rather than
the primary cap in parallel with the transformer terminals to
minimize the transformers exposure to high amplitude HF oscillations.

Malcolm