Re: NST failure modes

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Gerry,

At 11:27 PM 7/1/2003 -0600, you wrote:
>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.

You may want to see the paper at:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html

http://hot-streamer-dot-com/TeslaCoils/TeslaCoils.htm

It has a transformer model in it that works well for me.  You should also 
see Tero Ranta's paper at:

http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/CurrentLimitedTransformers/NSTModel.htm

Simply reducing the primary to secondary coupling a little will simulate 
the current limiting of the NST very well.


>
>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.

Typically in the past, NST were always run at the series resonance point 
for greatest power.  Left open (for only 100mS!), they could hit about 80kV 
before self limiting effects like saturation will occur.  Of course, the 
voltage at that point would almost always short the secondary and cause 
carbon arcs that would fail the transformer.

Today we try to run in the LTR or Larger Than Resonant point.  We keep 
adding capacitance beyond the resonant point so the voltage drops back down 
to the rating but we also charge the maximum cap size for the most energy 
per bang.  Since the spark gap "pulses" the coil, we also take advantage of 
inductive kick effects to suck out much of the NST's stored inductive 
energy (1/2LI^2) too.  See:

http://www.richieburnett.co.uk/tesla.shtml

http://www.richieburnett.co.uk/indkick.html#kick



>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?

If the cap and NST inductance are resonant to 60Hz and the "load" is lost 
(as it often is), then the voltage skyrockets and blows the NST.  This 
probably accounts for 90% of reported NST failures.

With no filtering, the say 40,000Vp-p 250kHz signal is present on the NST 
output windings.  It has been theorized that the signal will tend to drop 
across the outer few high voltage winding layers causing shorting.  A 
common NST filter circuit is at:

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

NSTs tend to be old and have lived a rough life for perhaps 30 years before 
we stress them super hard.  Typically they are removed due to problems anyway.

With a large capacitor on the output, NSTs can draw higher than normal 
currents and this can sometimes saturate the current limiting shunts.  This 
effectively removes the current limiting in a nasty uncontrolled way that 
can damage the transformer.  I simple input fuse can protect against this.

Cheers,

         Terry


>
>Thanks from newbie,
>
>Gerry