Attach NST Output to NST Input

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Liam David
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Attach NST Output to NST Input

Post by Liam David »

This is probably not a good idea...
Let's say I were to hook up the output of a 3kV NST to the primary of a 15kV NST core submerged in oil. Would this produce really high voltage, or do something else unintended?
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Re: Attach NST Output to NST Input

Post by Jeroen Vriesman »

Very bad idea.

the maximum output of a 15kV NST is just a little bit more than 15kV, above that the isolation will give up.

.....and many more reasons why your idea won't work.
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Liam David
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Re: Attach NST Output to NST Input

Post by Liam David »

So, what are those reasons?

But what if you removed the core and put it under oil? Wouldn't that provide adequate insulation?
Last edited by Liam David on Sun Dec 28, 2014 11:02 pm, edited 1 time in total.
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Noah C Hoppis
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Re: Attach NST Output to NST Input

Post by Noah C Hoppis »

NSTs are great factory limited Hv sources but they are also complex, sensitive little snowflakes. They tend to internally short fi prodded above ~140 Vac. The main mode of failure in this scheme would be internal shorting due to over-volting, and unless you were willing to rebuild an NST (thereby defeating the point) there isn't much you can do about it.
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Rich Feldman
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Re: Attach NST Output to NST Input

Post by Rich Feldman »

To begin with, magnetic saturation of the transformer core. That depends only on construction details, primary voltage, and frequency.

A transformer has primary current even with no load (zero secondary current).
It's called the magnetizing current, and is normally a small fraction of rated operating current.
Just enough to cyclically magnetize the core, so the induced voltage in primary winding (we could call it "back EMF") matches the applied voltage.

The induced voltage is limited by core saturation -- the point at which the core material can't be magnetized any more. If you apply more primary voltage (at the same frequency), the magnetizing current rapidly goes up to destructive levels.
sat.PNG
from http://www.bookcracker.com/transformer/

If you took your 15 kV NST and tried to get 30 kV by connecting primary to 240 volts, you would immediately trip the 240 volt circuit breaker. The situation would be closer to a short circuit than to a mere overload.

If you connected the secondary of NST1 to the primary of NST2, things would be less dramatic. NST1 secondary current is self-limiting, and won't exceed the nameplate value (probably 20 or 30 mA). If that's less than the normal magnetizing current of NST2, then the second transformer will happily operate at _less_ than its rated voltage.

Exercise for Liam: measure the magnetizing current of your NST at normal voltage with no load. If you don't have a meter with an AC mA range, you can measure the AC voltage across a low-ohm sense resistor in series with the NST primary winding. Please tell us your face-to-face mentor is competent to help. For extra credit, see how the primary current changes when the secondary is shorted, or is lighting a luminous tube (e.g. unballasted fluorescent lamp), or a Jacob's Ladder.
Last edited by Rich Feldman on Sun Dec 28, 2014 11:35 pm, edited 1 time in total.
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Liam David
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Re: Attach NST Output to NST Input

Post by Liam David »

Thank you Rich for your helpful explanation and the graph. If I were to increase the voltage, I would have to decrease the frequency in order to prevent saturating the core and drawing too many amps, right?
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Rich Feldman
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Re: Attach NST Output to NST Input

Post by Rich Feldman »

Hi Liam.
Pay attention to Noah, who appears to speak from experience.
I have apparently been lucky never to break a working NST.
Can't say the same for ferroresonant CV transformer units.

To avoid saturation as you up the voltage, you would have to up the frequency.
The parameter being stressed is maximum magnetic flux swing in webers.
Same thing as the volt-second integral (per half cycle, per turn) in every winding around that flux.
"Switch Mode" transformers are run faster than 20 kHz, allowing whole volts per turn from small flux swings in ferrite cores.

As others have hinted, you can't increase the voltage by much without overstressing the high voltage insulation.
It's already potted in tar, so I don't see where oil immersion would help. I bet it would make a gooey mess.
All models are wrong; some models are useful. -- George Box
Charles Vorbach
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Re: Attach NST Output to NST Input

Post by Charles Vorbach »

It's pretty easy to connect NST in parallel though to get more current and then possible to use a CW multiplier to get the voltage you need.


Here is a link showing how to phase NSTs:
http://youtu.be/KSlQjw9OPG4
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Re: Attach NST Output to NST Input

Post by Richard Hull »

Get more current via parrallel connection? True! Voltage will forever be limited due to center taps, however. voltage multiplier effectiveness is related to capacitor stage values. Very large capacity needed to support stage effectiveness. It can be done, of course. Unfortunately, most here are not up to the task or have the money or supplies needed to make it happen. The issue usually rests with inexperience and lack of good reliable metering to know what you actually have at the output under load. Only the real go-getters stand much of a chance at this effort. When finished, if successful, they will have a very dangerous power source.

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