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Re: [TCML] 1 Mhz ,1 kw

Hi Gary,
I agree that safety gaps are a necessity and should prevent the failures if set properly.
There is of course more to it than quick zero crossing passings. If the drift speed relative to sync is slow enough, it can hover around the low zero crossing area for a decent length of time. It's probably true that NST's can be used with ARSG's, but there may be cases where it's troubling as the safety gap may fire frequently (and the more it fires, the lower the breakdown voltage across the safety gap due to heat and material, so the frequency of breakdown increases).
I remember back when some coilers using ARSG's with NST's were having failures with the two components, enough so I think that the correlation began to stand out and it soon became a "bad" thing to do on the TCML. It's possible that some of these NST failures did not use safety gaps. It's also possible that even though some did use safety gaps, the frequency of the safety gap firing caused the coiler to open the gap up too wide for NST protection.
In other words, a safety gap is more likely to become very annoying on some ARSG's. As a result, coilers seem to always open up the gap (and thus the NST's are in danger). This may be the _**real**_ reason for NST failures with ARSG's. 

Take care,
Bart

Lau, Gary wrote:

I think it's important to understand the real reason (IMHO) that ARSG's _might_ cause NST failures.

That zero-voltage crossings can occur is not in itself a problem.  They happen all the time with static gaps.

The problem is when users don't have a properly set safety gap in parallel with the RSG, AND they have a speed control on the RSG motor and run it significantly below 120BPS (or forget to turn the motor on before the NST is on).  Under these conditions, yes indeed, mains resonance will cause the cap voltage to ring up to voltages that will damage the NST and/or cap.

But the solution is easy - ALWAYS have a properly set safety gap in parallel with the main gap, for when things go wrong.  The safety gap in a Terry filter (or the MOV's) will accomplish this as well, though I prefer to have a separate 2-terminal safety gap built right onto my RSG (see http://www.laushaus.com/tesla/sync_gap.htm; it's a sync gap but same reasoning).  If one has a properly set safety gap, I don't believe that ARSG's are any more likely to damage an NST than anything else.

Regards, Gary Lau
MA, USA


-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of bartb
Sent: Monday, March 23, 2009 9:54 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] 1 Mhz ,1 kw

Hi Dex,

One other thing regarding the ARSG. It's not good for NST's. Breakdown
will occur throughout the 60Hz charging waveform and it will breakout
whenever the cap reaches the breakdown voltage. Zero crossing occurs in
the ARSG and the voltage is allowed to climb to the next break, at which
point the cap (especially if STR) is allowed to charge to a higher
potential than the transformer output voltage. That extreme voltage can
be lethal to the secondary windings inside an NST which simply doesn't
handle excessive high voltages well. So again, a potential transformer
would be a better choice here if you build an ARSG.

Take care,
Bart

Dex Dexter wrote:

Thanks for the photo.
I think power source for the coil I want to build is a potential transformer.
And I'm thinking of use ASRG with 300-400 PPS.Cp about 10 nF.
Still thinking of how to realize good 40 kV capacitor bank for the purpose.

Dex


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