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60 Hz resonance


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From:  Robert W. Stephens [SMTP:rwstephens-at-headwaters-dot-com]
Sent:  Saturday, February 21, 1998 10:19 PM
To:  tesla-at-pupman-dot-com
Subject:  (Fwd) Re: my twin

All,

In a recent private exchange with Jim Fosse I came up with the 
following reply which I thought might contain information of interest 
to the list.  I am reprinting it here.

Robert W. Stephens
_____________________________________________________

Jim,

You are now pushing the envelope as they say.  Congrats!

You mentioned 2" arcs off the primary circuit.  I found that by using 
a certain value of air core choke between each HV terminal of a 15-at-30 
neon xfmer and the system spark gap, there seemed to be a very high 
voltage generated across the rf chokes.  I could draw as you say, 2 
inch sparks to ground from either rf choke terminal at the ends where 
they went to the system gap.  Even from the neon transformer terminal 
you could pull at least an inch of blue snappy spark.  I concluded that I was 
somehow shock exciting the chokes and neon secondary L with their associated 
parallel distributed C to ring at their natural RF frequency so 
created, every time the system gap closed to fire the main coil 
circuit.  I haven't tried to analyze this phenomenon further, but am 
well aware of it and whenever it happens I change my component values 
to "spoil" the effect which I am certain will destroy your neon xfmer 
very quickly. I suppose a series R inseries with the xfmer output and 
the RF choke would help a lot here.  I haven't seen this happen yet in a pole pig
size powered coil, but on these I haven't been using RF chokes in the HV 
supply line.  I am more concerned on these bigger systems with 
keeping the sucker away from 60 Hz resonance in the power circuit, or 
resonance at multiples of 60.  The total package, both the current 
limiting reactance and variac on the primary of the pig, and the 
system Tesla cap on the secondary side will conspire to form a low 
frequency resonance.  

On my twin I've recently substituted a nice little 9500-0-9500 center 
tapped, 4.5 kVA transformer(which worked superbly) with a single 
ended 16000V-at-10 kVA pig.  I found that the system safety gap across 
the capacitor was suddenly going nuts, blasting frequenly over a 1.25 inch air 
gap.  This had not occured before with the smaller, higher impedance 
supply transformer.  I grabbed a audio signal generator and o-scope 
and determined that at 1/2 variac setting (just where the safety gap 
would start to blast) and above the pig went into 
high-Q resonance at within 4 Hz of the 6th harmonic of 60 Hz.  This 
was causing (I'm guessing because I haven't scoped the output bushing 
of the pig during operation) that the 60 Hz waveform became distorted 
and a portion of this waveform must greatly exceed my normal charging 
voltage.  If I insert series R (1K ohms at 600watts) with the pig, 
this resonance is swamped out (verified by instrument measurements) 
and the safety gap firing is almost completely stopped.  At least a 
very noticeable improvement was obtained.

A list member has posted about a condition where he employed coax cable to run 
the HV from a pig in a control cabinet to a large TC.  He had a real 
voltage rise which caused arcover at the pig bushing to ground.  He 
explained that this was a Blumline effect from the coaxial cable, the 
voltage crest returning to the transformer when the rotary break 
closed.  That would only explain a 2X supply doubling and it sounded 
like there was a lot more voltage being generated than that to flash 
over the pig bushing to me.  I think he may have been seeing a combo 
effect with the RF resonance that I think you are seeing with your 
present setup occuring as well.  

Waddayathink?

BTW, I've been kicked off the Tesla list for my ISP bouncing too many 
messages.  You think YOU HAVE COMPUTER PROBLEMS!  : )

Cheers,

Robert W. Stephens
Director
Lindsay Scientific Co.
RR1 Shelburne, ON Canada L0N-1S5
Tel: 1-519-925-1771   Fax: 
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