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Re: Top Toroid



Tesla List wrote:
> 
> Subscriber: Esondrmn-at-aol-dot-com Thu Feb 13 22:09:05 1997
> Date: Thu, 13 Feb 1997 18:25:48 -0500 (EST)
> From: Esondrmn-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: Top Toroid
> 
> In a message dated 97-02-13 03:47:34 EST, you write:
> 
> << Ed,
> 
>  Running _any_ cap at higher break rates in Tela Coil service will
>  certainly stress it more. If it has any internal weakness (especially a
>  poor/higher resistance interconnection), it may fail from the additional
>  heating and vibrational stress. However, ANY cap, if pushed far enough,
>  will fail. High break rates (>600 BPS), coupled with high TC primary
>  currents (i.e., relatively low primary L) are a recipe for cap failure.
> 
>  Safe coilin' to you, Ed!
> 
>  -- Bert --
>   >>
> 
> Bert,
> 
> What do you call low?  When the C.P. cap failed, I would guess the primary
> was tapped at about 80 microheneries.  I was running at about 7 kva at 14,400
> volts.  As I said, the break rate may have been as high as 1,000 bps -
> probably just beating the hell out of the cap.
> 
> Ed Sonderman

Ed,

"Low" inductance is a relative parameter versus tank cap size. Primaries
with relatively few turns can have deceptively low inductance, since the
inductance scales as the square of the number of turns. Less primary
inductance means more peak primary current flow "through" the cap each
time the gap fires. For example, for a typical pancake primary, with a 9
inch inner diameter, 0.625" turn-turn spacing, driven from a 0.025 uF
cap and a gap breakdown voltage of 15 KV generates the following peak
primary currents. The factor of increase in the degree of ohmic heating
versus the 12 turn primary is shown in the far right column.  You can
eaisly see that a 4:1 decrease in turns results in a 16:1 increase in
ohmic heating alone. 

    (Turns)     (uH)    (Amp)  (Amp^2)   Ohmic
       N         Lp      Ip     Ip^2    Heating 
      ===       ====    ====    ====    =======  
       3        4.1     1475  2175625   15.9X
       6       15.9      755   570025    4.2X
       9       36.0      501   251001    1.8X 
      12       66.0      370   136900    1.0X
       
Unfortunately, caps are destroyed by many other factors, such as voltage
overstress, dielectric heating, internal mechanical fatigue, and partial
breakdowns of the dielectric fluid or the diectric itself. All of these
tend to be a functions of the energy per "bang", % of voltage reversal,
maximum surge current, and _rep rate_. In general, doubling the rep rate
will double the dissipative losses since you're processing twice the
average power. A high rep rate, combined with extended runtime, can be a
"cap-killing" combo; increased power dissipation generates more heat
than can be conducted away, causing the temperature to rise
catastrophically. This is a problem that typically occurs deep inside
the capacitor.   

BTW, 80 uH is not particularly "low" for a 0.025 uF cap. It was more
likely that the high rep-rate caused excessive internal heating and
failure.  BTW, is Is your new CP cap now working "solidly" in your
system?

Safe coilin' to you, Ed!

-- Bert --