RE: Ok, what is an LTR cap.

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Deano,

	The real key here is that in an NST the primary to secondary coupling is a
little loose so the secondary is sort of free to ring up.

	In a pig, with very tight coupling, this degree of freedom would have to
be the ballast.  So if you rang the secondary up into ultra high voltages,
the primary would follow via the turns ratio.

	It all sounds sort of nasty.  Personally "i" would try in on a computer
first ;-))  

	Off hand, I would think you would divide the ballast inductance by the
turns ratio squared to get the transformed impedance value of the ballast
as it looks on the secondary side.  Then choose your capacitor to resonate
at 60Hz with that value.

At 220VAC and 10kW at 60 Hz, we could choose a ballast of Xl = 4.84 ohms or
12.84mH.  With a 14400 pig that transforms to 3uH.  To resonate at 60 Hz
the cap value is 2.34F.  An impossibly high cap value...

So it would appear that there is no chance of getting resonant or LTR
charging from a pig.  However, if I should have multiplied the square of
the turns ratio instead of dividing, I would get 127.5nF...  If I have made
that error (which the book on my lap says I didn't...) then the resonate
case would be easy...  Perhaps others could please double check my
reasoning and math here...

I would think such a resonant system would make one darn good bang!!

Cheers,

	Terry



At 10:22 PM 8/25/00 -0500, you wrote:
>Hi Everyone,
>
>I have been following this thread with some interest.
>
>snip
>> >  > > however (and OBIT's), can use LTR's. Could you imagine the
>> joules for a
>> > pig
>> >  >
>> >  > > using an LTR? I see smoke, fire, wooosh!!! Well, for most
>> of us, not
>> > Greg,
>> >  > > Bill, Hull, etc... or you Chris (yet?).
>> >  >
>> >  > Actually, Terry's MMC Calc gives me .351uF.
>> >
>> >  For a 240/14.4kv, 694mA, 20,749 ohm pig? Terry is .351uF
>> correct? I though a
>> >
>> >  matching reosnant cap size should be 0.128uF. Maybe my math is
>> wrong here.
>> >  Someone please clarify?
>> >   >>
>> > As I found out the hard way, you can also get into a 60 hz
>> resonant condition
>> >
>> > with probabaly any cap by varying the inductance in the primary used as
>> > ballast.  I am using .05uf with a 14.4  5 kva distribution
>> transformer and it
>> >
>> > was resonant with the particular welder that I was using.  It was an
>> > uncontrollable condition.  Maybe with a sync rotary gap, but
>> not with my non
>> > sync rotary.
>> >
>> > Ed Sonderman
>>
>>
>>
>>
>> Yes, I remember the situation. Your transformer is resonant at
>> .064uF, so you
>snip
>
>So what would happen if one hooked up a pig to a TC primary circuit with a
>"resonant" cap,
>say 0.064 for a 5KVA or 0.128 for a 10KVA, then hooked the low voltage side
>to a 240V source,
>and did not put any ballast in the circuit, just depended on the "impedance"
>of the pig as if it were a NST?
>
>Hint:
>(excerpt of a post on the subject from Malcolm)
>
> <          resonant charging calcs really only apply to a
>ballasted system. The faceplate rating on a pig might be 10kW
>but there is nothing to stop you drawing 100kW from it on a
>transient basis as the leakage inductance is so low (well it
>should be - it's supposed to mimic a voltage source with the
>mains behind it).>
>
>"I" think the fuse or circuit breaker, whatever over current protection is
>upstream would blow.
>
>For my money, the distribution transformer (AKA pig) is close enough to an
>_ideal_
>transformer to not worry about any of its parameters (that is DC resistance,
>inductance of the windings, coupling coefficient, etc. etc.) except for the
>turns ratio.
>Of course if you are planning to overload it for a long time, then you might
>want to
>consider the "% impedance", but that would be a rarity for our application.
>
>"I" think that the only meaningful impedance seen by the TC primary circuit,
>spark gap, etc.
>is the impedance of the ballast as "reflected" by the transformer.
>
>Suppose someone wanted to run in "Mains resonance mode". In order to match
>the pig to the tank circuit, an inductor with X(L) = X(C)/n^2 ( where n is
>the turns ratio of the transformer, and C is the tank cap, the reactance's
>of course calculated at line frequency) would be inserted in series with the
>low voltage winding of the pig.
>If the inductance were larger, then it would be a LTR cap. situation.
>If the inductance were smaller, then it would be a STR cap. situation.
>
>For the record, I try to run mine LTR.
>
>Just my humble opinion,
>later
>deano
>
>