Re: Ok, what is an LTR cap.

["Barton B. Anderson" <tesla123-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Terry, 

Tesla list wrote: 

>
> Original poster: Terry Fritz <twftesla-at-uswest-dot-net> 
> <snip> 
> 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.




This would go in the opposite direction, wouldn't it? The transformed impedance
should go up, not down. The ballast inductance is in series with L1's
inductance. They should add together for the total primary inductance with L1
providing the turns ratio for transformer impedance. 

>
> 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... 
>  




Using your values and math as you layed out here, I arrived at the same
numbers, but I don't think this is right. However, assuming the ballast and L1
are in series, Xl = 9.68 or 25.68mH. WIth a 14.4kv pig, that transforms to 110H
(I multiplied the ratio squared, not divided). The transformed impedance is
41,474 ohms and the cap value to resonate at 60Hz is 64nF. 

Looking through some past notes, I found that I run my pig ballast (best arc
length) near 12mH when using my 60nF cap. (I had marked the ballast position,
then later measured the ballast inductance at that setting using an LCR meter).
Note the longest arc length ballast inductance is also near the resonant
condition. 

>
> 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...




Terry, we know 128nF is the resonant cap size for the pig (unballasted). You
chose to use L1's inductance for a ballast value (without adding L1). You
should then arrive at 128nF cap size because the inductance value equals L1's
stand-alone value). It "appears" that the square of the turns ratio must be
multiplied and not divided. I think this also provides a method to calculate a
ballast inductance for any cap size used that would bring the tranformer into
resonance with the cap. 

For example, using Ed Sonderman's .05uF cap size, 240V/5KVA transformer (60:1)
which happens to be 60Hz resonant at 64nF: for ease of math >>> 2*pi*f = 377
and 60^2 = 3600) 

Z = (1/.05uF) / 377 = 53,050 ohms 

L2 = 53,050 / 377 = 140H 

L1 + Ballast = 140H / 3600 = 39.08mH 

Ballast = 39.08mH - (L1 inductance of 30.56mH) = 8.52mH variac position. 

On a 40A variac, this is probably somewhere around mid winding (I think). 
  
>
> I would think such a resonant system would make one darn good bang!!




I think a greater than resonant cap size can be used as long as the coil can
handle the energy of the bang. 

Take care, 
Bart 

>
> 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 
> > 
> >