Re: Ok, what is an LTR cap.

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

Hi David and Bart,

	Yes, I should have multiplied instead of dividing.  I seemed to have
misinterpreted the formula in the book.  I should have just gone with what
I thought was correct instead of looking it up to be sure. :-)))

	Indeed the pig can have a substantial resonant rise.  I am not sure L1
should be added to the ballast equation since it is tightly coupled to the
secondary??  I will think on that.

	MicroSim suggest that the LTR size is around 200nF but I need to look at
that more.  The line and variac inductance may play a big part in all this too.

Cheers,

	Terry
 


At 10:41 PM 8/25/00 -0500, you wrote: 
>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 
>> 
>>
>