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Re: LTR Pig Project - Long Post!
Original poster: "Terry Fritz by way of Terry Fritz <twftesla-at-uswest-dot-net>" <terrellf-at-qwest-dot-net>
At 02:52 PM 2/16/2001 -0700, you wrote:
> I would
>say that one does not want to run a large degree of LTR in a pig
>because you want a high voltage for better primary efficiency, and
>you want to keep the power factor good.
LTR coils should still fire at the full rated transformer voltage. You
basically just keep loading up capacitance until you can't charge anymore C
to the full firing voltage.
>Consider Kevin's coil. His pig combo rating is 14.4kV at 20kVA. His
>cap is 0.138uF or something similar. He draws 100 amps from a
>240 volt line. He runs at 120 bps. I just did the calcs, and a
>matched sized cap for Kevin's coil would be 0.31uF or so, and
>a typical LTR sized cap would be about 0.6uF or so. This is
>huge cap, and I don't recommend anything that large. His cap
>seems to be less than 1/2 the resonant size. (If you calc the
>reso-size based on the pig's specs.) You may want to check
>my calcs here in case I made an error. But if his reso-size
>would be 0.31uF, I certainly would not suggest using anything
>larger than 0.375uF or so. Since Kevin's cap may be about
>1/2 the reso-size, it's possible that his power factor might not
>be that good, but I have no real idea if it is or not. It is also
>possible that Kevin's coil may work much better with a 0.375uF
>cap, if it greatly improves the power factor. It's too bad we don't
>know what the power factor is on Kevin's coil. Then again, it is
>possible that a reso or LTR cap is NG for a big coil. It may hurt
>the quenching too much or something, who knows. Maybe
>someday someone will build a big coil with LTR.
Forgive as I think out loud about Kevin's coil...
240 volts at 100 amps gives us 24000 watts. We'll assume we can add power
factor caps until the PF is good and we can use all 24kVA.
24000 / 120BPS gives a 200 Joules per bang. The firing voltage for a
14.4kV transformer is 14400 x SQRT(2) = 20365 volts. 200 Joules = 1/2 x C
x V^2 so the LTR cap size is 964.5nF. Yeah!! :-))
If his present coil is running 138nF then he would have to increase the BPS
to match the LTR case. 120 x 964.5 / 138 = 839BPS. However that is async!
There for he instantly has to double that to match the sync case for 1677
BPS (async gaps deliver 1/2 the power of sync gaps of the same BPS do to
firing on less than full voltage).
So to match our theoretical 120BPS 964.5nF LTR coil he has to run his
138nF coil at 1677BPS. Since the RMS current in the primary cap is pretty
much proportional to the BPS, his "little" 139nF cap has to take 14 times
the current as the big 964.5nF LTR cap! If Kevin knows his breakrate, we
can pretty much figure out his power factor since 1/2 x C x V^2 x BPS is
the real power input. Once the cap is charged, LTR coils are just like any
other coil, I would think the low BPS would help power arcing in the gap
One should also consider Jeff's Model 8J which is very close to a true big
LTR coil. It works darn good too!! I am not sure if he does much with
power factor or really optimized it for LTR (I think it was built before
the LTR days) but it turned out well.
>Well, a low bps coil will indeed use a more expensive cap, but if
>it outperforms, it may be worth the cost.
As in the example above, if the RMS current is only 1/14, then the cost of
the cap may be dramatically less. Rating TC caps by RMS current is very
important!! In the past, it has been very hard to do but now we can get
pretty close. The fact that the big cap manufacturers eyes get big when
you mention 90 amps RMS to them suggests that some of those exploding caps
of the past could have been avoided if we only had known. However, the
"right" cap's price may have made our eyes get big 8-)