Re: 15kVA 12.6kV pole pig (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 16 Jun 2007 10:37:02 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: 15kVA  12.6kV pole pig (fwd)

Hi Tim,

It sounds like your tank capacitor and current limiter may be resonating 
at the mains frequency. If your ballast is set to short circuit current 
limit at ~40A and you're using a 14.4 kV distribution transformer, a 0.1 
uF cap will resonate at just about 60 Hz. This situation disappears when 
you only use half of the LV side winding (in effect you change the 
system to go from Resonant to Larger Than Resonant (LTR) mode.

The root cause is using a current limiter set to 40 amps with your 0.1 
uF tank cap. Because the combination is mains resonant, this mode of 
operation can be a "cap killer" if your SRSG motor fails. The loud 
humming sound strongly suggests that, in addition, you may also be 
saturating your variac current limiter. This is quite likely if you 
haven't inserted any air gaps in the variac core(s). Ballast saturation 
will also cause significant power arcing in your rotary gap. The higher 
impedance of the potential transformers used in your previous systems 
may have masked both of the above problems.

For improved results, you'll want to readjust your current limiting 
ballast so that, for any pig voltage setting, the system always operates 
in Smaller Than Resonant (STR) mode. For smooth operation and to reduce 
power arcing, it's also important that your ballast inductor does not 
saturate under maximum current draw. To avoid mains resonance problems, 
you'll need to significantly lower your ballast inductance so that your 
current limit (without saturating) is no less than 60 amps under full 
input voltage. Since this may "cook" the variac brushes in your existing 
current limiting scheme, you may also want to consider making a separate 
inductive ballast using #10 - 12 AWG house wire wrapped around soft iron 
wire, welding rods, or transformer laminations, or use 1-2 500 foot 
rolls of #10 - #12 housewire (as is, on the spool). Unfortunately, 
because of the tank cap value you're using, a standard 200A welder may 
still cause mains resonance problems if used as a ballast...

Bert
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Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 16 Jun 2007 01:57:34 -0400
> From: timjroche@xxxxxxx
> To: tesla@xxxxxxxxxx
> Subject: 15kVA  12.6kV pole pig
> 
> i have recently went from two Westinghouse PT's (about 5kVA) to a 15kVA
> pole XFR. Holy Squirrelly!! i am running SRSG 3600 RPM 4flying (.5"
> tung.)/2 station (.5" german silver). 0.1 uf hipotronics 70kV RMS tank
> cap.
> 
> my biggest issue, besides the obvious current limiting, is getting the gap
> to quench. went to 8 pole flying and basically made a jacobs ladder around
> the rotor. like no quench at all. so back to 4 flying. am i way off or is
> this a impedance problem on the LV side of pole XFR? BTW - i get nothing
> but 60 cycle hum or saturation using 220 VAC RMS across the LV windings.
> yet if i hook up my 220 VAC RMS with one leg on the CT or hooking it up
> like 110 VAC RMS, i get much better performance. for those interested, i
> was using a 1256 variac as voltage control and a 1156D as phase ajustment.
> Now i have put a 3-stack 60 amp inline current limit. i first put 220
> across it and turned the wipers till i was pulling 40 amp. then put it in
> series. it is still only a 3 to 4 foot streamer. with my PT's, i can get 2
> -3 streamers 6 to 7 foot long.
> 
> Thoughts? 
> Suggestings? 
> Ideas?
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