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RE: Ballasting for pole pig/Many questions

Original poster: "David Dean by way of Terry Fritz <twftesla-at-qwest-dot-net>" <deano-at-corridor-dot-net>

Hi Harvey,

I will try to answer as many of your Questions as I can.

> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Saturday, May 26, 2001 12:18 PM
> To: tesla-at-pupman-dot-com
> Subject: Ballasting for pole pig/Many questions
> Original poster: "harvey norris by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <harvich-at-yahoo-dot-com>
> I am entirely unfamiliar with ballasting requirements
> for a 15 k pole pig. I understand that the current
> must be limited on the primary side, correct?

It is usually done that way because it is much easier to implement. It could
be done on the secondary (high voltage) side, but would require much higher
inductance, and the entire coil (ballast inductor) would have to be able to
withstand the high voltage of the secondary as well as HF kickbacks from the
tank circuit.

> Since
> two 150 volt variacs  are available can I use two of
> these in series for a control of a 240 volt input by
> simultaneously increasing current for both variacs?

Yes, this can be done, but as I have no experience with the method, I will
leave it for someone who has done so to comment on how to go about it. Off
the cuff I would say that the best way would be to bring neutral from the
supply to the center tap between the variacs, and the two 120V lines to the
other two input terminals, respectively. The load (ballast in series with LV
windings of pig) would be taken between the wipers. That way if the voltage
was a bit unequal on the output of each variac, the neutral would have a
return current, and the center connection between the variacs would remain
at 0V potential. With no neutral connection, the voltage may not divide
evenly across the variacs which could have one of them seeing higher than
rated voltage and going into saturation. But like I said, I have not tried
it, so take it with a grain of salt.

> I also have several large 3 phase transformers, would
> it be better to use their primaries or secondaries as
> ballast? Would I be limited to only using one of the
> phases or could two in series be used? Forgive the
> many questions, this is new territory here for me.

Whether you use primaries or secondaries or both would not matter. Whichever
will get you closest to the required inductance. As the transformer would be
acting as a large closed core inductor, you could use one, two, or even
three phases of windings so long as the windings are phased properly with
respect to each other, it will work. The most common connection would be to
use the outer two legs for the ballast, leaving the center leg unconnected.

> I also have an air core inductance system of .15 henry
> consisting of thirty 14 gauge, 500 ft coils in series
> that comes to approximately 40 ohms, but if these as
> just coils were used wouldnt that limit the current to
> that made by the impedance of the .15 henry coil
> system? Wouldnt that lower the current input lower
> than what my needed requirements for primary current
> at the pole pig would be? Using X(L)=2 pi(Freq)L=56.5
> ohms would that not limit the current input to some ~
> 240/56.5= 4.24 amps/ where this pole pig should easily
> handle twice that amperage as primary input. Is it
> safe to say that 14 gauge wire is a poor choice for a
> ballasting wire, as one shouldnt want more than 8 amps
> conduction through this wire for ballasting. This
> could be done by merely using half of the coils.

I assume those coils are air core...
You should be able to get to just about any desired current limiting with
those by hooking them in series/parallel combinations. Another advantage
would be the elimination of non-linearity caused by an iron core. Though 8
Amps would be a safe and sane max. current rating for 14AWG wire tightly
wound on a spool, one could double or even triple that so long as the runs
are kept short enough to prevent the wire from getting hot enough to exceed
the temp. rating of the insulation. For example, in appendix B table B-310-L
of the 1999 NEC, 14AWG with THHN insulation is rated 21 amps. (however there
is a max over current protection of 15A in any case for 14AWG).

> Since we are limiting current by addition of
> reactance, why couldnt capacitive reactance be used
> instead? Is there usually a problem with a capacitive
> reactance resonating with the inductive reactance of
> the pig primary, thus establishiong more current
> conduction and not less?

Since the TC tank circuit presents a capacitive load, using capacitive
current limiting would have a very bad effect on the overall power factor.
Inductive ballast is preferred, as the power factor will be much closer to

> What about placing cap banks in parallel instead of
> series for a sort of power factor correction for the
> pig. Would this not be the best approach? Finally
> should I use the KVA rating to establish the maximum
> allowable amperage consumption for the primary?
Power factor correction will not limit current.
If you should choose your ballast inductor so that its reflected impedance
is within the range of .75 to 1.5 times that of the tank cap. at the mains
freq. the power factor (of the system) will be close enough to 1 that you
will not need, or want PFC caps.

The KVA rating of the pig can be safely ignored. Use the current rating of
the weakest link in the chain (power source, variac rating, etc.) for the
absolute max. current. I use the power requirements of the tank circuit (cap
size, charging voltage, bps) to determine the current I want to limit to.

Hope this helps a little,


> Thanx for any replies, I am in the dark on this
> subject... HDN
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