# Re: Pig Ballast Question (fwd)

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---------- Forwarded message ----------
Date: Sat, 18 Jul 1998 15:16:34 -0500 (CDT)
From: Larry Bud Melman <gasman-at-althea.a-line-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Pig Ballast Question (fwd)

I have only a limited background in electronics and physics.
Would you explain what the 'complex conjugate' of the load is?
Sounds like you're talking about imaginary numbers...  :-)

Clay

> You can get well over the rated 5 kVA using only resistive ballast if
> you choose the resistance appropriately.  Comments are interspersed
> below.
>
> Jim Monte
>
> >>A fellow coiler told me that if I use a purely resistive ballast, I'll
> >only get about half the rated power out of the pole pig.  In other
> >words, if I use a 5KW space heater as a ballast, the pig will only yield
> >about 2.5KVA output.  Any truth to this?  Sounds screwy to me.
> >>
> >>Greg
> >>
> >Yes, this is true.
> >
> >Assume a 5KVA transformer with a 5KW heat strip in series with the
> >primary. (240 volt side).
> >(The 5KW heater is a "pure" resistance so phase shift is "0", so KW =
> >KVA.)
> >When the transformer is unloaded the "pri." will pull about 2 amps or
> >so, so most of the voltage will
> >be dropped across the transformer pri. and very little across the
> >resistor, so output voltage will be almost
> >rated voltage. When the transformer is fully loaded the current draw
> >goes up, the drop across the resistor
> >goes up till it equals almost the input voltage, and all the power is
> >dissipated in the resistor as heat.
>
>   True so far.
>
> >Maximum power transfer will be when the loading on the transformer is
> >such that its impedance as seen by the power supply is exactly equal to
> >the resistance of the heater, that is the input voltage is divided
>
>   For maximum power transfer to the load, the supply should have an
>   impedance equal to the complex conjugate of the load.  But you
>   REALLY do not want to have max power transfer to the distribution
>   transformer since that is far more power than it can handle on an
>   average basis.  The point of ballasting is to limit power delivery.
>
> >evenly between them. One half of the power is dissipated in the heater
> >as heat, one half of the power is delivered to the transformer to do
> >work. A nice way to make a pole pig coexist with a 30 amp circuit
> >breaker.

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