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Re: Pig Ballast Question (fwd)



---------- Forwarded message ----------
Date: Mon, 13 Jul 98 11:52:43 EDT
From: Jim Monte <JDM95003-at-UCONNVM.UCONN.EDU>
To: tesla-at-pupman-dot-com
Subject: Re: Pig Ballast Question


Hi,

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

>From: Tesla List <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
>Date: Sunday, July 12, 1998 1:56 AM
>Subject: Pig Ballast Question (fwd)
>
>
>>
>>
>>---------- Forwarded message ----------
>>Date: Fri, 10 Jul 1998 22:17:00 +-100
>>From: "Gregory R. Hunter" <ghunter-at-enterprise-dot-net>
>>To: 'Tesla List' <tesla-at-pupman-dot-com>
>>Subject: Pig Ballast Question
>>
>>Dear List,
>>
>>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.
>
>