Re: Ballast Puzzle

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason Johnson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <hvjjohnson13-at-hotmail-dot-com>


>
> Pardon my ignorance, but I thought transformers will 'reflect' the
impedance
> of the secondary back to the primary.  Can one not ballast the secondary
side
> of the the transformer to effectively limit the current of the primary?

You are correct in that transformers like pole pigs reflect impedances onto
the primary that appear on the secondary side, and transformers can indeed
be ballasted on the secondary side to limit current flow. The main problem
with this is that the impedance is reflected through the turns ratio and in
order to get the same amount of ballasting on say a 14.4kv pole pig, you
would have to use 60 times the amount of inductance, and even though it
would only be carrying an amp or so it would need much more insulation.

 I
> know this impedance matching is done when driving a speaker from a tube
> amplifier, but how could this be done for TC's(thinking pole pig or
similar)?
>   It seems like a resistor placed on the transformer output could ballast
the
> transformer without dissipating too much of the power(thinking I^2R
losses).

Well to ballast a14.4kv pole pig to 500 ma you would need 28.8kohm, and it
would still have to handle 7.2 KVA, and it would drop alot of voltage.

>   Another question springs to mind as well:   If one uses inductive
> ballasting to limit the current(primary of transformer), would power
factor
> correction be of use here?   If so, where would  one place the capacitor?
> between the ballast and transformer or 'in front' of the ballast inductor?
> Mike
>


Well, one of the main design goals for most large coils is a good power
factor. The power factor is determined by three main components; the ballast
inductance, the tank capacitor, and the breakrate of the gap. With
reasonable components you can adjust the breakrate, ballast inductance, and
with MMCs the capacitance quite easily to acheive a good power factor
without PFC. If however you needed PFC (you shouldn't, provided you haven't
got anything way out of whack) it would go before the ballast.


<< Jason R. Johnson >>
G-3 #1129
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