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eHam Forums => Elmers => Topic started by: K3PRN on February 04, 2010, 05:16:33 AM



Title: Transformers
Post by: K3PRN on February 04, 2010, 05:16:33 AM
Discussion about small "pole pigs',the
high voltage transformers on telephone poles.I am in the Baltimore, Annapolis Maryland area.


Title: RE: Transformers
Post by: KE3WD on February 04, 2010, 05:22:31 AM
The commone pole pig is an autoformer.  It thus does not isolate from the AC line. 

While some have used these for linear amplification or other high voltage supplies, they can be a very dangerous situation because of the above.



Title: RE: Transformers
Post by: K9YLI on February 04, 2010, 09:16:31 AM
to use a pole pig for an amplifier supply, they are usually used backwards.

use the 230  output side for input and say the  4400
input side for output.
the danger of an auto transformer is
 in if the load winding opens, the entire input
voltage is impressed accross the output, and the input winding becomes a big inductor in series.
But using it backwards, the worst that can happen
is having your high voltage side drop down to 230,
or 120 depending on  transformer and how its wired.
not much hazard that-a-way...
original configuration failure having  4400 applied laccross you 230 line  would be a bear.


Title: RE: Transformers
Post by: KE3WD on February 04, 2010, 03:42:33 PM
Well we can all see that someone doesn't give a you-know-what about such otherwise common electrical observances as hipot testing or coming into contact with a chassis that is suddenly at Line potential...


Title: RE: Transformers
Post by: K4DPK on February 04, 2010, 04:46:48 PM
I've use quite a few pole transformers over the years.  I have never seen one that was wound in an autoformer configuration.  They have all had separate primary and secondary windings.

It seems to me this would be universal, because otherwise ther would be a direct connection between the high voltage transmission line and the breaker panel, with only a winding (with possibility of a short) in between.

I have three pole transformers in here right now, none of them show continuity between the primary and secondary.

Phil C. Sr.
k4dpk


Title: RE: Transformers
Post by: K4DPK on February 04, 2010, 05:13:32 PM
Before the PCB uproar, pole transformers were much sought and used in the ham community as plate transformers.  In general, these were better built and more substantial, owing to their expected use, than their ICAS counterparts.

Nowadays, because of government meddling, the power companies usually won't dare sell surplus transformers.  EPA requires them to keep track and document the disposition.

But, when you can get one, they make a very hefty and economical HV supply.  I wish they were more available.

Not sure what the OP's question was, but first you have to take it out of the can and let the oil drain out of the windings.  We used to put them under a wash tub, with the xfmr sitting on boards and cardboard to soak up the oil.  The tub would keep the rain out, and the summer sun would heat the inside like an oven.  When it no longer seeped oil, you could use it.  This process might take a week.

You should use a 12 volt transformer and connect it to the primary, then make measurements on all the windings to identify them and the taps.  You can extrapolate the ratings from this information.  Usually, you can still read the kva rating on the side of the can.  5 kva is about the smallest you will find any more, but it's ideal for a good stiff supply.

Phil C. Sr.
k4dpk


Title: RE: Transformers
Post by: W8JI on February 05, 2010, 02:06:56 AM
Well we can all see that someone doesn't give a you-know-what about such otherwise common electrical observances as hipot testing or coming into contact with a chassis that is suddenly at Line potential...

Actually you are incorrect and K4DPK is accurate. In the USA virtually all "pole pigs" have a fully isolated and floating primary and secondary. This is because we have such a mixture of feeds, including three phase systems of various types.

Some foreign countries with very little three phase have transformers with a grounded HV primary, but even those are not "auto-transformers". They are normal isolated winding transformers in a housing with a single HV feed insulator.

They make darned good plate transformers when reverse fed if the voltage works out alright.

Tom


Title: RE: Transformers
Post by: KE3WD on February 05, 2010, 06:42:50 AM
My apologies, the info I have is based on esperience with only two pole pigs of unknown origin.  They were indeed autoformers, though I don't know why nor where they came from and it was ages ago.  (One of 'em worked in the amp anyway, the other was a spare.  I just inserted two back-to-back filament xformers between the primary and the wall to yield isolation). 

PCBs? 

Look, when i go, they'll blame it on the pipe tobacco no matter what I die of anyway (grin). 



Title: RE: Transformers
Post by: K8SOR on February 05, 2010, 07:23:21 AM
Boy, I'd like to see the size of those filament transformers--they would have to be as big as the pole pig.???