[Amps] HV transformer and Variac

[Jim Thomson]

Date: Tue, 21 Jul 2015 14:55:24 -0700
From: "Hardy Landskov" <n7rt at cox.net>
To: "'Steve Wright'" <stevewrightnz at gmail.com>
Cc: <amps at contesting.com>
Subject: Re: [Amps] HV transformer and Variac

Steve,
Don't be surprised if you see large swings in plate voltage. A "pole pig" is
not designed to be run backwards. It wants secondary current to be low and
primary current be very high. I ran one as a kid in California because
that's all I could afford. It worked but I only used it on CW.
73 Hardy N7RT

## I have never seen  bad regulation in a hypersil pole-pig.  I checked this out a few yrs
ago and 90% of the available xfmrs could be run at 100% of their ratings...run backwards.
The remaining  10%  could be run backwards at 90% of their ratings. 

##  The problem is..with a 7200 vac xfmr,  with a fwb, you end up with  10.1 kvdc no load. 
And 5.1 kv with just 110 vac applied to the 220 v primary.   To get it down to say 3800 vdc,
you would have to drop the primary  down to just 82 vac.  

## About the lowest v  I have seen on a pole pig is the 4160 / 4800 vac variety.   You will still
require a heavy duty variac to run the mess.  Use too small a variac, and V regulation will suffer. 

## The difference between the hypersil pole pigs and say the dahl plate xfmrs is... on the pole pigs,
the primary is the 7200 vac winding.... which is called the primary.   The primary always goes on the inside
of the core..with the secondary wound on top. Sec of course is the   240 /120 vac side. 

##  on the dahl the 240 side is the primary..and is wound 1st..with the HV sec wound on top. 

##  On a pole pig supply I built years ago,,,,we used a 10 kva ccs  type, 4800 vac sec. removed
from oil and weighed 120 lbs.   6700 vdc no load,   I had an extra dpdt 50 amp relay in there, so I could
feed 120 vac into the   240 vac primary, to drop the B+  down to  3350 vdc.     All the relay did was toggle
between  neutral and one of the hot legs.   Regulation was good either way.   Its currently being used 
on a  YC-156 amp...with a 150 uf filter.  

##  two identical  0-130 vac @  20A variacs can be ganged  together...to operate on 240 vac. 
A longer shaft is used.  I used AL tubing...and simply bolted the variacs nose to tail, flush to each other.
A longer shaft used, and only one knob of course.  Both variacs cranked fully CCW, b4 tightening down the 
shaft set screws. 
I have seen as many as 3 x variacs nose to tail... for 208  3 phase service. ( 3 x 0-240 vac variacs used). 

##  since the  pole pig has the HV winding on the core 1st.... with the 220 vac on top.... there is no way 
to remove  HV turns. 

##  u might want to try and find a lower HV winding if a pole pig is to be used.  Flip side is... u could easily test 
what u have with a variac, and a  simple FWB diode board, and an oil cap... + a B+  voltmeter.   Don’t use too small
ga wire in the 240 side, ditto with any relays etc.   You don’t want V drops  on the low V side.   Since u only need
aprx 82 vac to run ur xfmr,  IF u can find a BIG  0-120 vac variac,   it may work well.   Since u need a FWB + HV cap
anyway, it’s a easy test .... provided u have a variac. 

Jim  VE7RF