paralleling vasriacs continued...

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Tesla729-at-cs-dot-com>

Hey all,

I've made a little more progress in trying to parallel my two
large GE variacs. Well, maybe not really progress but let's
just say that I'm figuring out what's wrong :-) Evidently, the
coils are not identical as they seem to appear. The brush
assemblies are different but the actual core and coil assem-
blies "looks" the same. However, after adjusting and se-
curing both wiper arms to the fully CCW position my DMM
shows about 6 volts of difference between the two brushes. 
When I rotate the wiper toward the center position, the volt-
diffence drops to near zero but as I continue to turn the wip-
er toward the full CW postion (voltage max), the voltage dif-
ference once again increases until it reachs approximately
6 volts in the fully CW position. If I try to offset the wiper of 
one of the variacs so that I have a near zero voltage difference
when they are turned to the fully CCW position, then the volt-
age difference increases linearly with the CW turn of the wi-
per(s) until at a fully CW position there is a 12 volt difference
between the two brushes. SO, it appears that the variacs are
not "exactly" identical. BTW,  I wired both of the variacs for
240 volts in the entire coils and not to part of the coils . In 
other words, I wired them to only be able to obtain 0 - 240
volts out with 240 volts in as opposed to 0 - 280 volts out
with 240 volts in. So it looks like I'll have to use a paralleling
choke that can nuetralize a +/- 6 volt gradient. 16 turns of
# 10 AWG wire on an old MOT core for each variac seems 
to help in paralleling the variac outputs,  but # 10 isn't big
enough (it gets HOT) and I can't squeeze that many truns of 
a larger guage wire inside the small core windows. So it ap-
pears that I'll have to find a somewhat larger core maybe an
old variac core.....


Coiling in Memphis,
David Rieben