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Transformer loading


Since being on the net for the last week or so, I've seen several opinions
regarding *proper* transformer loading.  While not professing to know
anywhere near everything, we at Allanson have a great deal of experience
which has led us to summarize some general guidelines.  I thought I would
share these with the group.  Perhaps someone will find them helpful.

Virtually all ferromagnetic neon transformers are designed to work
effectively within a certain limited set of operating parameters.  That's
why there's not one size to fit all loads.  If the transformer is too big
for the load, excess heat is generated which may cause permature failure of
the unit.  If the load is larger than for which the transformer is designed,
all sorts of bad things take place.  Things like high voltage spikes, excess
corona, distorted waveforms, etc.  These kind of things tend to punch holes
through insulation materials therefore causing shorts in the secondary coils.

We're asked all the time for recommendations regarding transformer selection
for unusual jobs.  Unfortunately, this is not simple since there are usually
many variables to consider.  Not only do we need to know tube load but we
need to know tube fill pressures, processing conditions, electrodes,
proximity of tubes to dead metal,  lengths of gto/conduit (if used), primary
input voltage, ambient temperatures, ad infinitum.

So, here's how we start.  We recommend that one starts with the ANSI footage
chart (that's what Allanson uses).  Barring that, go back to the
manufacturer of the transformer to ascertain that you and he use the same
chart.  Start from there and confirm variables such as tube diameter,
footage, *fill pressure* and current rating.
I must first stress...follow the recommendations and instructions of the
manufacturer.  It seems like something so simple, and I am almost
embarrassed to mention it but we see so many instances where someone has
violated the obvious.

We suggest to inquirers that they do not overload transformers.  That means
don't use more footage than the chart allows.  We also suggest that
*generally* one should not underload a transformer by more than 20% to be
conservative.  If there is no other alternative to underloading, one risks
premature failure of the transformer due to buildup of excess heat.

For further confirmation of proper transformer selection, we run a voltage
check of the operating voltage on the secondary side of the transformer.
This can be accomplished with a Multimeter (like a Fluke 83) and a
high-voltage probe.  If probed on the secondary terminals of the transformer
(while energized and connected to the chosen tube load) a *proper* reading
should be a total of one-half of the rated open-circuit voltage of the
transformer.  For a midpoint-balanced design, that means each secondary
terminal should read one quarter of the labelled open circuit voltage of the
transformer.

Well, this has gotten longer than I originally intended and I haven't
scratched the surface.  In summary, it should be stressed again that all
transformers have a proper *operating window* and shouldn't be run outside
of the window unless one wants to consider the consequences.  Barring that,
one must change other variables within the system.  If fill pressures are
adjusted off what is called for in the footage chart, this absolutely has to
be considered for proper transformer selection.   

If anyone wants further explaination, it would be best if we sent you our
Loading Guidelines and had a long chat about it.  Thanks for your interest.

Eric Johnson
Director of Marketing
Allanson Transformers