[Hallicrafters] Power transformer temperatures

[Bill]

High voltages do not lead to excessive heat.  However, I think the
poster meant to say, he believes voltages higher than the design or
normal voltage leads to higher heat generation.

This is true provided current demand remains the same.  I just want
to say one thing.  As a very large transformer design engineer, transformers
are probably the most ruggedly designed electrical apparatus that has ever
been
invented.

Transformers are meant to get hot, very hot, too hot to touch and are
designed to handle this heat without failure.  Again, however, excessive
heat can shorten expected service lifetime and does lead to premature
failure
of transformers or any other electrical apparatus.  Heat is the destroyer of
insulation and hence electrical equipment.  Insulation dries out and
impregnated
varnish will become brittle.  This will eventually lead to winding movement
and
with the 60hz frequency and changing magnetic field, the wire will rub
adjoining
components 120 times a second and eventually cause turn to turn failures or
turn
to core (ground) failures.

Transformers, motors, circuit breakers etc. are designed with a + - 10 %
service factor.  That is for over voltages and over currents.  Therefore,
with a transformer designed for 110vac (nominal), a 7-10 volt increase will
bring you up to 117-120vac.  This should not present a problem for any well
made transformer except as the poster stated, the dissipation of additional
heat due to the higher voltages.

Enough heat to damage the xfmr?  Who knows without knowing the original
specs
on the transformer in use.  In my professional opinion and experience,
higher
heat in the transformer will shorten a transformers expected useful service
life.  But, like all transformers they do have a finite life span.  In the
industrial world, for large dry type transformers, that is about 40 years.
I think for intermittent duty such as xfmrs in ham gear, that span should at
least double, disregarding environmental factors of course.

K2WH


-----Original Message-----
From: hallicrafters-admin at mailman.qth.net
[mailto:hallicrafters-admin at mailman.qth.net]On Behalf Of k6uuz at juno.com
Sent: Friday, April 04, 2003 10:28 AM
To: magnuson at mac.com
Cc: hallicrafters at mailman.qth.net; magnuson at mac.com
Subject: Re: [Hallicrafters] Power transformer temperatures


Let me get on my soap box. I feel that high voltage leads to excessive
temperatures and subsequent failures. Most of the tube-type equipment was
manufactured when the line voltage was 110 to 117 VAC. Running it on
today's 122 VAC can lead to failures. Some people think that because your
equipment may be rated for something like 105 to 125 VAC you are within
limits and safe. When a range is listed it usually is the design voltage
+/- some amount. In this case the design voltage would be 115 VAC. If you
have any doubts, measure the filament and B+ voltages and compare to the
service manual. A quick and easy way is to pull a tube and measure the
voltage on the socket filament pin. If it is more than 6.3 or 12.6 volts
you are running over rating. You can get info on building or buying a
simple but efficient voltage reducer from the BAMA site, about 1/2 way
down the page. Thanks for the bandwidth.

 Ed Richards

On Thu, 3 Apr 2003 18:46:02 -0800 Waldo Magnuson <magnuson at mac.com>
writes:
> Does anyone know what the normal range of temperatures is for power
> transformers?  I've been monitoring the temperature on an audio
> amplifier (a Craftsmen RC-2 which drives a pair of 6V6s).  I measure
>
> the temp at the mid-point on top of the power transformer with my
> wife's digital food probe and it has been reading up to 120.0 deg. F
>
> after a couple of hours.  Is this normal, or hot, or cool, or ?  Is
> there any guess on how much the cover on my SX-42 power transformer
> increases its temperature.  I've heard that over time the
> temperature
> tends to weaken the insulation amoung the transformer windings much
> in
> the same way as it dries out capacitors.  Any thoughts?  Thanks.
> Skip Magnuson
>
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