Wow, Thanks to everyone who answered my posting, both to the lists, and
personally.
I received lots of interesting ideas on how to solve my problem of
drying out transformers which have been exposed to moisture. Thanks to
all of you who took the time to respond. I really appreciate the help.
Since I posted my original query to 3 different mailing lists, and I
also received direct mail responses, I thought I would summarize what I
have received so far, for the benefit of anyone else who may run into
this issue.
The ideas fall into two main categories. Internal Self-Heating, using a
current flowing in one of the windings to heat the transformer from the
inside out, and, External Heating, applying heat from some external
source until the unit has dried from the outside in.
Obviously from the accounts below, both methods are known to work, so I
am trying to make the best choice for my circumstances. Someone else
may have much different priorities. So far, I am leaning toward the
Internal Self-heating route, because I think I can control the process
more closely, and still get the job done rapidly. Speed is important
for me, as I am trying to get this thing on the air for the upcoming
top-band season. I am currently thinking that the best method for me
might be to use AC from the mains thru a Variac and the secondary, with
an ammeter on the primary to measure current.
For those of you with experience with this, I have a question or two.
First, Steve Hobensack, suggested that I use DC instead of AC, as did
Bob Bruhns and Bill Hawkins, however, they also pointed out that this
would have a tendency to magnetize the core, and even suggested a couple
of fixes for that problem. However, I would rather not have to deal
with the magnetization problem at all. What advantage is there to using
DC for this purpose?
My second issue is that of sealing the newly dried transformer once I
have finished. Different postings mentioned variously HV varnish,
polyester laminating resin, polyester varnish, etc. What is the actual
terminology and can you recommend sources?
Last question, where can I get access to a "megger" (without having to
invest a whole lot of money for a one or two time use)? Or, can they be
had inexpensively somewhere?
Ok, that's it for my new questions, the following is the summary of the
received responses, along with credit to those folks who volunteered
information. Again, thanks to you all for the input, and as usual, I
learned a lot in the process. The BOLD and italics were all added by
me. Italics are used to show that the person's original post indicated
that they had previous success with the method described.
Internal Self-heating - All of these suggestions centered on shorting
one or the other windings of the transformer, and applying power to the
other, until sufficient current was drawn to cause the transformer to
begin to heat from the inside. There were multiple variations on this
theme, including the following:
Steve Thompson [g8gsq@ic24.net]
Self-heating? An untried idea - short one of the
windings, and apply a little voltage to the other with a variac and let
the thing warm up gradually, increasing the voltage until it sits at
50C/120F for a few days. My guess would be to short the primary and
apply volts to the secondary.
Dfmich@aol.com
I have not done this but had a "brainstorm".
How about shorting the secondary of the xfmr with a 1 ohm resistor.
Apply AC to the primary with a variac and adjust the volts in to get
rated current in the secondary as indicated by the voltage drop across
the 1 ohm resistor. (you'd have to pick a resistor wattage to withstand
the current/dissipation). The primary voltage will probably be fairly
low; and the voltages in general will be low so no arcing should occur.
Let the whole shebang set for say a week. hopefully it will get warm and
the heat will chase out the moisture. Your can feel the thing to
determine if any seemingly significant heat is being generated. If not,
try something else!
Dave Brown [tractorb@ihug.co.nz]
If you know the secondary current rating or can
guess it approximately then you are all set. Disconnect all connections
to the transformer-both primary and secondary. Short the secondary with
an AC ammeter that will be used to indicate the rated current flowing in
the secondary. Run the primary up on a variac - CAREFULLY! - until you
see the rated current indicated on the ammeter. It's not all that
critical -as long as the current is around the right value it will do
the business. In this configuration you are way below the overall VA
rating for the xfmr as a whole so a bit of extra dissipation in the
secondary isn't going to matter. A few percent high or low is fine. The
secondary resistance is used to dissipate power at the rated current in
the secondary and thus heat up the transformer from the inside out.
Then just leave everything like that for a week, or however long you
want to. Don't disconnect anything without first winding the variac
right down!
Steve Hobensack [stevehobensack@hotmail.com]
It may be hard to pump enough ac current through
a high voltage secondary that comes from a variac. You can use a
variable source of DC, crank it up until the 100 pound piece
of iron will take in about 50 watts. Fifty watts
is quite hot. For smaller pieces of iron, use only a few watts.
Bill Hawkins [bill@iaxs.net]
You could try self-heating by dissipating some
DC watts in the windings. The DC will magnetize the core, so run AC from
a series light bulb through the winding until the bulb dims. If the bulb
doesn't dim then reduce the magnetization by using half the DC current
in the other direction for a few seconds. Repeat swapping using half the
previous current until a 100 watt bulb is dim on AC. I've never tried
this, but that's what ought to happen.
How do you know when it's dry? Use a 500 volt
megger to the case and stop when the megohms won't reduce any more.
Bob Bruhns [bbruhns@erols.com]
Low-voltage DC on the windings would do it. It
would be tricky to figure a current that would produce enough heat
without toasting anything, though. Probably something around the
maximum operating current would do it, over time. I would put the
primary and secondary windings in series, and heat the whole core. You
might try to arrange the connections to approximately cancel
the DC flux in the core. Put the core in a
blanket so the heat builds up, and monitor its temperature. My guess is
it will take days to build up the heat. You want to heat it up so it is
warm to hot to the touch, (maybe 140F / 60C), but you don't want to
scorch it.
.
John Lyles [jtml@losalamos.com]
You could put a short on the secondary
terminals, and run up the primary voltage with a variac, to where you
get the full current flowing in the short. Voltage will be, of course,
low, and this will heat the transformer up without overloading it. Be
sure to not exceed the rated DC current, and maybe to be safe, run it
only 50-75% of that. Other than this, you could put a plywood or
polyethylene sheet tent around it, and heat it with a radiant heater of
any type. All you need is 120 deg F or so for so many days.
John Lawson [jpl15@panix.com]
I second this, with a slight caveat - I've
used this technique over the years for transformers and chokes that have
been in damp places, out in the weather, (or in a few cases actually
submerged) - however I terminate the secondaries in some appropriate
resistance - sometimes various lamps work nicely for this -and then use
a Variac on the primary, and use plenty of patience!
If there's a lot of water, you have to take it
very slow as it's sure to arc no matter what you do - this causes
insulation damage and super-heated steam in the windings. But this is
by far the best method, since it heats 'from the inside out' - all the
major utilities use this when drying out Really Big Iron - even to
desiccate the insulating oil. But I don't think I'd short the
secondaries - this might tend to concentrate excessive currents at any
'weak spots'.
Gene Bigham [jbigham2@kc.rr.com]
Variac in the primary, slowly bring up to full
potential over time. I did this with a huge Gates transformer that went
under water during a flood.
R. Measures [r@somis.org]
I heat large xfmrs by shorting the primary with
an AC ampere meter and connecting the secondary to a variac that is
connected to the electric mains. I increase the sec. V until I arrive
at the normal pri. current. I put a folded in half bath towel over the
unit to hold in heat. After a few hours, the xfmr should be almost too
hot to touch. Continue for a few days. At this point, the xfmr should
be dried out. At this point, polyester laminating resin can be poured
into the windings to fill the air spaces. When the resin hardens, the
xfmr will not absorb moisture, it will be able
to get rid of internal heat better, and the insulation will be
improved.- note - xfmr potting is covered in:
http://www.somis.org/D-amplifiers3.html
External Heating - These methods are based on applying heat from the
outside of the transformer or choke until it is sufficiently dried.
Several approaches: Custom oven for the purpose; use the heat from the
sun; simulate the sun (sun-lamp); and electric space heaters. Again, I
received several variations on the theme, including success stories.
Custom-built Oven
Larry Will [lhwill@verizon.net]
Here is the section in my article from ER
Magazine on rebuilding an RCA BTA-1R1 on the homemade oven I used with
great success. All you need is a thermostatically controlled hotplate a
thermometer and a hood.
A check of all the iron with a 1000V 1000 megohm
ohmmeter showed excessive leakage (less than 1000 megs) in the
modulation reactor, the driver plate transformer, and the control
transformer. The control transformer makes 110VAC for relays and lamps
from the 240V input. The smaller transformers were baked in the kitchen
oven at 140 degrees F for 5 hours. For the very heavy 50 henry
modulation reactor, I fashioned a homemade "oven" using a
thermostatically controlled single burner hotplate and a hood made from
an old water heater jacket (Figure 1). This allowed me to cook the
transformer out in the garage near the rig. An oven thermometer allowed
me to set the
oven temperature to 140 degrees F and after 25
hours of "baking", the leakage was cured. All of these transformers and
chokes were then dipped in transformer varnish to re-seal the winding
from new moisture incursion.
Solar Heating
John Coleman ARS WA5BXO [wa5bxo2005@pctechref.com]
I have a modulation XFMR that was under
muddy water for 6 months.
It was let to stand in the sun for about 3
months under shed. I then measured 1000 ohms from any winding to
another and to ground. I took the end caps off exposing the coil, rust
and mud. Washed it out some more and decided to put it on the shelf in a
un-air conditioned office that no one used, where the temp would reach
120-140 every day in the summer. I was thinking I would rewind it some
day. But alas, 6 months later all the readings were infinite. To be
safe it was mounted on a wooden shelf in the XMTR and has been working
for the last 7-8 years. Modulator is four 813s with 2500 volts on the
plates, 800 on the screens and about -100 on the grids. This modulator
will pump out some PTP voltage. I was really surprised at the come back
of
this XFMR.
HAROLD B MANDEL [ka1xo@juno.com]
Do you live in a sunny area? Take the xfmr and
put it in over a
"parabolic" reflector made out of cardboard with
tinfoil on it, focusing the sun's energy on the iron, which is sitting
on some 2x4's so the underside is raised. A day in the sun with this
will dry it out well. Get some HV varnish and goop it up real well after
the iron is real warm. Turn it over and goop it up from the bottom up
after the top layer dries some. A few layers of HV varnish will soak in
to the innards with the core and winding nice and warm.
Joe A. Taylor [n4nas1@hotmail.com]
I'm sure may be too simplistic to work but.....
why not put the xformer in the trunk of a car and leave it there,
outside in the sun, for a week or month. Here in KY that works for
most everything. I've dried apples this way ;-)
Electric Space Heaters
Bob Bruhns [bbruhns@erols.com]
Another thought is to get a few electric space
heaters and point them at the unit, and let them heat it up. Again,
monitor its temperature. With this kind of
heating power, you really need to watch it
closely
Will Matney [craxd1@verizon.net]
You can use an electric space heater too. One
like the old "milk-barn" types works well. Just set the transformer up
with its layers open to the heater about 6-8 inches away. You may have
to turn it once exposing the other open side. I'd advise doing this
though where nothing could be caught on fire for some odd reason as this
takes a while. The heater itself should pose no problems if it has an
active tip-over switch. Let it cook for a few days and it should be as
dry as a bone. It needs to be pretty hot to the touch but not to where
it would actually burn the fish paper. Baking temperature in ovens is
around 300-350 degrees but remember that's for wet varnish. A
through-temp of around 120-150 degrees should do it. The main thing is
to dry out the paper layer insulation and any moisture that may be deep
inside.
k7fm [k7fm@teleport.com]
Put a heater on it. That will warm up the iron,
which will warm all else. Leave the heater on for a day or so. Even a
close floodlight would help.
Sun Lamps
Richard Dillman [ddillman@igc.org] - Restoring KPH
experience
Our solution was to remove them from the
transmitters and bake them under sun lamps for extended periods - a week
or sometimes several weeks. We monitored progress with a megger.
Eventually those transformers came back to life and they are in use
today.
mikea [mikea@mikea.ath.cx]
I thought about sunlamps, and that seemed like a
reasonable approach. Then I thought about electric blankets. That's
cheaper, involves less energy, and so will take longer, but it's easy to
do: just wrap Mr. Xfmr up in one or two blankets set to high, and wait a
while. He'll be all dry and toasty warm in a week or three.
StephenTetorka@cs.com
how about a 500 watt light bulb in a covered
box?
Sealed system with desiccant
Morris Dillingham [mdilli@trip.net]
How about placing them in a sealed container
with desiccant? The desiccant will absorb the moisture. I can't tell
you where to find the desiccant.
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