Don,
I can't really comment on nothing but using the space heaters as I have
actually done this. I used the same process to dry out TV chassis that had been
in a flood and then washed out. After drying, the TV worked just fine and
played for several years. The transformers I done worked right on.
You mentioned varnish. The thing you have to have here is an air dry type. They
do make air dry transformer varnish which you can buy by the gallon. However,
plain old polurathane varnish will do, the thinner the better. When it's thin
enough, you can simply pour it into the open coil. Too thick and it wont
penatrate the tight layers. This is air dry so using it after the dry out with
the heater is okay. Just let it air dry an additional 2-3 days after you use
the polyurathane, no heat needed unless you want to speed up the process. You
may want to turn the transformer over and pour in the varnish from each side of
the open coil to assure it penatrates fully into it. The other way would be buy
enough varnish to fill a large tub and douse the whole transformer in it. A
small vibrator can be used to vibrate the tub and make the polyurathane really
soak in and make any trapped air come out of the coil. This is done on
manufacturing small lot runs (1-4) of transformers in shops u
sing air dry varnish. Doing this though, you still should let it soak for at
least one day or until you no longer see any bubbles floating to the top. I
made a small vibrator out of a small 120 VDC permanant magnet motor. I mounted
a small eccentric weight to the shaft and mounted the whole assembly to the
side of the tub. I used a small variac into a rectifier to set the speed of the
motor. You could use an old windshield wiper motor to do this and use a
variable 12 VDC supply to vary it, or just let it run wide open if it dont
shake the tub too much. To make the eccentric, just use a shaft collar with a
set screw. Braze or weld on a small weight to one side of the collar, it dont
take much. Something about like a 3/4-10 nut would be plenty big enough for the
weight. Set the tub up on a couple of blocks of wood or a couple of 2 X 4's as
setting it on concrete will drive you nuts from the noise.
Best,
Will
*********** REPLY SEPARATOR ***********
On 9/18/05 at 8:04 PM Don Lemley wrote:
>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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