Re: rusty transformers

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
news:4A4C2091.6CAF2413@xxxxxxxxxxxxxxxxxxxxx

{lots of stuff snipped throughout}

There are suppliers selling "electrical varnish" that pongs badly while
using it and which hardens in air, but not under layers of wire and
insulation unless heated at 125C for 4 hours. I have no idae if its
Glyptal.

The ideal materials will not harden at all at room temperature, although
Glyptal makes a number of alkyds that will air dry or bake.


Wattyl Estapol ...
It doesn't seem to really go very hard after heating. Sort of mushy,
and
then its easy to over heat it and its brittle.

It is probably designed to air dry.

....Motor rewinding shop...
There is a guy here who I did visit for just that purpose but he didn't
ever use a vacuum chamber for the motors he re-wound because was no
need. The varnish just soaks in easily to motor windings because there
are no flat thin windings between cheeks with insulation tightly
fitting
to prevent vanish ingress. I hunted around for someone with a vac
chamber but no luck.
So I made a vac chamber.

The trouble is that I can submerge a warmed up item in a can of varnish
within the vac chamber, and draw the vacuum, but the big risk is that
the varnish boils at the lower pressure then gets into the vaccum pump,
where it causes grief to the piston and its sealing rings and valves.
YUK!. I stuffed an old fridge compressor I was using backwards. Now I
have a 1HP painter's compressor, and I don't want to *** that up.
The vacuum I get is only 95%, but good enough.

I don't know if Harbor Freight is in Australia, but they are an importer
of Chinese tools of "reasonable" quality, not junk but still not
professional quality. They sell a venturi pump that runs on compressed
air much like in a chemical lab where they use a water driven aspirator.
They claim it achieves around 28 inches of mercury vacuum using 4.2 cubic
feet per minute of 90 PSI air. It cost around $15 US. It is very
difficult to hurt one of these.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96677

You don't really need all that much vacuum. It is best if you go through
several cycles of vacuum/release/vacuum/release. This will eliminate
much of the trapped air and fill the void spaces with resin. You really
should avoid boiling the resin.

Well, sometimes the guys at the motor re-winders are not all that
friendly. The ones I knew are rather surly, and always busy worrying
about everything, and never take time off during working hours for
coffee and a dougnut. After working hours they rush off home; they
can't
bear to stay another minute. And the soak method is unreliable for PT
or
OPT. One gave me 4L can of varnish for its cost price, but next time I
enquired he said he didn't want to give me any. They don't like having
to keep ordering it in bulk more often. The slighytest thing thay did
for me always cost money, so I figured I needed to be independant.

All I can suggest here is to find another shop with a worker who is into
hollow state audio.

And you have not ansered my question, what's the best goop that you can
paint with a paint brush on while winding neat layers, and then heat
afterwards to a temperature not exceeding say 60C? It needs to be able
to adhere to polythene or polyester. I'm used to the stuff that hardens
at 125C after 4+ hours. Very smelly while whatever solvent is boiled
off
and whatever else. For my oven, I use an old electric frypan with a lid
that has been increased in height to take largish PT with say a 2kW
rating. The temp control appears to be OK and if I set it at 260F the
varnish does appear to go dry and hard under the outer layers of clear
coloured insulation around an item. If it ain't baked enough, you can
see that the varnish is still liquid and squishy under the insulation.
Further within the winding things would be worse.
I wanted something that didn't need such a high temp and thus gave me
some leeway against melting insulation.

Adherence to polyethylene and polyester is going to be a problem.
Polyethylene (polythene) does not have a high service temperature anyway.
High density polyethylene has a heat deflection temperature of slightly
less than 80 C and a Vicat softening point of 125 C. Polyethylene
terephthalate (polyester) has a heat deflection temperature of
approximately 115 C and a maximum service temperature of 110 C. The only
way you are going to get a strong bond to PE or PET is to surface oxidize
the plastics with a corona dischange.

I have used Wattyl 7008 floor varnish which comes equal cans of part A
and partB which you mix as you need it. Its also terrible to use while
winding because it gets everywhere and on your hands and stinks like
hell. You need a can of methylated spirts and cloth to keep cleaning
hands and your fingers are black for a week afterwards. Lord knows what
the fumes do to lungs.
Its good stuff though because its pot life is 8 hours, and you paint it
on generously and you can clamp up layered tranny winding between
blocks
of wood to make sure the height of winding gets down to what you've
calculated so the E&I lams will fit in later. Next day the clamps are
removed, and the winding stays down to the clamped dimensions, and
insertion of E&I is easy.
But it doesn't stick very well to other plastics used for interlayer
insulation, so in an OPT you can get some howl because there is still
some microscopic vibration possible between where things have little
adherance. What appears to howl ther least is a compound that does not
harden completely, but which stays a bit gooey, sort of like tars or
heavy waxes, so that itm is damped rather than glued together. Tars of
course are usless for varnishing; they are far too viscous but are
ideal
for potting compound, and they are cheap.
Trouble is the local road emoleum used here is liquid at 60C, and even
at 30C it will run slowly out of a pot used for potting a tranny. The
roofer's pitch won't run at 30C or even 60C, but you need to heat it to
150C to use it, so when its poured in around a tranny in a can you have
to allow a big space so it gets in without setting hard as it cools
against items in the can.
I tried waxes, but they melt too easily, and you get pools of it under
amps on a warm day. I tried styrene as a hardener but it did virtually
nothing to improve matters. Corboba wax was not much better and it
shrank a lot after it had cooled.
The real mc'coy potting compound is a terribler price!.
Good stuff, but like the casting resin I now use for potting, but if
you
ever need to re-wind the tranny its extremely difficult to remove the
core. For a usless tranny with a shorted turn or open winding I
generally place the itm in a wood fire after drilling a few holes to
let
vapors under pressure get out without explosions. When I cansee the
core
is just dull read then next day the mess will fall apart and I can
recover the lams. But it doesn't work with toroids or C-cores because
the lams are strips glued together with epoxy glue of some kind, but I
am not sure what though.
I have an open fireplace with a door that closes down and the terrible
smelling smoke goes up the chimney late at night. I don't do it often.
From what I have observed, the cores magnetic performance does not
suffer from the heating. They are used later in chokes where core
material is not critical. The fire vaporizes any plastics, and the wire
can be easily cut free for the re-cycle bin.
The varnish between lams is also vaporized and the tranny is easy to
dismantle.
So if ever a house with a huge collection of boat anchor electronics
burns down, at least there will be a pile of cores somewhere for that
dedicated old bugger somewhere who will beging to re-build.

I gave this a lot of thought today and I think I may have something
useful for you to consider.

Have you thought about a two-part, condensation-polymerization-cure
silicone? General Electric and Dow make a range of materials with
differing pot lives, viscosity, service temperature, etc. Most of these
will cure at room temperature, and they will bond to PE and PET better
than most materials will. The end product is elastomeric (rubber like)
which should be excellent in suppressing vibration. Many have service
temperatures over 200 C after curing. You can get cures from a few
minutes to several hours. There are low viscosity liquids for potting to
thixotropic gels that flow when painted on, but gel when the shear is
stopped thereby not running on vertical surfaces.

I suggest looking for RTV (room temperature vulcanizing) silicones on the
Internet. GE and Dow make scores of varieties so make a list of the
necessary properties, and the preferred properties and contact the
maufacturers. It might be a good idea to request brochures and
datasheets before you make your selection. If you don't act like Phil,
you might even get some free samples!

73, Barry WA4VZQ


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