[Boatanchors] Re: [Ausmilsig] RE: GB> Potting of old power transformers.

[Brian Clarke]

Hi fellas,

I used to be production manager of a transformer manufacturer. 

Pitch is one of those unusual products, like glass, that is a supercooled 
liquid at normal temperatures - it flows under pressure and high 
temperature, it cracks at low temperatures and shatters under sudden 
shock loading - that's why it's such a poor cement for our roads when 
large trucks running very high tyre pressures travel over it. Its main 
redeeming feature is that road workers, for whom their next IQ point 
would be infinitely higher than their previous, can lay it - and so they are 
the economic rationalist's choice.

Pitch is neither very good at preventing ingress of water nor at conducting
heat from the transformer. Polychlorylbiphenols [PCBs] were absolutely 
brilliant at both jobs - but are now banned because of their carcinogenicity. 
Oils, in general, are better at heat removal because you can use both 
conduction and convection - pitch can only use conduction. Nonetheless, 
modern transformer oils are pretty good. I have made several cases for 
potting small transformers and have not had a leak. What I do is to bring all 
leads out through the top which is held on as for a tappet cover on a car - 
bolts through a flange and a cork gasket. There are some brilliant silicone 
products that are oil resistant and can be used in place of a gasket. 

Where can you get the correct oil? I just front up to the local electricity 
distribution authority with a clean can and ask - they use thousands of 
litres a year - they'll probably give you a few litres.

Make sure that when you finally seal up the transformer you have dried it 
completely; any remaining water in the windings or insulation or in any 
trapped air, will fall to the bottom - water has a higher specific gravity than 
all oils and will possibly damage the transformer core.

However, I have a concern about how much heat you think you need 
to dissipate. For small devices like radio receivers, air cooling should 
suffice, and consequently, an open framed transformer is better at 
dissipating heat than a fully enclosed one. However, if you have to enclose 
the transformer to attenuate magnetic coupling, you could look at punched 
steel plate, the type with many rows of 1/8" diam holes. A good transformer 
should not generate more than about 2% of rated kVA as waste heat - 1% 
in copper losses and 1% in iron losses; for a poor transformer, this might 
rise to as much as 10%. If you need to get rid of more than that, you have 
a transformer design problem - eg, not enough good quality iron, poor 
stacking of the laminations, not enough primary inductance. Often, I have 
found that transformers designed for American equipment [60 Hz] have 
very high magnetising current at 50 Hz = elevated copper losses and 
eventual failure of the insulation and probably damage to the lams as well.

I've sent this to the Boatanchors list because there has been some concern 
about transformer replacement expressed there as well.

73 de Brian, VK2GCE.
  From: Brian Goldsmith 
  To: 'Chris Gill' ; ausmilsig at smartgroups.com 
  Subject: [Ausmilsig] RE: GB> Potting of old power transformers.

  -----Original Message-----
  From: Chris Gill

  G'Day Brian,
  I will watch this and see what the consensus is, I'm inclined towards making
  the case fully sealed and oil filling. I need to rewind my AR 88 transformer
  also so can you give me some details of what you did?, I haven't stripped it
  down yet so I would like to hear of your experiences.
  Regards,


  *** Hello Chris,hope you are well.
          IMHO it would be very difficult to hermetically seal the can and its
  bottom plate to prevent the loss of a liquid oil. However, in your
  climate, given the information I have been able to gather, I would suggest
  that you find yourself a local pitch or "asphalt" supplier and, after
  rewinding the transformer, pot it completely, (after a bake out period) but do
  not include any oil filling.
  I have just bought some unclad PCB fibre glass material to make a cheeked
  bobbin that will enable me to wind the appropriate primary and secondaries
  turns onto it with Mylar insulation between windings. I will forward you the
  ultimate dimensions for the "LF" transformer but if yours is an AR88D type
  then you may find the core size is different.
      My AR88LF had a 117/234 Volt, 25 to 60 Cycle transformer fitted and I
  have reverse engineered the turns and wire gauges of the primary and various
  secondaries.These details will be different to those required for the AR88
  or AR 88D which used a series type winding of 110-125-150-210-240 Volts.

  Let me know what type you have, especially any info on the bakelite base
  plate.

  Brian Goldsmith.

  I need to rewind my AR 88 transformer also so can you give me some 
  details of what you did?, I haven't stripped it down yet so I would like to 
  hear of your experiences.
  Regards,
  Chris.

  ----- Original Message -----
  From: "Brian Goldsmith" <brian.goldsmith at echo1.com.au>
  To: <glowbugs at piobaire.mines.uidaho.edu>
  Sent: Sunday, August 08, 2004 8:16 PM
  Subject: GB> Potting of old power transformers.

  > Greetings to all.
  > I have had to rewind the power transformer of my AR88.Originally the
  > transformer was "potted" in pitch, primarily I believe, to kept moisture
  > out. What is the view of list members on repotting the rewound transformer
  > taking into account the use of modern insulants for the winding wire? Is
  > their any evidence to suggest the potting "mix" is needed to remove heat
  > from the core/winding? Etc,Etc!
  >
  >      All views considered!!
  >
  >                  Brian Goldsmith.