Cheap and easy vacuum impregnation

[wrenchone]

To do vacuum impregantion right, you need a non-solvent-based varnish. That way when you pull a vacuum on the varnish, entrained gas boils out, but you don't boil the solvent in the varnish. A commonly used varnish for vacuum impregnation is a solventless polyester varnish like Dolph CC-1105.

One way of increasing the impregnation efficiency without pulling a vacuum on the part is to heat it up in an oven to ~100C long enough so that the part is hot all the way through. This expands the air in all the cracks, crevices, and voids. The part is then dunked into the cool varnish. This does boil the varnish immediately next to the part a bit, but it also cools down the air in those cracks, crevices, etc., and causes the varnish to be drawn into the part. This works pretty well for small SMPS transformers. I haven't tried it on larger parts though. I've actually used it to quell winding buzz in power factor correction chokes, and the technique caused the varnish to significantly penetrate the choke winding.

 

[richwalters]

One way of increasing the impregnation efficiency without pulling a vacuum on the part is to heat it up in an oven to ~100C long enough so that the part is hot all the way through.

Don't overcook the broth up too much: tendency for manufacturers in hard times might use lower temp wire to save costs. Enamelled copper insulation ratings at a glance; the inner core of a working power EI tranny would be considerably higher. delta temp-rise depends on size.

Class O 90°C
Class A 105°C
Class B 130°C
Class E 120°C iec
Class F 155°C
Class H 180°C
Class K 200°C
Class M 220°C
Class C 240°C+

The enamel ratings are peppered with specs. Many audio trannies use lower °C rating materials when transformers are unlikely to feel hot when touched with the backhand. A p-p o/p would be a candidate. An air cooled welding tranny might use class H upwards for obvious reasons-.

richy

 

[wrenchone]

Most manufacturers I know of use class B wire with solderable nylon/polyurethane coating. This is an industry standard. Anything else (enamel, for example)is a pain to terminate in mass production, as you have to scrape it or use a chemical stripper which then must be completely cleaned off for reliability's sake.. The only dodgy points might be the input/output leads, and even 90C rated cheap PVC wire will stand a reasonable soak at 100C without hurt. I do agree, though - don't go overboard with the heating - 100C will get the job done just fine. You want a sufficient cold/hot difference to displace a lot of the air in the voids. We actually have used a toaster oven and a convection oven to bake our prototype magnetics at work, and they come out peachy. The smell of baking varnish fills the lab - mmmmm, just like mom used to make....

Old magnetics might be a different story.there you'll find the enameled wire and funky cotton-covered brittle leads.

 

[slafferty]

Hi wrenchone,

Do you know where an individual could order an affordable quantity of Dolph CC-1105? The smallest package I could find is 1-gallon at $125:

http://www.chemcenters.com/154669

Steve L.

 

[wrenchone]

One gallon is pretty much the smallest size of transformer varnish you'll see for sale. CC1105 is good stuff, but it requires higher curing temperatures than your average baking varnish. It does need baking to set it up properly.

The baggie technique described here is actually not too bad, as you are not exposing too much of the varnish to vacuum. What you want to avoid is a transformer sitting in a pot of varnish, with a vacuum pulling on it - you'll be looking at a big, frothy mess. Heating up the transformer beforehand is also good, - not only are you reducing the density of air that must be displaced, the warm transformer will also reduce the varnish viscosity for better flow. Still, there is the potential for a big, big mess without care at each step.