Plate Transformers and VPI (Vacuum Pressure Impregnation) In Detail

Dr. Transformer:  Welcome back to the Power Magnetics Institute, my friends!  Our customers have been asking for a clarification of the VPI and plate transformers post we discussed earlier.

Vacuum Pressure Impregnation (VPI) is the process of treating the finished coils with varnish by placing it a vacuum chamber.  First, the transformer is placed in an oven and heated to approximately 275 ˚F. This heat treatment drives out moisture from the insulation which could otherwise compromise the insulation.

After heating, the transformer is placed into the VPI hot. The advantage of doing it this way is that it makes it easier for the varnish to penetrate the insulation to the maximum extent possible since the viscosity of the varnish goes down as it is heated by the coil.

Next, the coil is placed in the vacuum chamber and a vacuum is pulled ‘dry’ on the transformer.   The varnish is then admitted into the chamber and the vacuum is pulled ‘wet’.  Finally, the chamber is pressurized to 100psi completely filling the coil and insulation with varnish, and then varnish is finally pushed out of the chamber.    This results in a coil that is totally saturated with varnish (with atmosphere dip it is only partial).  The insulation will then have its maximum dielectric strength and will not absorb moisture (key to longevity).  Also, because varnish is a good adhesive, it has better mechanical strength.  In addition, because varnish is a good thermal conductor, it runs significantly cooler.

The benefit of VPI on the plate transformer is that it reduces the temperature rise by approximately 20%, making it run appreciably cooler and quieter.  And the result of that is greater reliability and extended life.  This is recognized throughout the transformer industry and is backed up by numerous studies

We hope this information is helpful to you as you consider your options for your future designs.  We believe that your transformers should all be treated with VPI, particularly given your application.

Power Magnetics has been manufacturing plate transformers for the furnace industry for 30+ years and has an unmatched record of reliability in the field. Our customers cannot afford the price of a field failure with their customers which we are told can easily exceed 10x the price of the plate transformer.

As always, Dr. Transformer signing off and wishing you the best! If you have a question, we are always here to help!

 

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