Varnishing Methods: A White Paper – Jonathan Lerner Mitchell Electronics
Why Does One Want To Apply Varnish On Transformers?
It is desirable that the spaces between the coverings and the turns of the electrical winding be thoroughly impregnated and coated with the varnish and the varnish coating is then thoroughly cured to provide optimum insulating and thermal conducting properties as well as being imperviousness to moisture and sufficient strength to resist fracture of the coating.
Varnishing Methods Recap
To Recap: After the coil is wound the best practice is to impregnate it with some sort of insulating varnish which hardens after filling. There are four good reasons for this: First, it protects the wire from movement and possible mechanical damage. Second, it prevents the entrance of moisture and foreign matter which might corrode the wire or cause insulation deterioration. Third, it increases the dielectric strength of fibrous insulating materials. Fourth, it assists in heat dissipation from the coil.
Three Types Of Varnishing Methods
What are three types of varnish finishing methods for transformers?
- Air Dry
- VPI (a VIP method)
90% of transformers manufacturer’s in today’s modern world of instant satisfaction use the cheap and quick air dry method. Basically a varnishing method, is when varnish is applied to the transformers as a conformal coating and once the unit dries they just pack it up and ship it out. If you don’t care about quality and having the best possible product inside your own product then this is probably what you are currently using. Sad to say that nowadays transformer winding is fast becoming a lost art.
MEC – Make, Shake &Bake
Our registered varnish impregnation process is very special, just like the quality that goes into every one of our products during its production. This process has been passed down, generation to generation and is a time honored tradition at Mitchell Electronics.Why then does the MEC – Make, Shake,&Bake process create a better unit?The first step is that we preheat the transformers to drive out trapped moisture. Any trapped moisture will shorten the life of certain materials. For example in Kraft paper there would be chemical decomposition through oxidation of the cellulose. How does this effect your units reliability in practical terms?Think of the design life of a power transformer between 30 to 35 years.In fact the typical Time to Failure of a large generator transformer (assume working at constant full load) is 16 to 25 years and a transmission or distribution transformer(working at half load or less) can last from 38 to 65 years.The actual life of a transformer is determined by ageing of the cellulosic insulation in the form of paper on conductors and leads, and of pressboard used for inter-turn or intersection spacers.As the cellulose ages the length of the glucose chain slowly reduces due to chainschism from 1200 molecules to about 200 molecules when it no longer hassufficient mechanical strength to be viable. Thermal (heat) cured (as opposed to air dry) insulation helps in the curing process and preheated windings lower the viscosity of the insulation varnish that will be applied, allowing the varnish to flow more easily through the windings.Having greater fill of varnish ultimately will lead to better disparage of heat buildup and create a much better temperature rise rating for your transformers.The secret sauce is in using the right temperature and bake time mix.At Mitchell Electronics we do it correctly and as a result, your transformer will last much longer than our competitor s transformers with customer reviews raving about incredible mechanical and electrical performance.
What is VPI (Vacuum Pressure Impregnation)?
VPI resin is not really different from our regular insulating varnish, when people say “VPI resin,” they are usually referring to the application method being used rather than the type of resin or varnish.Vacuum pressure impregnation refers to the process in which a prepared winding is placed into a chamber where the air pressure is reduced to a very low level. The resin is then fed into the chamber until the winding is completely submerged. After soaking in the resin for some time (this varies depending on the winding and end-use application), the coil is removed from the vacuum chamber, and the resin is drained. The winding is baked until the resin cures, which is usually done overnight when our regular production lines are shut down. In general, VPI windings will have much better quality impregnation compared to non-VPI windings. We recommend this process when extreme quality and special circumstances call for the best available method available.
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