Why is varnish sometimes applied to transformer windings?

Why is varnish sometimes applied to transformer windings?

Published on Jun 28, 2021 by Skipper

What is varnish?

Varnish is a protective coating/paint that is often used in the furniture industry to give a finishing and protect wood. In electrical and electronics industry, there are varnishes called ‘insulating varnishes’ that aid in proper operation of devices such as motors.

What is insulating varnish?

Sometimes, insulating varnish is also used on the windings of transformers. This is usually done in low tension or low power transformers, where the only cooling source is often the air. In high tension transformers or transformers who can utilize oil for cooling, varnishing is rare.

Why is varnish applied to transformer windings?

Note that varnish is not applied to only transformer windings, but only in specific cases (usually low tension or low power transformers).

There are multiple reasons as to why someone would want to varnish the windings of a transformer —

  1. The varnish coating applied to the whole winding serves as a protective barrier against moisture and therefore lowers the moisture absorption of the winding. This also makes it more robust to dust and other undesirable foreign particles.
  2. The insulating varnish also helps to increase the insulation of winding, as its name suggests. It enhances not just the general insulating properties, but the thermal properties as well.
  3. Applying varnish to the whole windings binds it together lightly, which makes it more resistant to physical/mechanical damage and/or deformation.
  4. Sometimes, coils are packed very tightly. Varnish helps by preventing the wear and tear damage that would arise in such scenarios by the coils rubbing against each other.


We have learnt that applying varnish on transformer windings can help it function more efficiently as varnish does not only improve it’s functional capabilities but also it’s electrical, thermal and mechanical characteristics while reducing the wear and tear damages.

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