How do you dry a winding before varnishing?

Why dry a winding before varnishing?

The insulating materials in a winding absorb moisture from the air. Slot paper, binding tape and old varnish layers hold water, especially in motors stored in damp places. If you varnish a damp winding, the varnish traps that moisture inside.

Trapped moisture causes several problems:

• Low insulation resistance: the megger reading stays low even after varnishing.
• Bubbles and voids: moisture turns to steam when heated and leaves bubbles and pores in the varnish film.
• Poor adhesion: varnish does not bond well to a damp surface and lifts over time.
• Drying problems: some varnishes dry slowly or incompletely on a damp surface.

For other causes of drying problems, see our guide on why winding varnish won't dry.

1. Clean first

Clean the winding before drying it. Varnish will not hold on dirt and oil, and a dirty surface also holds more moisture.

• Dust and dry dirt: blow it off with dry, oil-free compressed air. Make sure the compressor's water and oil separator is working.
• Oil and grease: wipe with a suitable cleaner that does not attack the insulation. Let the cleaner evaporate completely.
• Carbon and soot: on burnt windings and machines with brush dust, make sure no conductive residue is left.
• Heavily soiled or washed windings: if the winding has been washed, drying takes much longer; do not rush it.

A freshly wound stator usually only needs dusting. If you are re-varnishing an old winding, cleaning matters even more.

2. Preheating and drying methods

Oven drying

An oven is the most controlled method. The winding is held in the oven for a period of time and the moisture is driven out. The oven must be ventilated; in a closed oven the steam stays inside and drying slows down.

There is no single correct temperature. Many shops preheat well below the winding's thermal class, often around 100 °C. Treat this only as a general idea. What matters is not exceeding the winding's thermal class and following the recommendation in the TDS of the varnish you will use.

Heat lamps and hot air

Without an oven, you can use infrared heat lamps or a hot air blower. Heat is not spread evenly with these methods. Do not place lamps too close, turn the winding from time to time and avoid overheating one spot.

Current heating

On larger motors, the winding can be heated from the inside by passing a controlled current through it at low voltage. This pushes moisture outward and works well. It should be done by someone experienced. Keep the current below the winding's rated current, monitor the temperature continuously and never leave it unattended.

3. How do you know it is dry?

You cannot judge dryness by eye or by touch. The most reliable way is an insulation resistance test (megger test).

1. Before drying, take a reading between the winding and the frame and write it down.
2. Keep measuring at intervals during drying, at the same test voltage. Low-voltage motors are usually tested with a 500 V or 1000 V megger.
3. As moisture comes out, the value may dip first and then rise.
4. When the value no longer changes over several readings, the winding is most likely dry.

Compare readings taken at the same temperature. Insulation resistance falls as temperature rises, so a lower reading on a hot winding and a higher one on a cold winding is normal. For a closer assessment you can also use the polarization index, the ratio of the 10-minute to the 1-minute reading.

4. Varnish while the winding is warm

Do not leave a dried winding standing. As it cools it starts to absorb moisture again. The best results come from varnishing while the winding is still warm.

• On a warm winding the varnish flows better and works in between the wires.
• If the winding is too hot, the solvent in the varnish flashes off quickly. A skin forms on the surface and voids and bubbles stay inside. Let the winding cool to a suitable temperature.
• Check the varnish TDS for the right temperature to start varnishing.

If you use air-drying varnish, see air-drying motor winding varnishes, or the OKA-501 motor insulating lacquer for quick jobs. If you have an oven, baking motor winding varnishes let you use the same oven for preheating and curing. For the difference between the two, read air-drying or baking varnish.

What about transformer windings?

Transformer windings are usually thicker and more tightly packed than motor windings. They contain paper and pressboard, which hold a lot of moisture. Drying therefore takes longer and heat reaches the inside slowly. Extending the time is safer than raising the temperature fast. Again, track dryness with a megger. For choosing a transformer varnish, see the transformer varnish page.

Common mistakes

• Varnishing a motor straight out of a damp store. It may look dry, but the insulation can still hold moisture.
• Drying in an unventilated oven. The steam cannot escape and drying drags on.
• Too much heat. Enamelled wire, paper and old varnish are damaged if heated above their class.
• Drying and then leaving it for hours. The winding pulls moisture back in as it cools.
• Pouring varnish on a very hot winding. You get skinning, bubbles and poor penetration.
• Cleaning with wet compressed air. You end up spraying water and oil onto the winding.
• Not letting the cleaner evaporate. Leftover solvent mixes with the varnish and spoils drying.
• Leaving current heating unattended. Uncontrolled current creates hot spots in the winding.
• Relying on guesswork. Confirm dryness with a megger.

To choose the right varnish, see our which winding varnish to use guide.