Why is insulation resistance low after varnishing?

What is insulation resistance and how is it measured?

Insulation resistance shows how well the winding is insulated from the frame (ground) and from the other phases. In the workshop it is usually measured with an insulation tester, often called a megger. The tester applies a DC test voltage to the winding and shows the result in megohms (MΩ). The higher the value, the sounder the insulation.

The test voltage and the acceptance value depend on the machine's rated voltage, its size and the standard you follow. Standards such as IEEE 43 and the manufacturer's instructions apply here. We do not give a fixed limit in this article; what matters is finding out why the value is lower than expected.

Keep one thing in mind when reading the result: insulation resistance is very sensitive to temperature. As the winding warms up, resistance drops. A winding fresh out of the oven will read higher once it cools. Compare readings taken at similar temperatures.

Why is the reading low after varnishing?

1. Moisture left in the winding

This is the most common cause. If a winding that was stripped, washed or stored in a damp place is varnished without drying, the moisture stays under the varnish. The varnish then keeps it from escaping. The result is a low, unstable insulation reading. That is why a winding should always be preheated and dried before varnishing.

2. The varnish has not dried or cured

Varnish that still holds solvent, or has not finished curing, will not give its full insulation value. You see this when a baking varnish is left to air dry or the oven time was too short. An air-drying varnish also dries slowly when applied thick in a cold, damp room. See our guide on why winding varnish does not dry for details.

3. The test was done too early

If you test a winding straight out of the oven, the value will look low. That is not always a fault. Let the winding cool to room temperature, then test again.

4. Dirt, carbon dust and oil

Brush dust, metal swarf, oil and old varnish residue can form a conductive path on the surface. If this dirt is sealed under the varnish, the problem becomes permanent. Leakage often shows up at the terminal box, the terminal board and the lead wires. Clean the winding before varnishing, and wipe and dry the terminals before testing.

5. Damaged wire or slot insulation

Enamelled wire scratched during winding, a torn slot liner or an end turn rubbing on the frame will cause a low reading. Varnish does not repair this kind of mechanical damage. If only one phase reads low and drying does not change it, this is the most likely cause.

6. Wrong or too much thinner

An incompatible thinner can spoil the cure. Too much thinner makes the film thin, so too little varnish stays in the winding. Always thin a varnish with its own thinner. See our thinner ratio guide and our thinner range.

Step-by-step diagnosis and fix

1. Disconnect the leads: Isolate the motor from the supply and other equipment. Clean and dry the terminal box.
2. Test each phase: Test each phase to frame, then phase to phase. If all are low, suspect moisture or curing. If one phase is low, mechanical damage is more likely.
3. Note the temperature: Test with the winding at room temperature and record the temperature.
4. Dry again: If the value is low, dry the winding in the oven. If you used a baking varnish, re-bake it under the conditions given in its TDS.
5. Cool and retest: If the value rises clearly after drying, the cause was moisture. If it does not, inspect the winding in detail.
6. Keep records: Keep the before and after readings for each motor. The trend over time tells you more than a single reading.

For a closer look, the polarization index (the ratio of the 10-minute reading to the 1-minute reading) is also used. It helps tell surface leakage apart from moisture.

How to prevent it next time

• Clean, preheat and dry the winding before varnishing.
• Apply the varnish while the winding is still warm but not too hot; the varnish penetrates better.
• Let excess varnish drain off; thick build-up slows drying.
• Follow the varnish's own curing conditions. Always bake a baking varnish. To choose, see air-drying or baking varnish.
• Do not leave a freshly varnished motor in a damp place.
• For motors that will run in damp or chemical environments, choose a more resistant system.

Which varnish?

Workshops with an oven can use the baking varnish OR-300, which penetrates the winding well and cures fully in the oven. Workshops without an oven can work with air-drying varnishes such as OR-100 and OR-102. For compressor motors that need class H and high chemical resistance, OR-114 is the usual choice. Drying and baking conditions are given in each product's TDS. If you are not sure which varnish suits you, read our varnish selection guide or contact us.