Six tips for testing motor insulation systems

Insulation resistance (IR) tests are useful for motor insulation systems and users should take steps such as recording conditions and watching for factors that may have an impact on the test to ensure maximum accuracy.


Although there are many ways to assess motor insulation systems, the insulation resistance (IR) test remains a useful tool for determining if a motor should be removed from or placed into service. When conducting the test and evaluating the results, consider these six tips.

Table 1: Guidelines for dc voltages to be applied during insulation resistance tests. Courtesy: EASA, IEEE

1. Discharge for safety and accuracy. Discharge the winding to ground before and after IR testing. A good rule of thumb is to ground the winding for 15 minutes or four times the test time, whichever is greater.

2. Winding configuration for testing. If possible, isolate and test each phase separately and ground the two phases not under test. This allows for testing of the phase-to-phase insulation as well as the ground insulation.

3. Record important test conditions. Document the ambient temperature, relative humidity, dew point, winding temperature, time since service, test voltage and connection arrangement.

4. Simple test procedure. Energize the winding for one minute at an appropriate voltage (see Table 1) and correct the measured IR value to 40°C by multiplying by a KT from Table 2. Most modern insulation falls under the "Thermosetting" category. Use the "Thermoplastic" values if you know it is a much older winding with an asphaltic or shellac mica system.

Table 2: Kt versus temperature for thermoplastic and thermosetting insulation stator winding systems. Courtesy: EASA, IEEE

5. Watch for factors that can affect the IR test. During the test, watch out for these four factors that can have an impact on the test and its results:

  • Surface contamination such as oil, dust and salts may be conductive, causing lower IR. Cleaning and drying the winding will usually improve the IR.
  • If the winding temperature is at or below the dew point, moisture may accumulate on the surface, causing low IR values. When the winding is otherwise in good condition, drying will resolve this issue.
  • Winding IR decreases when winding temperature increases, which is why correction to a baseline temperature is recommended.
  • Voltage magnitude and existing charge also can affect IR, but selecting the voltage from Table 1 and following proper grounding practices before testing should eliminate this issue.

Table 3: Recommended minimum insulation resistance values at 40 °C (all values in MΩ). Courtesy: EASA, IEEE

6. Evaluate the test results. Table 3 shows the recommended values for the corrected IR. Lower values warrant further investigation before the unit is placed into service. Some machines do operate below these values. Having historical data on-hand is valuable.

Mike Howell is a technical support specialist at the Electrical Apparatus Service Association (EASA), a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media,


Key Concepts

  • Insulation resistance (IR) tests are useful for determining whether a motor should be removed.
  • During the test, the user should discharge for safety and accuracy and record important test conditions.
  • Users should also be aware of factors that might affect the IR test.

Consider this

What other aspects should users watch for while performing an IR test on a motor?

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