Use cable testing to assess a motor's health

Regular cable testing on motors are designed to help determine if any scheduled maintenance may need to take place and using insulation resistance or polarization index testing can provide some answers.


Regular cable testing on motors are designed to help determine if any scheduled maintenance may need to take place and using insulation resistance or polarization index testing can provide some answers. Courtesy: David Manney, L&S ElectricDo not underestimate the importance of keeping a facility running at any given time. There are a myriad of circumstances that result in unexpected downtime. Taking care of equipment is one of the best ways to reduce surprises. Avoid those surprises and downtime with routine maintenance and cable testing. Among the ways to test cables are polarization index (PI) and insulation resistance (IR) testing.

Two similar ways of cable testing

When looking at cable testing, there is confusion sometimes because IR testing is like PI testing. The two test work well together. Seeing deterioration during an IR test provides justification to perform the PI test.

They both use the same voltage, but there is one primary difference between the two: time. IR testing lasts one minute. PI testing takes over ten minutes. Those extra nine minutes provide more detail about any current conduction or leakage current.

Polarization index testing

Polarization index testing checks various aspects of a motor. These aspects include:

  • Motor fitness (or health)
  • Amount of moisture present
  • Suitability to the operation
  • Insulation deterioration.

The PI test is reliable and provides accurate results. These results make it important in determining the performance of motors. PI testing equals the ratio of 10-minute to 1-minute IR value of a machine's windings. Test any motors with the high index for their windings. In the case of induction motors, 2.0 is the value of the smallest PI.

Insulation resistance testing

Insulation resistance testing determines the resistance of the insulation system within electrical equipment. An IR test measures the distance between two points separated by insulation. The test determines the amount of resistance the insulation allows.

A dielectric withstand test, or hipot, provides a more thorough examination of the insulation than an IR test, so IR tests are not performed often.

Outside influences

Operating in harsh environments may reduce the lifespan, but stator windings should last up to 20 years. One environmental example that shortens motor life is excessive heat. Issues sneak their way into the insulation.

The more time passes, the more often these problems occur. After a time, the insulation becomes brittle as it oxidizes and dehydrates. Disintegration occurs under certain conditions, such as excessive vibration or shock.

The PI locates any new area of deterioration within the stator winding. It determines the fitness of the motor and detects moisture better than a standard insulation resistance test.

These reasons link the health of the motor and insulation quality. Regular cable testing helps determine if any scheduled maintenance may need to take place. 

- David Manney is a marketing administrator at L&S Electric. This article originally appeared on L&S Electric Watts New BlogL&S Electric is a CFE Media content partner.

JIM , CA, United States, 02/28/17 03:15 PM:

Hipot testing can be a destructive test if it fails, and typically applied to cables, not motor or transformer windings. Choose your shutdown timing carefully, and be prepared for a repair.
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