Polarization index testing benefits

Polarization index (PI) testing is an extension of the insulation resistance test and is designed to check specific issues in a motor such as moisture and insulation deterioration.
By David Manney, L&S Electric July 10, 2017

Image courtesy: Bob VavraTo ensure the optimum performance and condition of machinery, it is important to conduct certain tests that can also prevent costly repairs and unexpected downtime. For instance, polarization index (PI) testing is one of these techniques that can help machines to function properly. 

Overview of PI testing

PI testing is designed to check specific issues in a motor including moisture, motor fitness, suitability of the operation, and the gradual insulation deterioration of the machinery. In fact, it is an effective and efficient way to evaluate the performance of a motor. Machines with the higher index for the windings are ideal candidates for this type of test. A minimum PI value of 2.0 is necessary for induction motors.

The life and performance of a motor depend largely on the windings’ insulation conditions. This is why it is necessary to determine the insulation condition through tests such as the polar index tests and insulation resistance (IR) testing.

To put it simply, polar index testing is more of an extension of the IR test. During an IR test, users will be able to identify and assess the conductors’ insulator resistance to the rotor or the stator. If the resultant resistance comes with a low value, this means the motor is undergoing deterioration. Consider every reading from the PI since the values all pertain to the insulator of the motor and its condition.

Performing PI testing

To gain success from PI testing, it is important to follow these nine steps involved in this process:

  1. Remove internal and external connections before the testing begins.
  2. Discharge the windings to the grounded value.
  3. Apply a maximum dc voltage between the stator and conductor in the windings.
  4. Use a megohmmeter to measure the resistance.
  5. Obtain a reading at one and 10 minutes so the final value can be determined.
  6. Turn the power supply off after the readings have been received.
  7. Use the PI test formula to calculate the final resistance.
  8. The entire test duration must be performed within 10 minutes.
  9. Take note of the winding temperature at all times. Perform this portion since the test is of single- and three-phase induction motors. This is why the windings should be grounded based on the testing procedure performed.

As mentioned earlier, the polar index test aims to determine the condition of the motor’s insulators. Since the winding insulation impacts the durability and integrity of a motor, it is important to keep this area in check to avoid untimely downtime and massive expenses for repairs. Hence, it is important to make sure that some factors that impact the insulation resistance are under control including the type of insulation, surface area, the age of the material, as well as contamination and moisture.

Insulation resistance also involves various components such as the capacitance, leakage, conduction, and absorption. The capacitance is usually the first to be determined at the onset of the test. As for the conduction, this can stay in zero because of the windings’ dry condition. As for the leakage current, this area remains constant during the polar index test.

Benefits and uses

There are several reasons why it is important to conduct PI testing. One of these is to take note of the absorption current, which provides information on the insulation’s condition and health. Users should also learn more about how the ground impacts motor insulation. This is why the PI testing ratio comes with a 10-minute to a 1-minute resistance.

In addition to these functions, users can also identify the time needed for the polarization of insulation molecules to resist current flow. When adding the current to the windings, molecules begin to align. The alignment rate provides details about the condition of the insulation.

Since the evaluation of the insulation resistance takes place between the conductors and ground during this test, users can obtain more accurate results from testing taped and sealed coils. Just be aware of air exposure skewing the results due to humidity, contamination, and the conductor surface.

To check the condition of a motor’s insulators, it is best to conduct the PI test properly. By doing so, the motor’s lifespan and durability can be prolonged while preventing unnecessary downtime, loss of profits, and expensive repairs.

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