Polarization Index Testing, or PI testing, is an important and often misunderstood motor test, and it is often misapplied in practice. PI is an insulation resistance test where the voltage is applied to the motor and the 10 minute value is divided by the 1 minute value for the PI index or ratio. PI tests are useful for identifying contaminants and physical changes in the motor insulation. Interpreting the test results correctly is critical because motors vary widely depending on type, so why do PI testing and how do we avoid misapplying the test? Misapplication is likely a result of a standards change. Various ways of testing insulation resistance have been around since the 1800s and they mostly worked to a point. More recently, the Institute of Electrical and Electronics Engineers realized that a simple megome test alone wasn't enough, so the IEEE 43 standard went through a major update in 2000 to accommodate changes in insulation systems, temperature and dew point limitations, trending tests, types of windings, and more. The net effect of these revisions is that PI tests are far more comprehensive, but also more challenging to conduct than in the past. A properly conducted PI test is like a doctor diagnosing a patient using an X-ray rather than using a stethoscope. You have to look deeper and you have to look at the data. That means graphing the results with a good mego meter or other graphical recording instrument. A good PI test will exhibit a smooth curve that levels off at the top as shown here. In a bad PI test, motor windings that have insulation problems will show limited absorption, electrical discharge patterns, and other anomalies as we see here. This pattern indicates contamination, embrittlement, and other problems with the motor insulation. Now, those are two basic examples, but for a complete picture of motor insulation health in the field or in the shop, the PI test should be used in conjunction with other methods, such as step voltage testing. Trending the test data yields the most accurate results. In addition to this, we combine PI trending data with other testing methods, such as those available in a Baker machine or a PDMA tester. We use this approach for a more complete picture of motor insulation health. In summary, updates to the IEEE 43 standard necessitated a change in how to perform testing and in how we should think about that data. PI tests require more interpretation than in the past. The polarization index is not a standalone acceptance test. You must combine PI with other testing methods. Graphical recording of the polarization index will yield the best results. We should consider doing PI tests every few months to detect potential areas of failure before they become problems, especially on your mission critical motors. For more information about PI testing or to get started on a testing program, contact us at ecoonline.com.