Research to correct misconceptions regarding significance of PI and IR testing of motors

Assurance of electric motor adequacy whether new, installed, refurbished, or repaired is based on a combination of quality controls and a comprehensive program of tests. Typical tests used by industry include Polarization Index (PI) and Insulation Resistance (IR) Tests per the guidance of standards such as IEEE 43 “Recommended Practice for Testing Insulation Resistance of Rotating Machinery”. Dividing the IR value after ten minutes by the value for one minute of test provides a typical “Polarization Index”, a measure of whether or not the absorption current decreases with time. A low PI of 1 to 1.5 is typically considered not acceptable based on practices and insulation systems developed many decades ago. A PI of 2 to 4 is typically considered acceptable, although a minimum of 3 is preferred. This acceptance criteria, however, does not apply to today´s modern motor insulation systems exhibiting very high TR values, especially Vacuum Pressure Impregnated windings. Test results have shown that 1 minute and 10 minute IR readings taken on new and rewound motors are infinity on the typical test instruments used, effectively resulting in a PI of 1. Conditions for conducting PI and IR tests on motors are considered including the inappropriateness of the industry “standard” temperature correction factor used in IR testing