FOUR RECOMMENDED ELECTRICAL TESTS We recommend the following four offline electrical tests that focus on stator winding insulation. They provide you with a complete assessment of the stator winding insulation’s integrity, aging, and contamination levels in a machine, which is why we consider them to be the most vital testing procedures. DC insulation resistance (IR) and polarization index (PI) These combined tests provide you with a quick stator winding integrity check. › Why it’s important: It detects general problems immediately. It is sensitive to surface moisture and contamination, which affect insulation integrity. › What it tells you: If the IR value is low, the insulation is likely damp and/or dirty. The PI is the ratio of insulation resistance over 10 minutes vs. 1 minute. Based on international standards, the PI value should be 2 or higher in modern insulation systems. › The bottom line: It’s the first line of defense for deciding if it’s even safe to energize the machine for further testing. › Frequency: Annually or before startup after a long shutdown period. › Applicable standards: IEC 60034-27-4 and IEEE 43 Dissipation factor (tan delta) / power factor This AC test assesses overall insulation quality and health. › Why it’s important: In a perfect world, insulation acts like a pure capacitor. As it ages, it can become more resistive. Detecting increased insulation losses early supports condition-based maintenance decisions. › What it tells you: The dissipation/power factor test measures the ratio of resistive current to capacitive current (IR/IC). By assessing these dielectric losses in the insulation system, its overall condition and aging or contamination levels can be derived. A phase-to-phase comparison immediately shows you if the winding’s parts have problems. › The bottom line: Dissipation/power factor measurements provide a quantitative indicator of insulation health, enabling early deterioration detection and preventing costly rotating electrical machine failures. › Frequency: Every 1 to 3 years › Applicable standards: IEEE 286 and IEC 60034-27-3 1 2 16
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