We often hear the terms preventive maintenance (PM) and predictive maintenance (PdM) of electric motors. However, it’s far less often that we give consideration to the tasks associated with these methods of maintaining motor operation and extending operating service life. This article will address some of the more common activities associated with PM and PdM, with the focus on three-phase squirrel cage motors.

Insulation resistance is affected by several variables: the type of insulation, age of the material, surface area, moisture and contamination. Insulation resistance can be described as being made up of four components: Leakage, capacitance, conduction and absorption. Capacitance normally only affects the first few seconds of the Polarization Index (PI) test; conduction should be essentially zero if the windings are dry; and leakage current is constant over time. The PI test is useful because the remaining variable - absorption current - indicates the health of the insulation. The PI test is used to determine the condition of motor insulation to ground.