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Use polarization index test to determine condition/health of motor insulation

Publication date: 
September 2000
Chuck Yung, EASA Senior Technical Support Specialist

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.

Winding insulation
Motor testing