A capacitive inrush current brings the potential in the motor up to the test voltage by charging it. This current drops quickly and reaches zero within a few seconds after the test voltage is reached. For large motors with high capacitance, the inrush current is high. Total leakage current failure limits must be set high enough to avoid tripping the limit during this initial phase of the test. For more information on the subject of capacitive inrush current and how to avoid tripping a limit, see DC hipot test methods.
Absorption current polarizes the insulation. This current also goes to zero or very near zero within 30 seconds to 1 min in random wound motors. Form wound motors take much longer due to the layers of insulation used between turns. The change in absorption current over time is what is used to calculate PI and DAR ratios in an insulation resistance measurement.
Conductance current flows between the copper conductors and ground through the bulk of the insulation. This current is usually zero if the motor is new or undamaged. As the motor insulation ages and cracks or is damaged conductance current may flow depending on the test voltage applied. Conductance current tends to accelerate with increasing voltage. This current is sometimes referred to as leakage current or as part of the leakage current.
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According to IEEE 43, surface leakage is the current flowing in the dirt on the surface of the windings to ground. It is called surface conduction current in other standards. A dirtier motor has higher leakage current and a lower megohm result. There may be an increase in the surface leakage current on motors with a stress-control coating on the the end-windings. After 1 minute with a random wound motor or 5-10 minutes with a form wound motor, the surface leakage current is typically the only current remaining unless the insulation is weak or damaged.
The total current is the sum of the 4 currents. An motor and insulation tester measures total current. The total current equals or is very close to the surface leakage current at the end of the insulation resistance measurement. This gives the operator a good measure of how dirty or contaminated the motor. It also alerts the operator to a possible catastrophic connection from the windings to ground.
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