What is it used for?
A megohm measurement is used to measure the winding’s Insulation Resistance (IR) to ground. The megohm result indicates how dirty, contaminated or wet the windings are (all referred to as “dirty”). If the megohms are below certain levels, the motor should be scheduled for reconditioning. Typically the lower the megohm number is, the “dirtier” the windings are.
PI (Polarization Index) and DA (Dielectric Absorption) are ratios of megohm measurements done at different times. PI is the ratio of the megohms at 10 minutes and 1 minute. DAR is the ratio of the megohms at 1 minute and 30 seconds. PI is most commonly used with form coil machines, and usually includes a megohm time series graph with multiple recorded megohm values. The graph can provide additional information about the condition of the insulation system and indicate if the insulation is brittle or delaminated. DAR and PI results may be tracked over time in maintenance and reliability programs.
Important: The megohm test results also tell the test operator if the windings can be over-voltage tested with a Hipot and Surge test.
How it works
(for more details see Insulation Resistance Tests under Test Technologies):
Megohm: A voltage potential is applied to the windings. The voltage and resulting current to ground is measured, and the resistance in megohms is calculated using Ohm’s Law. 1,000,000Ω = 1megohm.
For a megohm measurement on a used winding, most of the leakage current is often surface currents running in the “dirt” on the outside of the windings. The ground insulation certainly can be weak with conduction current flowing through it to ground. The insulation resistance can be burned or otherwise damaged leading to a low megohm number. There can also be a slowly decaying insulation absorption current. However, in many cases a low megohm test reading is the result of “dirty” windings. Therefore, the insulation resistance or megohm measurement is sometimes referred to as the “dirt test”. For more details click here.
PI, Polarization Index: This test is mainly used on form wound motors and generators. Megohm data is recorded over the 10 minutes, and the resulting graph and PI ratio can provide additional information about the winding insulation resistance as indicated above. The number of recorded data points can be programmed in the iTIG II motor tester.
DA or DAR test: Dielectric Absorption Ratio is the ratio of the megohms at 1 minute divided by the megohms at 30 seconds. This test is also called the Alternative PI test in the IEEE 43 standard. It is typically used when the leakage current measured stabilizes within 1 minute. If this happens, the PI test is useless, the ratio becomes 1, and the DAR should be used instead if a ratio test is required or useful.