Short answer: DAR (dielectric absorption ratio) and PI (polarization index) are both ratios taken from a timed insulation resistance test with a megohmmeter. DAR is the resistance at 60 seconds divided by the resistance at 30 seconds — a one-minute check. PI is the resistance at 10 minutes divided by the resistance at 1 minute — a ten-minute… Read More: DAR vs PI: Reading Insulation Absorption Ratios »
Most people who run a Polarization Index test can tell you the formula: 10-minute IR divided by 1-minute IR. Fewer can tell you why that ratio is meaningful, where it stops being meaningful, or what a result of 1.1 on a modern epoxy-mica winding actually implies — compared to the same reading on an asphaltic machine from 1965.… Read More: Polarization Index Testing of Motor Windings »
Three numbers come off an insulation test: the spot reading, the ratio, and the trend. Most people look at the first one and stop. The spot reading is the weakest of the three. It swings with temperature and surface moisture and tells you almost nothing about the winding underneath. The ratio — DAR or PI — filters most… Read More: How to Read Megger Test Results: Good, Bad, and Marginal »
The polarization index test is the most reliable way to assess insulation condition on motors, generators, and other rotating machinery. It goes deeper than a standard 60-second megger reading and gives you a result that’s independent of temperature and equipment size. If you only have time for one insulation test on a critical motor, make it the PI… Read More: Polarization Index Test: What It Is, How to Do It,… »