SKF AWAIV-12, Руководство пользователя

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Effects of temperature
Temperature has a strong effect on megohm readings because insulation resistance varies
inversely with temperature on an exponential basis (IEEE 43 has a very good description of
this effect).
Insulation resistance drops in half for every 10 °C (18 °F) rise in temperature. Therefore,
before making any judgments regarding the health of a motor’s insulation based on a trend
of past megohm measurements, all measurements used in the trend should be
compensated or corrected for temperature. The temperature compensation of the insulation
resistance means the user must convert all the IR measurements used in the analysis to the
same temperature. The recommended temperature to use is 40 °C (104 °F). Use the
following formula to make the calculation:
For example, if an insulation resistance/megohm value is 5,000 megohms at 30 °C (85 °F),
the compensated IR value at 40 °C (104 °F) is 2,500 megohms.
Some insulating materials developed in recent years for wire insulation do not readily
polarize. For example, the newer inverter grade wire insulation does not significantly
polarize. As recommended in IEEE 43, if the one minute insulation resistance is greater than
5,000 megohms, the PI measurement may not be meaningful. In these situations, the
leakage current is often very low – almost zero. Such low leakage currents are difficult to
accurately measure and, as a result, instrument errors become very evident. However, you
must use judgment before declaring the PI test to be meaningless. The indication of
damaged insulation based on the PI test can be a very low leakage current and a low PI
value.
Dielectric absorption (DA) test
We often substitute the dielectric absorption (DA) test for the PI test for the following
reasons:
• Some insulation systems do not polarize, or polarize so fast the process is not
observed
• Some motors are so small that a PI test will offer no useful information
• Some motors have such a small total current leakage that it is not possible to resolve
the polarization current
• Sometimes users do not have or do not want to take the time to perform the full
requisite 10-minute PI test
The DA test is basically a shortened version of the PI test. Instead of forming the ratio of
insulation resistances at 10 minutes and one minute, the DA test is the IR ratio at three
minutes and 30 seconds:
There are no accepted minimum or maximum values of the DA test, and the DA value often
appears to be subject to trends.
122 SKF Static Motor Analyzer—Baker AWA-IV User Manual
Motor testing theory and reference