Motor Insulation Resistance Test Results

Record initial and corrected values on motor testing record sheet.

Motor insulation resistance test results.

A common rule of thumb is insulation resistance changes by a factor of two for each 10 degrc change. Insulation resistance should be approximately one megohm for each 1 000 volts of operating voltage with a minimum value of one megohm. Minimum insulation resistance of the winding to ground is measured with 500 v dc. This test recognizes the fact that good insulation will show a gradually increasing ir after the test voltage is applied.

Determine rules for the actual minimum insulation resistance value of an electric motor because resistance varies according to method of construction condition of insulation material used rated voltage size and type. After the connections are made the test voltage is applied and the ir is read at two different times. Winding dc resistance test follow usace procedure three sets of results total a to b phase b to c phase c to a phase correct values to 40 c using the formula in step 8. For example suppose you measured 100 megohms with an insulation temperature of 30 degrc.

Maximum insulation resistance should be measured with 500 v dc with the windings at a operating temperature of 80 120 c depending on the motor type and efficiency. Usually either 30 and 60 sec or 60 sec and 10 min. For example a motor rated at 2 400 volts should have a minimum insulation resistance of 2 4 megohms. All values obtained from winding dc resistance test should be below 1 megohm and similar in value.

So to compare new readings with previous ones you ll have to correct your readings to some base temperature.