When we perform insulation resistance tester,temperature winding effected the accuracy of Mega Ohms reading.The basic theory is when temperature increase,the insulation resistance decrease due to physical factor of material for winding.
To get a accurate reading for insulation resistance ( Mega Ohms ),we need to ensure the minimum temperature during measurement.
According to IEEE 43,the temperature correction should be to 40 °C and the correction factor for temperature is such that the minimum insulation resistance value is doubled for every 10 °C decrease in winding temperature.
Temperature winding factor
For example 1 unit induction motor running 24 hour and suddenly it failure.We need to perform insulation resistance measurement to check winding condition,but the temperature of winding around 120 °C.So we must let it cooling down until reach 60 °C to 40 °C before perform insulation resistance test.
Another method if we cannot wait the winding temperature cool until 40 °C,we can use the formula for temperature correction factor.I explain detail below complete with graph and example.
1) Graph Insulation resistance versus Temperature
2) Formula for winding temperature correction
R40 °C = Kt40 °C x Rt
R40 °C = insulation resistance corrected to 40 °C
Rt = measured insulation resistance in MΩ at t °C
Kt40 °C = correction factor according to the graph above
Example for calculation :-
We perform measurement for insulation resistance and get a value 3 MΩ and winding temperature is 70 °C.So the calculation is :
We obtain from the graph : Kt40 °C at 70 °C = 7
Kt40 °C = 7
Rt = 3 MΩ
R40 °C = 7 x 3 MΩ
R40 °C = 21 MΩ