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Insulation Resistance Value Required For Operation
In general, it is difficult to give a minimum of permis¬sible insulation resistance value for starting since it varies with the type, rating and size of the motor. However, as a simple standard for daily maintenance, the following equation may be considered for the minimum value.
Rm = E + 1
Rm : recommended minimum insulation resistance at 40ºC of the machine winding, in MΩ
E : Motor rated voltage, in KV
When applying the Rm as given by above equation, the temperature correction of measured insulation resistance should be made by use of the following equation if the winding is not at a temperature of 40ºC.
Rc ; insulation resistance corrected to 40ºC, in MΩ
Rt . measured insulation resistance at a temp. t ,in MΩ
A¬0 : insulation resistance correction factor
(See Fig. 1-2 Temperature Correction Curve)
Fig.1.2-a Temperature correction curve