Insulation resistance of armature windings

Insulation resistance of the windings of an electric machine may be used as a barometer for indicating the condition of the windings. On the basis of tests on many machines of different types it is recommended that all insulation resistance readings be taken after the application of 500 volts (d-c) for one minute. The slope of the resistance-time curve at that point gives a combined indication of the moisture content and the quality of the insulation. In this paper are given formulas for the insulation resistance of different types of windings derived from the winding insulation dimensions and expressed in terms of the rated voltage, capacity, and speed of the machine. By the use of nomographs, insulation resistances can be determined in about 15 second´s time; 5 of these prepared for different types of machines are included in the paper.