Abstract
In the field of rolling mill large motors, we have usually adopted DC machines, since they have both high responsibility and over-load endurance.
Recently AC machines become more popular, since maintaining and controlling AC machines becomes easier in progress of power electronics and control theories. However, many old DC machines are still in operation at steel works, so we have to make precise diagnosis of isolation deterioration.
In this report we have theoretically studied how isolation resistance is affected by humidity, and correlation between isolation resistance and humidity has been made clear by our experiment.
Analysis of the maintenance data on DC machines that have been observed for more than twenty years at Nagoya works, proves that isolation deterioration depends on humidity.
So we propose the chart “%Mg-m value” as the diagnosis method of isolation deterioration. We have tested the breakdown voltage of DC machines when they were renewed after twenty-nine years operation and observed that breakdown voltage is the same to the %Mg-m value. These results proved that this method is effective for estimation of isolation deterioration for large DC machines.
