Continuous condition monitoring of the electrical insulation of AC motors without disconnection

A new method of monitoring the condition of the electrical insulation of the stator windings of AC motors and increasing the sensitivity of current protection against phase-to-ground faults are proposed. The method is based on the introduction of an additional magnetomotive force (m.m.f.) in a magnetic core of zero sequence current transformers. Frequency of this m.m.f. is different from the network power-frequency. The working algorithm of the device of control and protection of the feeders with the AC motors and the results of its tests in industrial environment are given.