Abstract: An electric motor monitoring circuit apparatus for an electric motor has a three phase electric motor having an electric motor starter circuit. An insulation monitoring circuit is coupled to the electric motor and monitors insulation leakage of the electric motor and inhibits activation of the electric motor starter circuit when an insulation leakage exceeds a preselected threshold. A three phase monitor circuit is coupled to the electric motor for monitoring phase sequence in voltage amplitude for the electric motor.

Abstract: A system for automatically monitoring the insulation leakage resistance of an electric motor for a short period prior to operation thereof having timing means for delaying application of the motor control voltage to the motor and for applying a high voltage direct current to the windings of the motor during such period. A sense resistor is connected between the motor frame and the high voltage supply ground, and the leakage current therethrough is monitored by a comparator. When the leakage current exceeds a preselected threshold, an alarm is initiated and operation of the motor is inhibited.

Abstract: A low voltage power supply powers a high impedance, high voltage dc supply. An electronic switch serves to automatically connect the high voltage output to the windings of a motor when the motor is non-operating. A lead connects from the frame of the motor to a sense resistor connected to the high voltage system ground. A plurality of variable threshold comparators is connected across the sense resistor. Leakage of the motor windings insulation results in a current flow through the sense resistor producing a voltage drop proportional to the leakage. Each comparator is adjustable to produce an indication of a value of leakage resistance at which the comparator produces an output.