Single channel apparatus for on-line monitoring of three-phase ac motor

Abstract

An on-line monitor for electrical faults in the stator of an ac motor has a single di/dt current transformer or three separate di/dt current transformers coupled to the three phase conductors of the ac supply. The windings of the transformer, or transformers, are selected to generate a single sensed signal containing a linear combination of components of current in the three phase conductors weighted relatively so that algebraic differences between components from any two phase conductors is non-zero, such as preferably 2A+B-C or 3A+2B+C. The single sensed current signal is bandpass filtered to generate a pulse signal which is applied to an analyzer. The analyzer, preferably implemented by a microcontroller, generates an output based upon a combination of the amplitude and frequency of occurrence of the current pulse signal including a time attenuated accumulation of the pulses. A similar pulse signal is generated from a single sensed voltage signal and compared to the pulse signal generated from the current sensor to verify that the detected events originate in the motor and not from upstream in the ac supply. The monitor is coupled to the phase conductors between the local disconnect switch and the motor, and preferably, is mounted in the disconnect housing.