Abstract: There is disclosed a monitoring apparatus for a multiphase electrical system of very simple and reliable circuit design capable of monitoring such a system and producing fault signals to operate protective relays and for visual indication. A main component of the apparatus is a multiphase, full-wave bridge rectifier circuit having matched voltage dropping input resistors so that the output of the bridge is a low voltage fluctuating DC signal such that neither the semiconductor diode rectifiers of the full-wave bridge or other internal components of the monitoring apparatus are exposed to high power line voltages. Phase unbalance in the apparatus is detected by evaluating the ratio of the peak to trough voltage of the fluctuating DC signal produced by the full-wave bridge.

Abstract: Electrical and electronic circuitry offering protection to motors, compressors and the like, from conditions of low energizing voltage or momentary voltage interruptions resulting in excessive mechanical load (locked rotor). When the undesirable condition is present, an internal relay prevents the equipment from being energized - transient conditions being allowed for by means of suitable time delay circuitry. The protection device provides significant improvement in safety and economy by preventing equipment from burning up.