Distinguishing Motor Starts from Short Circuits Through Phase-Angle Measurements

The Pittsburgh Research Center (PRC)4 investigated how the starting of induction motors may cause nuisance tripping of short-circuit protection on coal mine power systems. This research had a threefold purpose: (1) to determine the range of typical values for power system characteristics that affect short circuits and motor starts on high-voltage longwalls, (2) to identify how motor-start waveforms differ from those for short circuits, and (3) to devise a method to provide short-circuit protection without intentional time delays to account for motor starts. Distribution voltage, transformer impedance, power center location, and motor size were found to critically influence the magnitude of short-circuit and motor-start currents on high-voltage longwalls. An attribute of motor-start signatures that distinguished them from short circuits was the relatively large phase angle between voltage and current. Electronic circuitry was designed to detect phase angles and react to momentarily disable circuit breaker action for motor starts. A prototype was successfully evaluated with an induction motor in the laboratory. By minimizing intentional time delays in short-circuit protection, this technology will help ensure that surface temperatures of energized electrical apparatus will not exceed gas or dust ignition thresholds when such faults occur.