Method and apparatus for multi-function electronic motor protection

Abstract

A multi-function protection system (8) for protecting electrical equipment such as a screw compressor motor is shown having a housing (10, 20) mounting current sensing toroids (CT1, CT2, CT3) and associated circuitry as well as associated circuitry for temperature sensing PTC resistors (S1, S2, S3) adapted to be placed in heat conductive relation with respective windings of the motor. The circuitry is connected to an analog-to-digital converter or ADC (U6) which provides a binary coded input to a micro-controller (U5). The micro-controller performs protection sub-routines to control an output switch (Q4) which in turn controls the state of energization of a relay (K1) coupled to the screw compressor motor. The circuitry includes a current unbalance section (30a, 30b), an over temperature section (32), a low voltage cut-out circuit (33), a current transducer section (35), a signal conditioning circuit (36a, 36b, 36c), a micro-controller section (40) incorporating a shorted sensor protection system, current overload calibration section (41) and fault diagnostic section (42). A fault bank routine filters transient voltages resulting from industrial or environmental electromagnetic interference (EMI) for current overload, current imbalance and over temperature transient voltages to distinguish real fault conditions from EMI transients.