Using temperature, voltage, and/or speed measurements to improve trending of induction motor RMS currents in process control and diagnostics

Trending RMS current in an induction motor is often performed in industrial processes because of the availability and easy measurement of the parameter. This current can deviate widely with temperature, voltage and speed, making it difficult to determine process changes that are to be detected. Current deviations due to normal operation of the system can be corrected by applying the motor model equations to adjust the current so that the trending variable is more robust to normal variations in motor driven processes yet still detects abnormal process occurrences. A full motor model (requiring all steady state circuit parameters) is applied to correct the measured current, independently for voltage, speed and temperature changes in the motor driven process showing much reduced trended variable variation for normal process condition changes. A simplified model is also presented. This model requires only nameplate motor data and no load current as equation parameters and is shown to also reduce trended variable variation. Both the full motor model and the simplified model trended values are shown to detect motor winding partial failure that would not be detected by trending current alone.