Operating limits for drive condition monitoring using supply current signature analysis

A scheme has been proposed whereby the presence of mechanical faults in a variable speed motor drive is detected by monitoring the frequency content of the rectifier input current. This paper provides a detailed theoretical analysis of the proposed diagnostic method to determine the major influences on its resolution and accuracy. These include the influence of the resolution of the data acquisition systems for the controller i.e the analogue to digital controller used for current measurement, and the encoder used for speed measurement are investigated and also the bandwidths of the speed and current controllers, the rectifier and inverter circuit parameters and the motor load. Simulation results are used to demonstrate the limitations imposed by these drive characteristics and it is seen that the resolution of the speed encoder has the major influence on the diagnostic method.