Implementation of current derivative sensors for the sensorless control of AC drives

Abstract

The work carried out in this thesis focused on the implementation of current derivative sensors used in transient injection methods to extract a relevant position signal from asynchronous machines. During transient injection a voltage pulse, typically in the range of 10-30μs is applied followed by its inverse, during the zero vectors of the fundamental PWM pulse train. During such pulse application the current derivative measured is a function of the position dependent leakage inductances in machine. Through the extraction of the current derivative signals during pulse injection, the position of either the rotor or flux within the machine can be estimated. The first option for derivative measurement is the evaluation of the discrete differential quotient between two current samples, this introduces a systematic error and is very much a function of the sampling time used; high sampling rates are required which increase the processing power required in the drive. The preferred option would therefore be the direct current derivative measurement achieved by designing a sensor whose output is proportional to the derivative of the current passing through it. The main aim of this study was to implement such sensors, varying from those already proposed in literature to new ones and investigate their performance under transient injection conditions. These were then used to track the position of the resultant rotating flux vector within the machine. Results shown offer a comparative basis for the future implementation of such sensors into motor drives.