Direct torque control of permanent magnet synchronous machines - analysis and implementation

Abstract

The direct torque control (DTC) has become an accepted vector control method beside
\nthe current vector control. The DTC was first applied to asynchronous machines, and
\nhas later been applied also to synchronous machines. This thesis analyses the application
\nof the DTC to permanent magnet synchronous machines (PMSM).
\nIn order to take the full advantage of the DTC, the PMSM has to be properly dimensioned.
\nTherefore the effect of the motor parameters is analysed taking the control principle
\ninto account. Based on the analysis, a parameter selection procedure is presented.
\nThe analysis and the selection procedure utilize nonlinear optimization methods.
\nThe key element of a direct torque controlled drive is the estimation of the stator flux
\nlinkage. Different estimation methods – a combination of current and voltage models
\nand improved integration methods – are analysed. The effect of an incorrect measured
\nrotor angle in the current model is analysed and an error detection and compensation
\nmethod is presented. The dynamic performance of an earlier presented sensorless flux
\nestimation method is made better by improving the dynamic performance of the lowpass
\nfilter used and by adapting the correction of the flux linkage to torque changes.
\nA method for the estimation of the initial angle of the rotor is presented. The method
\nis based on measuring the inductance of the machine in several directions and fitting the
\nmeasurements into a model. The model is nonlinear with respect to the rotor angle and
\ntherefore a nonlinear least squares optimization method is needed in the procedure.
\nA commonly used current vector control scheme is the minimum current control. In
\nthe DTC the stator flux linkage reference is usually kept constant. Achieving the minimum
\ncurrent requires the control of the reference. An on-line method to perform the
\nminimization of the current by controlling the stator flux linkage reference is presented.
\nAlso, the control of the reference above the base speed is considered.
\nA new estimation flux linkage is introduced for the estimation of the parameters of
\nthe machine model. In order to utilize the flux linkage estimates in off-line parameter
\nestimation, the integration methods are improved. An adaptive correction is used in
\nthe same way as in the estimation of the controller stator flux linkage. The presented
\nparameter estimation methods are then used in a self-commissioning scheme.
\nThe proposed methods are tested with a laboratory drive, which consists of a commercial
\ninverter hardware with a modified software and several prototype PMSMs.