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

摘要

The direct torque control (DTC) has become an accepted vector control method besidethe current vector control. The DTC was first applied to asynchronous machines, andhas later been applied also to synchronous machines. This thesis analyses the applicationof the DTC to permanent magnet synchronous machines (PMSM).In order to take the full advantage of the DTC, the PMSM has to be properly dimensioned.Therefore the effect of the motor parameters is analysed taking the control principleinto account. Based on the analysis, a parameter selection procedure is presented.The analysis and the selection procedure utilize nonlinear optimization methods.The key element of a direct torque controlled drive is the estimation of the stator fluxlinkage. Different estimation methods – a combination of current and voltage modelsand improved integration methods – are analysed. The effect of an incorrect measuredrotor angle in the current model is analysed and an error detection and compensationmethod is presented. The dynamic performance of an earlier presented sensorless fluxestimation method is made better by improving the dynamic performance of the lowpassfilter used and by adapting the correction of the flux linkage to torque changes.A method for the estimation of the initial angle of the rotor is presented. The methodis based on measuring the inductance of the machine in several directions and fitting themeasurements into a model. The model is nonlinear with respect to the rotor angle andtherefore a nonlinear least squares optimization method is needed in the procedure.A commonly used current vector control scheme is the minimum current control. Inthe DTC the stator flux linkage reference is usually kept constant. Achieving the minimumcurrent requires the control of the reference. An on-line method to perform theminimization of the current by controlling the stator flux linkage reference is presented.Also, the control of the reference above the base speed is considered.A new estimation flux linkage is introduced for the estimation of the parameters ofthe machine model. In order to utilize the flux linkage estimates in off-line parameterestimation, the integration methods are improved. An adaptive correction is used inthe same way as in the estimation of the controller stator flux linkage. The presentedparameter estimation methods are then used in a self-commissioning scheme.The proposed methods are tested with a laboratory drive, which consists of a commercialinverter hardware with a modified software and several prototype PMSMs.