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Abstract
This thesis focuses mainly on improving the performance of direct torque control (DTC) for matrix converter driven interior permanent magnet synchronous machine (IPMSM) drive. Although DTC does not require continuous rotor position information, the rotor speed is required for speed feedback of the speed controller. If the rotor speed can be estimated from the measured variables for DTC, it not only eliminates the cost of the speed sensor, but also increases the reliability of the drive. Therefore, speed sensorless control and extraction of accurate rotor position information for reliable sensorless operation of a direct torque controlled IPM synchronous motor fed by a matrix converter (MC) over a wide speed range, including zero speed, are major concerns in the thesis.
To improve the performance of the matrix converter, a hybrid current commutation strategy was developed, which features a shorter commutation delay and thus enhances control continuity. A modified modulation with reduced switching losses was presented by using two-stage indirect space vector modulation theory and was evaluated by means of THD analysis.
A modified hysteresis direct torque control (HDTC) for the matrix converter was proposed and implemented on the developed laboratory prototype of matrix converter, which allows great reduction of input current harmonics without any adverse effect on the output performance or increasing the complexity of the system, in comparison with the classical DTC scheme. The good performance of the proposed DTC using an adaptive observer for speed and flux estimations has been verified by extensive experiments.
The associated problems with HDTC have also been investigated in this thesis. A direct torque and flux control (DTFC) scheme based on ISVM for MC-fed IPMSM drive was proposed to solve the problems, which features low torque and flux ripples and sinusoidal input/output currents while maintaining a constant switching frequency. The unity input power factor and fast dynamics were achieved with the input power factor correction and the overmodulation strategies respectively.
However, at extremely low speed, including zero speed, sensorless DTFC MC drives was still not achievable using the conventional adaptive flux observer. A modified adaptive flux observer has been presented in this thesis for the sensorless DTFC IPMSM MC drive. Inserting a speed correction term makes the adaptive observer robust to the stator resistance variation as well as the load disturbance. Combined with HF signal injection technique, the observer is capable of handling full-load at low speeds including standstill. Experimental verification has been carried out, confirming the improvement and effectiveness especially at very low speed and standstill with full load compared with the conventional observer. To achieve a wide speed sensorless operation, a changeover algorithm using a speed-dependent weighting function was implemented for smooth transitions from zero to the switchover speed at which the adaptive observer alone can accomplish reliable speed and flux estimations.
The abovementioned studies on the DTC technique combined with the advantages of matrix converters for IPM machine drives are applicable to many other types of machines. It is evident that the high-performance direct torque controlled matrix converter drive without any position or speed sensors and without any current controllers is an attractive and viable candidate for sensorless drives and potentially an alternative to VSI drives.