Electrical machines may exhibit various types of imbalances and undesirable harmonic distortions. These may increase the torque and flux ripples, acoustic noise, unbalanced three-phase currents, while also reducing efficiency. These types of imbalances and undesirable harmonic distortions cannot be controlled by using the conventional indirect torque control (ITC) and direct torque control (DTC) strategies. For some high-performance motion control, such as precision machine tools, robotics, and servo drives, low torque ripples are, however, obligatory. Nowadays, more studies have been conducted on the ITC strategy to control undesired current harmonics, such as double synchronic reference frames (DSRF), resonant controller, second order generalized integration, and reference current generation. Such strategies, however, can rarely be applied to DTC strategy.
In this research, the influence of asymmetric winding impedances, unbalanced back-EMF, and inverter nonlinearity in three-phase surface-mounted PMSMs has been systematically investigated by employing space vector modulations (SVM) based ITC and DTC strategies. This thesis firstly presents a modified ITC strategy by extracting the positive and negative sequence components in the stationary abc frame, and then a coordination transformation is used to control the machine in DSRF. This strategy provides faster dynamic response when compared with the conventional DSRF strategy, since the filters and the decoupling network are not required.
Due to the lack of research regarding the DTC strategy under unbalanced conditions, this research investigates and proposes modified cascaded and parallel DTC-SVM strategies. The conventional cascaded DTC strategy is investigated under balanced and unbalanced conditions. Then, a modified control strategy is introduced by adding two compensators (the conventional PI-controller with a resonant controller, and the use of the negative- and positive-sequence voltage vectors) to suppress the 2nd harmonic components in the torque and stator flux linkage.
Furthermore, for parallel DTC-SVM, the compensation of the 2nd and 6th harmonic components is investigated by means of either a resonant controller or an adaptive filter. In addition to the simplicity of the proposed strategies, these may also be able to significantly reduce the torque and flux ripples, while maintaining the merit of the fast dynamic response of the conventional DTC strategy even under variable fundamental frequency. Moreover, it has been proven that the compensation from using a resonant controller or an adaptive filter is parameter independent. Thus, regardless of unbalanced conditions, an effective torque ripple minimisation can still be achieved by properly selecting the dominant harmonic compensation.
