Modulation strategies of multilevel converters for low inductance machines in electric vehicle applications

This thesis presents a strategy for the control of an ironless axial flux permanent magnet motor for electric vehicle applications. The ironless axial flux permanent magnet motor offers a number of advantages over conventional permanent magnet machines, such as higher efficiency and higher power density. However, as a result of the ironless design, the machine has very low stator inductances which increases the difficulty of current ripple regulation. The multilevel cascaded H-bridge converter is proposed as the motor drive topology as it offers opportunities to achieve the most optimal harmonic performance. Conventional modulation schemes have been found to be not suitable for this application, requiring further research aimed at improving the harmonic performance of the converter. This thesis develops designs for new modulation strategies for the multilevel cascaded H-bridge converter with the objective of minimising carrier harmonic distortion. The harmonic performance of conventional modulation strategies are investigated, which offers insight into the current ripple performance. Two new modulation strategies are proposed which exploit an extra degree of freedom in the converter which has not been explored previously. The proposed scheme allows the harmonic performance to be further improved and provides extra flexibility in the harmonic optimisation objective. Lastly, simulation studies are presented, which confirms the performance of the proposed modulation strategies. The results show that the proposed modulation strategy is superior to that of conventional schemes. Experimental results are also provided, which verify the practical performance and implementation of the proposed schemes.