This study presents two improved modulation strategies for ultra-sparse matrix converter to solve the problem of high total harmonic distortion (THD) under the circumstance of low-voltage transfer ratio. When the load power is low, the first strategy reselects the medium and the lowest positive line-to-line voltages to synthesise a lower dc-link voltage. Thus, the same output performances with the increased voltage transfer ratio will be achieved, and the THD of output voltage is decreased effectively. The second strategy is realised by rearranging the sequences of active vectors in rectifier stage and by adding suitable zero space vectors in inverter stage to widen pulses. Thus, the number of narrow pulses is decreased and the THD of output voltage then will be further reduced. Under the condition of normal operation of the load, both proposed strategies can improve the input and load currents without increasing the switching losses. The improved strategies are proposed for the application of AC motor speed regulation, and can be easily implemented in software with no need of any additional hardware modifications. Experimental results are given to verify the feasibility and effectiveness of the proposed strategies.
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