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Multilevel converters with series connection of semiconductors allow power electronics to reach medium voltages (1-10 kV) with relatively standard components. The increase of the number of semiconductors provides extra degrees of freedom, which can be used to improve different characteristics. This paper is focused on variable-speed drives and it is shown that with the proposed multilevel direct torque control strategy (DiCoIF) the tradeoff between the performances of the drive (harmonic distortions, torque dynamics, voltage step gradients, etc.) and the switching frequency of the semiconductors is improved. Then, a slightly modified strategy reducing common-mode voltage and bearing currents is presented.
Date of Publication: Oct 2002