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This study deals with direct torque control (DTC) of induction motor drives when a multilevel inverter is involved. The proposed control technique is finalised to reduce the amplitude of torque oscillations and to improve the motor dynamic response regardless of operating speed conditions while assuring a satisfactory input current distortion. This is achieved by using a special look-up-table built with a novel approach. The control itself is based on the application in each sampling interval of one or two voltage vectors suitably selected, in order to ensure low electromagnetic torque ripple in steady-state operations and fast dynamic behaviour in transient operations. In the algorithm, the converter voltage vectors selection is carried out by using a novel look-up table whose entry points are computed by means of a specialised relation, starting from the outputs of multilevel hysteresis controllers. The suggested method, suitable for a generic multilevel inverter, is detailed for a neutral point clamped three-level inverter. The proposed algorithm is validated by several numeric a lr e s u l t s ,a n dc o m p a r e dw i t haD T Ca l gorithm based on a traditional threespeed range look-up tables. Finally, some experimental results, obtained with a scale laboratory setup, are shown, discussed and compared with the numerical ones.

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Electric propulsion represents the new research frontier in the future ship technology. There are different hybrid topologies requiring sizing criteria of components and related control strategies, finalized to maximize different objective functions: weight and space reduction on-board, fuel economy, pollution reduction or optimized efficiencies. In the paper sizing criteria of on-board sources and energy storage systems are proposed for a hybrid series architecture for a typical "passenger boat". Two different design criteria are proposed for a series architecture: a criterion aims to ensure that the engine-generator group runs at fixed speed and power, whereas the other consists to run the engine at variable power and speed. The procedure for an optimal design of an on-board sources is set up and validated by numerical applications on a 23 m long passenger boat having a displacement of 37 tons.