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The propellers have been used to generate the propulsion of ships with a long history. Many studies have been conducted to improve propulsion efficiency and to reduce vibration by optimizing the shape of propellers. Due to the shape of the stern of the hull, the inflow velocity entering the propeller plane has non-uniform characteristics, which is called a nominal wake. The propeller rotates in the wake, which generates vibration and noise. In terms of the vibration, the fundamental frequency corresponding to the rotation frequency of the propeller is evoked. Moreover, it includes harmonic frequencies caused by the number of propeller blades, which leads to torque ripple and thrust fluctuation. Conventional vessels used by internal combustion engine rotate the propeller directly. It can control only the fundamental torque of the propeller and is difficult to control torque ripple generated by harmonic frequency due to the shape of the propeller. On the other hand, recently, the development of electric propulsion ships using electric motors rather than internal combustion engines is increasing rapidly. The key feature of electric vessels is that the electric motor rotates the propeller, and it has the advantage of having a high speed controllable of the propeller torque. A Permanent Magnet Synchronous Motors (PMSM) and Induction Motors (IM) are used as propulsion motors of typical electric propulsion ships and PMSM is commonly used due to its high power density, large torque to inertia ratio, and high efficiency. A PMSM uses a current controller based on Field Oriented Control (FOC), which controls the stator winding current for speed and torque control of the propeller. A torque ripple is generated due to harmonic frequencies contained in the back electric magnetic force (EMF) by permanent magnets. To reduce the torque ripple, a harmonic injection method has been conducted. In this paper, the harmonic injection method of the motor is proposed to reduce harmonic frequencies due to the nominal wake and the torque ripple of a propeller. This method is applied by the numerical algorithm and CFD (Computational Fluid Dynamics) computation, dynamic simulation.
References
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| Keywords | Propeller vibration, Harmonic injection, FFT, Torque fluctuation, PMSM, Electric ship, CFD |
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