Dynamic modeling of the trans-rotary magnetic gear for the point-absorbing wave energy conversion systems

This paper studies dynamics of a point-absorbing Wave Energy Conversion System (WECS) that employs a Trans-Rotary Magnetic Gear (TROMAG) within its Power Take-Off (PTO) system. The TROMAG is a magnetic device, consisting of a rotor and a translator, intended to convert highforce, low-speed translation to high-speed, low-torque rotation. Translator of the TROMAG is coupled to the buoy and heaves along with it while its rotor is coupled to the rotor of a rotary machine. The adopted control strategy for the buoy determines the force by which the translator has to react to the wave excitation force. Whereas in the case of a Direct Drive Linear Generator (DDLG) the desired force can be directly generated by controlling the generator current, in the case of TROMAG the generator torque would go through certain dynamics to appear in the form of the translator force. Understanding the aforementioned dynamics is the purpose of this paper. A dynamic model, verified by three-dimensional (3D) finite element analysis (FEA), is presented for the TROMAG. The proposed nonlinear analytical model is then linearized and combined with the linearized model of a point absorbing WECS. Transfer functions are derived and the variations of the system characteristics with different parameters are studied.