Design and analysis of a semi-embedded six-phase offshore wave energy generator

The continuous depletion of fossil fuels has intensified the demand for renewable energy, among which wave energy is regarded as a promising option for offshore power generation due to its high energy density and wide availability. However, conventional three-phase permanent magnet synchronous machines suffer from limited fault tolerance, operational reliability, and efficiency, making them less suitable for the low-speed and highly fluctuating operating conditions of wave energy conversion systems. To address these challenges, this paper proposes a novel six-phase semi-embedded permanent magnet synchronous machine (SSPMSM) for direct-drive wave energy generation. The semi-embedded rotor structure improves mechanical robustness and enhances permanent magnet utilization by introducing a non-uniform air gap, thereby increasing electromagnetic torque capability and reducing torque ripple. Finite element analysis is conducted to systematically evaluate the electromagnetic performance of the proposed machine operating in generator mode, with comparisons made against a conventional non-embedded three-phase permanent magnet synchronous machine (NTPMSM) of identical dimensions. Simulation results indicate that, under rated low-speed operating conditions, the SSPMSM achieves higher efficiency and significantly lower copper loss than the NTPMSM. The cogging torque of the SSPMSM is reduced to only 29.6% of that of the NTPMSM, resulting in improved operational smoothness. Moreover, fault-tolerant analysis demonstrates that the proposed machine can maintain continuous operation under a single-phase fault, highlighting its superior robustness and reliability. These results demonstrate that the proposed SSPMSM offers clear advantages in electromagnetic performance, torque stability, and fault tolerance, making it a promising generator candidate for direct-drive wave energy conversion systems.