Abstract
Point absorber wave energy converters face critical survivability and efficiency challenges due to limited stroke constraints and destructive end-stop collisions in extreme seas. This study presents a 1 kW-scale winch-driven power take-off system with a 10 m active stroke, designed to geometrically decouple the internal drivetrain from constrained motion rails. To evaluate the drivetrain without hydrodynamic uncertainty, land-based crane trials imposed repeatable 10 m tether displacements, while the prescribed 1 m/s line velocity was selected to be broadly consistent with a 1:6 Froude-scaled representation of full-scale extreme vertical motion. The controlled actuation isolated the internal energy cascade and enabled stage-wise separation of speed-dependent mechanical losses from load-dependent electromagnetic losses across resistive loads of 4.5 to 45 Ω. Experiments identified an optimal internal conversion efficiency near 18 Ω, beyond which high-current effects degraded performance. Diagnostic analysis further showed that constant-parameter generator predictions failed when stator current exceeded the 5.16 A rated limit, causing over-predictions of current, torque, and electrical power due to armature reaction and magnetic saturation. To capture this behavior, a multi-domain physics-based model combined a no-load-calibrated viscous rotational damper with a current-dependent back electromotive force coefficient roll-off profile. This correction eliminated high-current prediction bias and restored agreement across the mechanical-to-electrical energy cascade. These findings demonstrate that single-point static tuning is insufficient under transient overloads. The validated model supports future upscaling and parameter-scheduled load control to maintain optimal operation, mitigate saturation, and maximize wave-to-wire energy extraction in irregular seas.