Permanent Magnet Generator Testing for Low-Power Off-Grid Oscillating Water Column Wave Energy Converters

This work concerns the development of an off-grid oscillating-water-column wave energy converter (WEC) equipped with internal batteries designed for powering sensors and autonomous vehicles. Calculating the generator counter torque from electrical measurements is advantageous due to mechanical constraints with torque meters in sea-faring prototypes. Mapping generator performance concerning load and speed enables advanced wave-to-wire modeling for controller and battery charger design. Experimental testing involved the WEC's permanent magnet generator, diode rectifier, and capacitive filter across combinations of rotational speeds and resistive loads, yielding the generator-rectifier electromechanical input-output relationship. This relationship yielded a steady-state efficiency with differences below 1% to measured values for representative loads, and it reproduces time series of the modeled variables in speed-varying conditions with good accuracy. Some drawbacks of passive rectification, compared to active, were assessed. The power factor ranged from 0.92 to 0.95, and generator-rectifier efficiency ranged from 0.83 to 0.93 over the range of 20 to 100% of the generator's continuous duty torque. Additionally, the maximum observed direct-axis current reached 8% of the demagnetization limit. Finally, the highest observed generator-rectifier efficiency of 91% occurred at the maximum tested speed of 3000 r/min.