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Generating Electricity From Wave Power Using Asynchronous Motors

01 July 2004





Ocean Power Delivery's 750kW 'Pelamis' wave energy converter - believed
to be the first commercial scale development in the sector - produced
electricity for the first time in April of this year, during sea trials
conducted in the North Sea off the Firth of Forth. The 120m long
converter is a semi-submersible, articulated structure consisting of
cylindrical steel sections linked by hinged joints. Moored at its nose,
Pelamis is free to swing about this point into the dominant wave
direction. Waves travel down its length, causing each section to
articulate; hydraulic rams at the section joints try to oppose this
movement and subsequently pump high-pressure fluid back to a hydraulic
motor via an accumulator.

The eight 125kW generators for this project were supplied by Crewe-based
AEG Electric Motors, and are based on the company's standard AM315
asynchronous electric motor. The rotor laminations and windings were
adapted to boost generator efficiency and each unit was IP68 sealed to
meet the needs of the application. The generators can be run up to
synchronous speed as electric motors, thus avoiding the need for
synchronisation. Once near synchronous speed, the hydraulic motor applies
positive torque to push the unit into generation mode.

Pelamis has excellent load-shedding and power-limiting capabilities that
allow the machine to survive even the most extreme wave conditions. It is
also able to maximise power capture in small seas thanks to an adaptive
controller that 'tunes' the system by varying the stiffness of each joint.














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