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PDD technology is prepared for wind turbine trials

13 November 2015

Magnomatics, a specialist working in the field of magnetic gear technology, is currently building proof of concept generators to demonstrate the application of its novel Pseudo Direct Drive (PDD) technology to wind turbines.

Analogy between mechanical and magnetic gear system

A magnetically-geared generator or Pseudo Direct Drive (PDD) offers a significant advance over conventional permanent magnet (PM) motor/generator technology, achieving a step-change in continuous torque density of up to eight times that of equivalently cooled PM machines while maintaining very high efficiency. This dramatic increase in torque density enables the motor to drive loads directly – particularly those loads involving a conventional motor and gearbox combination.

Suitable for applications involving high continuous torque at relatively low speed, PDD technology applications range from aerospace actuation, marine propulsion drives through to direct drive utility scale wind turbines and rail traction systems.

Magnomatics says its compact, lightweight PDD generator can be scaled to 10MW and beyond to deliver class-leading annual energy output and cost of energy. It offers a number of advantages over more conventional systems, including optimised servicing, reduced system complexity and reduced rotor stresses thanks to its passive ‘torque-fuse’ capability. Its compactness and light weight ensures flexibility in drive train architecture and enhanced transport options.

Principle of operation
A magnetic gear is a magnetic equivalent of a mechanical planetary system as shown in the illustration.  The torque is developed magnetically and there is no mechanical contact between the shafts. This removes the need for gear lubrication, filtration and the associated costs of maintenance.

By adding a stator around the gear a PDD generator is created that is a magnetic and mechanical integration of the gear and a permanent magnet generator. The single stage gear gives an uplift in drive train performance through increased generator input speed, reduced input torque and increased efficiency, whilst allowing the size and mass of the generator to be minimised.

Further benefits include inherent gear compliance that attenuates drive train oscillations and reduces acoustic noise, and the aforementioned ‘torque fuse’ which removes damaging torque overload conditions that can occur during low voltage ride-through events or grid faults. Significantly, no lubrication is required (except for the bearings) and the service interval can be extended accordingly.

Generator testing
A 300kW (20kNm) PDD has been tested on a dynamometer to give a class-leading power curve. This generator replicated the magnetic and mechanical stresses of a multi-megawatt generator.  This generator proved the potential to scale for use within a 220m diameter, 12.5MW turbine and is one of the innovations being investigated at 10 to 20 MW-scale as part of the EU-funded Innwind.EU project.

The measured data has confirmed the modelling techniques used to design and evaluate multi-megawatt systems.  The extremely high part-load efficiency of the generator required only convective air-cooling systems that increase reliability and give a low maintenance drive train.

One key aspect of the Cost of Energy (CoE) calculation for a wind turbine is the proportion of time the turbine is available to generate electrical power. Faults and downtime requiring service renders the output zero and this has a direct and proportional effect on the CoE when the initial capital investment is considered.

Historical data has been used to provide the input for the PDD generator and a particularly conservative approach adopted, where the combined ‘unreliability’ of a conventional generator and gearbox was summed to provide the expected unreliability of the PDD system.

Despite these very conservative assumptions regarding availability, the PDD has the same CoE as a class leading Doubly Fed Induction Generator (DFIG) system, which is beyond that of all other drive trains.

A PDD generator is particularly appropriate to utility-scale wind turbines.  The CoE should be as good as the leading DFIG technology (even with worst case reliability assumptions) and it offers significant improvements over other topologies, including 1 and 2 stage and direct drive permanent magnet generators.

The structure allows easy maintenance of the generator while initial studies indicate that it is scalable up to the current highest power levels envisaged of 10MW and beyond.  Magnomatics plans to build a 500kW demonstrator for evaluation in 2017.

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