The PMG is proposed as a wind turbine generator due to its property of self excitation. The stator winding is connected to the load, and is insulated.
They also call for special care when assembling, to avoid damage and accidents to personnel. Maintenance is generally restricted to bearing lubrication only.
A major problem is the necessity of maintaining the rotor temperature below the maximum operating temperature of the magnet, which may be limited by the Curie point of the magnetic material, and also by the thermal properties of the binding material in the case of powder metallurgy composites.
Large machines are reported (17 MW motors), but most research papers describing generators consider machines with ratings less than 10 kW. The synchronous nature of the operation causes problems of start, synchronisation, and voltage regulation.
Methods of control include full power frequency inverter, and cyclo-converter. Methods of improving voltage regulation include the application of full voltage, large capacitance capacitors.
Methods of Analysis of PMG
Models are reported, which are adjustable to allow for internal and surface mounted magnets, and for the current dependence of flux and inductance (Ojo & Cox, 1996).
The model takes no account of current waveform and gives inaccurate results. (Lampola, 2000) reports extensive Finite Element modelling of surface mounted, NdFeB, cylindrical air-gap PMG.
This may be supplemented by a linked circuit simulator. In (Polinder & Hoeijmakers, 1997) an analytic method is proposed, to determine the magnetic field distribution in a PMG. This is to ease the numerical problem of optimisation, as opposed to the Finite Element Method.
It is also claimed that the method provides insight into the relationship between dimensions and equivalent circuit parameters.
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