The Permanent Magnet Generator

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Figure 11 shows the cross-section of a typical Permanent Magnet Generator (PMG). The PMG differs from the Induction Generator in that the magnetisation is provided by a Permanent Magnet Pole System on the rotor, instead of taking excitation current from the armature winding terminals, as it is the case with the Induction Generator. This means that the mode of operation is synchronous, as opposed to asynchronous. That is to say, in the PMG, the output frequency bears a fixed relationship to the shaft speed, whereas in the mains connected IG, the frequency is closely related to the network frequency, being related by the slip. These differences will be discussed at length. However, it must be recognised at the outset that the differences have a significant effect on the operating characteristics and performance of the two generator types.

Permanent Magnet Machines

CONDUCTOR _ REGION

Figure 11. Cross-section of typical conventional Permanent Magnet Generator.

CONDUCTOR _ REGION

Figure 11. Cross-section of typical conventional Permanent Magnet Generator.

Permanent magnet machines may be set in several categories, those with surface mounted magnets, those with buried magnets, those with damper windings, etc., etc. All categories where data was found were considered, as each has some special features to offer. The search term 'Permanent Magnet Generator' produced an immediate list of some 50 articles, and standards dating from 1990 and later. In view of the small number of articles found, this was not reduced in any way, see Appendix A.2. Some 21 representative articles have actually been studied for this report. Other articles and references found have been included if they came to the notice of the author. This means that some significant articles may have been omitted from consideration. References [8, 9] in Appendix A.2 were ignored, as it concerned a high-speed, 400Hz generator, and cyclo converter. This combination was found interesting, but irrelevant to the case under consideration.

The PMG has been proposed as wind turbine generator in several research articles. Advantages include self-excitation, which allows operation at high power factor and efficiency. A disadvantage may be the synchronous operation, which causes a very stiff performance in the case of external short circuits, and when the wind speed is unsteady. This may lead to instabilities. Other applications are

Figure 12. Voltage regulation of 12 pole, outer stator PMG with surface mounted magnets (Mitcham & Grum, 1998).

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Figure 12. Voltage regulation of 12 pole, outer stator PMG with surface mounted magnets (Mitcham & Grum, 1998).

also considered in the literature, e.g. animal driven generators (Van Niekerk & Hancke, 1999). An overall description of the function of the PMG was not found in the literature search.

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Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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