The Utilization of Permanent Magnet Generators in Wind Turbines
Permanent Magnet Generators (PMG) sometimes also known as Permanent Magnet Synchronous Generators are believed to be the generators of choice in variable speed wind turbines because they are light weight and have high power density. The design and the assembly of the generator is relatively straightforward when compared to other type of generators. PMGs use permanent magnets in place of field windings and therefore are the most dependable wind power generators because they are independent from the grid and can operate very well even at the lowest of wind speeds.
Permanent magnet generator is a type of synchronous generator in which the excitation coil inside the rotor, is replaced by a system made up of permanent magnets which delivers a constant excitation field. The operation of the Permanent Magnet Generator (PMG) differs greatly from other synchronous generators. In a normal generator, voltage is controlled by means of excitation. In a Permanent Magnet Generator (PMG), the excitation is constant which is why, when the generator is charged, the voltage drops without the option to regulate. It is used in cases where it does not matter if the voltage drops a certain degree or when electronics are applied to the output of the generator. The advantages of Permanent Magnet Generator (PMG) include the elimination of commutator, slip rings and brushes so that the machines are rugged, reliable and simple. In wind turbine applications, as the actual wind speed is variable so they the permanent magnet generators (PMG) should operate at full variable speed. For which, they should be connected to the power grid through AC-DC-AC conversion by power converters. The generated AC power (with variable frequency and magnitude) is first rectified into fixed DC and then converted back into AC power (with fixed frequency and magnitude).
The most commonly used material for permanent magnets is neodymium-boron-iron NdFeB or samarium-cobalt. These materials provide a magnetic flux density in the air gap comparable to that produced with field windings, using a radial depth of magnet of less than 10 millimeters. Other magnet materials such as ferrite can be used, but with a considerable reduction in air-gap flux density and a corresponding increase in generator dimensions. The NdFeB magnets previously had a limited temperature stability which was the major obstacle in using the material in high power applications. The development of NdFeB grades with improved temperature and corrosion resistance in the past ten years has enabled the use of efficient permanent magnet machines even in relatively large applications, such as wind turbine generators.
In practice, there are three permanent magnet wind generator alternatives, direct-drive (DD) generators, medium-speed (MS) generators and high-speed (HS) generators. The use direct-drive (DD) generators, can eliminate troublesome gearboxes which cause the majority of wind turbine failures. Permanent Magnet Generators (PMG) can be classified into Radial Flux Permanent Magnet (RFPM) and Axial Flux Permanent Magnet (AFPM) generators. The AFPM generator has higher power density and can operate more efficiently at low speeds as compared to its radial counterpart. Moreover, for same power ratings, AFPM has smaller volume and lower mass as compared to RFPM. All the features of AFPM generator listed above make it the generator of choice for wind power generation system.
Significant advancements have been made in the field of wind power generators since the inception of utilization of wind for the production of electrical energy. For efficient power generation there must be a simple, light weight, variable speed generator for high energy yield. Permanent Magnet Generators (PMG) are probably the best choice for wind turbine applications as they offer a lot of flexibility in design because they do not require reactive magnetizing current and are connected to the grid through a full-scale power electronic converter and therefore are decoupled from the grid.
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