Job number one for most turbine manufacturers is to design the highest efficiency drive train that they possibly can. Now, after years of experience with many different industrial drive applications, one company, “The Switch,” has come up with an innovative drive arrangement called “The Switch Drive,” which is based upon a permanent magnet generator (PMG) and a special power converter. Even when the wind speed varies over a wide range, the turbine generates sufficient output to remain connected to the grid.
An optimized permanent magnet generator (PMG) does not need an energy source to feed the rotor; the magnets handle the excitation. Moreover, PMG efficiencies remain extremely high –close to nominal value – over a wide range of speeds so they can generate more output power with less input.
In the wind power industry, PMGs have sizeable advantages over conventional machines. For instance, they can be designed to achieve 98% efficiency. When used in low-speed, direct-drive applications, generator efficiency typically drops off, but a PMG does not use a gearbox that would otherwise drop the efficiency even more. Perhaps its biggest advantage is how it generates power under partial load – that is, when the wind is inconsistent. PMG efficiencies remain very high – close to nominal value – over a wide range of speeds, thereby generating much more energy.
In the belly of the PMG is a simple form of synchronous generator that does not need rotor excitation. Depending on the application, permanent magnets are placed on the rotor, for low-or medium-speed generators, or embedded in the rotor for high-speed generators. The permanent magnets create the excitation, which significantly contributes to greater efficiency, because this concept virtually eliminates the major source of rotor losses.
By driving the generator with an optimal power factor using the PMG arangement, stator losses are also minimized. And because PMGs do not require any separate excitation systems, it costs less, simplifies the system, and increases system efficiency. In addition, because permanent magnets are used, no slip rings are needed, which also significantly reduces maintenance costs.
By accommodating a wide range of speeds, a drive train based on the PMG configuration can run at the turbine’s optimized operating point. Control depends upon the optimum turbine curve and is not limited by the drive train, so it has better partial-load ratings. A standard machine, on the other hand, has a limited speed range where the turbine cannot fully adapt to actual site conditions.
PMGs meet the needs of modern wind farm turbines with power output in the range of 500 kW to 5 MW. The Switch recently produced and delivered a range of direct-drive, medium speed, and high-speed PMGs for OEM customers in Europe, China, and the US.
::DW::
Filed Under: Green engineering, Motion control • motor controls, Slip rings + rotary unions
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