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Extrait du livre blanc Génératrices entraînement direct hautes performances

Phase Motion Control technology for DD wind power Generals Wind power plants are characterized by mechanical energy being available at low speed with very significant torque. On the other hand, the physical size (and cost) of electric generators depends critically on the torque, not on the converted power. Consequently, the conversion of wind power into electric energy can be performed in two ways: 1. Traditional approach: in order to avoid the use of a large and massive generator, a geabox speed multiplier is interposed between the low speed turbine and a conventional high speed generator; 2. DD approach: a suitably large, but optimized, low speed generator is coupled directly to the low speed turbine. The gearbox solution, which requires just a composition of standard or semi standard technologies, was favored in the initial phase of wind power development. This solution, however, is saddled with important disadvantages: • Reliability: in the last 20 years, the average lifetime of wind rated gearboxes was found to be close to 11 years. A gearbox failure usually causes an out of service of over one year and 11 years is very close to the payback time of the typical wind farm investment • Availability: Due to internal friction and inertia, gearbox multiplied units cannot tarck wind gusts so invariable weather conditions, it was found that DD units produce substantially more electric power, all other conditions being equal • Maintenance and environment: large gearboxes require periodic maintenance and a significant volume of lubricant oil to be processed. On the advantage side, the entry barrier to this technology is lowest as it only needs semi standard parts to be assembled. The Direct Drive solution is intrinsically exempt from the above drawbacks, but it requires the use of a very large, dedicated electrical machine. The size and cost of the DD generator are the true limiting factors of this otherwise optimal and simplest solution. For this reason, a large technology development effort was deployed over the years to advance the design and the volumetric performance of slow DD generators by many specialist Companies. Phase Motion Control was in the forefront of the development of large, slow speed DD machines since the early nineties, developing and installing the first 10 m diameter, double balanced axial air gap machine in 1992-1995; research and development effort continued since. M0423.5 rev. 1 del 16.01.09 NC Phase Motion Control specializes in DD solutions exclusively as they recognize this technique as the only one capable of providing long life, maintenance free operation. Generally, large/very large machines are not self standing generators, rather they are integral to the wind turbine design. Consequently, Phase Company is active in the coengineering of wind turbine designs along with some of the major suppliers. This short overview describes the tradeoffs of DD design and some of the technological tools available to the wind turbine designer. Tradeoffs in machine integration and sizing. In general terms, all electrical machines are limited in terms of the amount of force that the magnetic system can control per unit airgap area. If the airgap diameter is fixed, this is converted in a limit of torque per stack length. This in turn implies that the lower the speed, the larger the generator for a given power. Unfortunately, wind turbines are generally limited in their blade tip speed, which means that the higher the turbine power, the lower the speed (Fig. 1) 6 4×10 6 2×10 6 30 20 10 0 1×10 6 2×10 6 3×10 6 4×10 6 0 6 5×10 Output power, W Figura 1: Torque and speed for DD generator vs power (note 1) 1 1 Data referred to: turbine efficiency 40%, tip speed 80 m/sec, rated power at wind speed=10 m/sec Shaft speed, rpm Shaft torque, Nm DD generator torque and speed vs power 6×10 Inspection of Fig 1 shows, for example, that the torque required, hence the size, of a 5 MW generator is not 5 times that of a 1 MW generator but over 11 times more due to the lower operating speed, all other parameters being equal. In order to optimize the performance, mass and cost of a DD generator Phase Motion Control developed a number of special solutions in the magnetic circuit that improve the available current density, magnet utilization factor, cooling and heat transfer, as well as stray loss. These techniques allow a significant improvement in useable air gap thrust per airgap surface area. Key parameters, however, depend on system choices that must be shared with the overall turbine design. In general terms, optimizing factors are the following: 1. Machine diameter 2. Airgap width selection, Bearing type and local deformation 3. Geometry (radial, double radial, axial, double axial, multi layer axial) 4. Cooling method Machine diameter In general, the larger the machine, the lower the magnetic mass required and the shorter the stack. A large diameter however brings about increased structural costs, as well as increased transportation and installation costs so an optimum must be found between electric machine cost and overall “mechanical” cost. The relationship between diameter and mass of active parts, all other factors (particularly the airgap) being equal, is shown in Fig. 2 Stack length, 2.5 and 6 MW, m 5 × 10 4 4 × 10 4 3 × 10 4 2 × 10 4 1 × 10 4 3 2 1 0 4 5 6 7 Airgap diameter, m Figura 2: Stack and active mass vs. diameter, constant airgap = 3.6 mm 0 8 Mass of active parts (dotted llines) 2.5 and 6 MW, kg DD stack and mass versus airgap diameter 4

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Génératrices entraînement direct hautes performances

Génératrices entraînement direct hautes performances


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