Page 451 - Wind Energy Handbook
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GEARBOX                                                                425


             of between about 1:31 and 1:88 are therefore required. Normally these large step-
             ups are achieved by three separate stages with ratios of between 1:3 and 1:5 each.
               The design of industrial fixed ratio gearboxes is a large subject in itself and well
             beyond the scope of the present work. However, it is important to recognize that
             the use of such gearboxes in wind turbines is a special application, because of the
             unusual environment and load characteristics, and the sections which follow focus
             on these aspects. Sections 7.4.2 to 7.4.6 consider variable loading, including drive
             train dynamics and the impact of emergency braking loads, and examine how gear
             fatigue design is adapted to take account of it. The relative benefits of parallel and
             epicyclic shaft arrangements are discussed in Section 7.4.7, while subsequent
             sections deal with noise reduction measures, and lubrication and cooling. A useful
             reference is the American Gear Manufacturers Association Information Sheet (1996)
             which covers the special requirements of wind turbine gearboxes in some detail.


             7.4.2  Variable loads during operation

             The torque level in a wind turbine gearbox will vary between zero and rated torque
             according to the wind speed, with excursions above rated on fixed-speed pitch-
             regulated machines due to slow pitch response. The short-term torque fluctuations
             will be subject to dynamic magnification to the extent that they excite drive train
             resonances (see Section 7.4.3 below). In addition there will be occasional much
             larger torques of short duration due to braking events, unless the brake is fitted to


               1200
                      Curves based on 8.5 m/s annual mean wind speed
                          and IEC Class A turbulence intensity
               1000
                                                       500 kW,
                                                        stall-
                                                      regulated
              Duration (h/annum/20 kW bin)  600
                                                       machine
                800





                400


                200
                                                            Two-bladed 500 kW, 40.4
                                                                 r.p.m.
                                                             pitch-regulated machine
                 0
                  0     100    200    300    400   500    600    700    800   900    1000
                                               Power output (kW)
             Figure 7.24  Load Duration Curves for 500 kW, Two-bladed Pitch-regulated and 500 kW,
             Stall-regulated Fixed-speed Machines
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