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1Rotor BrakeA mechanical breaking system is besides the aerodynamic breaking function of the rotor an unavoidable component of a wind turbine. It is part of themechanical drive train. The first task is to keep the rotor of a wind turbine in position when it is at a standstill. Locking the rotor is a must for servicing and repair work and is generally common practice during normal down times. Moreover, most turbines have locking bolts between rotor hub and nacelle for bridging extended periods of standstill and for servicing and repair work. Therotor can thus be secured in one or more positions.Rotor brakes are almost always disk brakes. Suitable disk brakes can frequently be adopted cost-effectively from existing production runs intended for other machines or vehicles .Against this background, the design of the rotor brake itself poses few problems. Nevertheless, the rotor brake presents the systems designer of a wind turbine with issues which have consequences for the entire system. The first and most important question is, which task the rotor brake is to fulfill within the operating concept. In the simplest case, its role is restricted to a mere holding function during rotor standstill. In this case, the brake must be dimensioned for the required holding torque of the rotor during standstill. This is determined in accordance with the aerodynamic forces calculated to occur at the assumed maximum wind speeds (Chapt.6.3.2).Apart from its function as a pure rotor parking brake, the rotor brake can 2also be dimensioned as a service brake. As long as the braking torque and braking power(thermal loading) can be absorbed, the mechanical rotor brake can be used as a second independent raking system in addition to aerodynamic rotor braking and the operational reliability of the wind turbine is considerably improved in this way. In small wind turbines, a mechanical rotor brake, which in cases of emergency prevents rotor runaway, has proved to be extraordinarily successful and is widely used today.With increasing turbine size, it becomes more and more difficult to meet this requirement. For a turbine with a rotor diameter of 60 to 80 m, the rotor brake takes on almost absurd dimensions if it is to brake the rotor torque and power during full-load operation. For this reason, the task of the rotor brake in large turbines is always restricted to the function of pure parking brake.Apart from the issue of the rotor brake’s task with respect to operations, there is the question of where in the drive train the rotor brake is best installed. The alternatives are for the rotor brake to be on the” low-speed” or on the” high-speed” side of the gearbox. In most turbines, efforts to keep the brake disk diameter as small as possible lead to the rotor brake being installed on the high-speed shaft, i. e. between gearbox and generator(Fig. 8.31). Owing to the higher rotational speed, the torque is one or even two orders of magnitude lower than at the slower rotor shaft, depending on the gear ratio.However, mounting the brake on the high-speed shaft has at least two disadvantages. It is inferior from the point of view of safety, since the braking 3function fails if the low-speed shaft or the gearbox break down. Moreover, the rotor must be held by the gears during a standstill. Gears react with increased wear of the tooth flanks to small oscillating movements, which are unavoidable in a stopped wind turbine due to air turbulence. In some turbines, it is attempted to solve this problem by no longer locking the rotor during standstill but by letting it” spin” at low speed.To avoid these disadvantages, the rotor brake was installed on the low-speed rotor shaft in some earlier systems. In small wind turbines a fully effective operating brake can be implemented with justifiable effort on the low-speed side, as long as design of the rotor shaft bearing assembly does not present an obstacle. The rotor brake on the low-speed side was a common feature of many earlier stall-controlled Danish wind turbines up to a power rating of about 100 kW in the Eighties. At that time it was considered to be an extra safetyelement even though the rotor brake was only designed as a parking brake.Installing the rotor brake on the slow side is much more problematic in large wind turbines, however. Even a parking brake already assumes a considerable size (Fig. 8.32).These disadvantages have led to the rotor brake being arranged on the high-speed side behind the gearbox in almost all new systems.8.8 GearboxThe conversion of the greatly differing rotational speeds of the rotor and the electric generator has given the designers of the first wind turbines many headaches. This situation led to costly low-speed generator designs and to 4hydraulic or pneumatic transmission systems to the generator (Chapt.8.1).Aerodynamicists made efforts to drive the rotor speed as highas possible in order to lower the gear ratio. It was assumed that costs would also increase considerably with increasing gear ratios, so that the development of rotors with extremely high tip-speed ratios was pushed forward.This situation has changed with the progress which has been made in gearbox technology. Today, high-performance gearboxes with gear ratios of up to 1:100 and more are available. In many areas of mechanical engineering, gearboxes are used which are suitable for deployment in wind turbines, as regards their technical concept, their efficiency and their operating life. The gearbox for the wind turbine has become a” vendor-supplied component”, which, with certain adaptations, can be taken from the standard product range of the gearbox manufacturers.Regardless of this favorable situation, the gearbox has been and still is a source of failures and defects in many wind turbines. The cause of these“gearbox problems” is not so much the gearbox itself, rather the correct dimensioning of the gearbox with regard to the load spectrum. In wind turbines, it is easy to underestimate the high dynamic loads to which the gearbox is subjected. Thus, in the early phase, many turbines had gearboxes whichwere undersized. Having learned their lessons, successful manufacturers equipped their turbines with ever stronger gearboxes and thus, in the course of development, empirically arrived at the right dimension.58.8.1Gearbox ConfigurationsToothed-wheel gearboxes are constructed in two different forms. One is the parallel shaft or spur-gear system, the other is the technically more elaborate planetary gearing. The gear ratio per single reduction is limited, so that the difference in diameter between the small and the large wheel does not become too unfavorable. Parallel-shaft-gear stages are built with a gear ratio of up to 1 :5, whereas planetary stages have a gear ratio of up to 1 : 12. Wind turbines generally require more than one stage. Fig. 8.33 shows what effects differentdesigns have on gearbox size, mass and relative cost [11].It is noteworthy that the three-stage planetary design has only a fraction of the overall mass of a comparable parallel shaft system. The relative costs are reduced to about one half. In the megawatt power class, the multi-stage planetary gearbox is, therefore, clearly superior. In smaller power classes, the comparison is not quite as unambiguous. In the range up to about 500 kW, parallel-shaft gear designs are often preferred for cost reasons.Small wind turbines are equipped with parallel-shaft gear systems.Theprevailingmodels are two-stage gearboxes which are commercially available from numerous manufacturers as modified universal transmissions (Fig. 8.34).In larger wind turbines, the planetary design definitely prevails. For outputs of several megawatts, two- or three-stage models are used (Fig. 8.35). Large gearboxes of this type are used, for example, in ship-building and several other 6fields of mechanical engineering, so that suitable gearboxes for large wind turbines can be derived from these production sources. Gearboxes with one planetary stage and two additional parallel-shaft stages are used in many late-model turbines (Fig. 8.36).With the additional parallel shaft, the primary and secondary shafts are no longer coaxial. This has the advantage that a hollow through shaft can be implemented more easily. In this way, power supply lines supplying power to the blade pitch drive, as well as measurement and control signals for the rotor, can be routed through the gearbox.In larger gearboxes, an auxiliary rotor drive is frequently flanged to the gearbox housing. Using this electric motor, the rotor can be turned slowly. Such an auxiliary unit is indispensable for assembly and maintenance work in large rotors. Gearbox lubrication is usually carried out via a central oil supply in the nacelle. As a rule, it also contains an oil cooler and a filter.In spite of indisputable advances having been achieved in the durability of the gearboxes, there is still “trouble with the gears” being experienced even in the latest wind turbines. Although it is possible to adapt gearboxes for wind turbines from other types of machine, they are subject to special demands which are often not encountered in other applications. Much negative experience in recent years has provided important insights into this issue:– Special attention must be devoted to the smooth running of the tooting. Particularly prominent gear meshing frequencies can cause resonances in the drive train.“Cheap” transmissions with simple tooting are unsuitable for use in 7wind turbines.– Oil leaks in the transmission are a particular problem. Labyrinth seals have proven more reliable than slipping type seals. In many cases, the housing flanges also showed leaks after some time. A box design with a top flange is apparently more advantageous than gearbox housings with flanges on the input and output side.– The quality of the lubrication has been found to be a decisive factor for the service life of the gearbox. Oil temperatures which are too high cause just as much damage as does contamination in the oil. Oil coolers and filters are indispensible for large gearboxes ands is the careful observance of oil change intervals.– The stiffness of the gearbox housing is an important criterion for its service life if the housing is integrated into the static design of the nacelle.Apart from these constructional measures, of course, the correct dimensioning has a decisive influence.8