Engine & Turbine Technology
shaft typically means that it will be operating within a more limited space envelope. Te energy which must be dissipated is the same wherever the brake is placed, meaning the total linking area must be the same. Also, the brake pad area must be sufficient to dissipate the heat generated during braking – particularly during emergency stops. Speed and available space quickly become limiting factors with regard to the maximum disc diameter, impacting on brake selection. A further consideration regarding brake position is the possibility of gear tooth damage. If brakes are installed on the gearbox output shaft and the turbine is stationary, gusts are likely to cause the rotor to transmit a rocking motion within the backlash of the input and output gears.
and under all climatic conditions. Factors which impact on the calculations include
rotor inertia, aerodynamic torque, rotor speed, the composition of the brake liner material and the thermal properties of the disc. But there are other factors to consider, too, such as
the effects of dirt on braking surfaces, condensation, oil on the braking surfaces, rubbing speed and pressure, and wear debris on the surface linings. Because wind turbines operate unmanned, it is not possible to monitor all of these conditions, so an allowance must be made when calculating a safe torque level.
Brakes for yaw control Under normal operational conditions, a horizontal axis wind turbine can be stopped by moving the blades out of the wind. Yaw brakes provide an effective means of smoothly controlling a wind turbine nacelle as it “rotates up wind”, or yaws. Tey are usually installed as drag brakes and operate by controlling back pressure, which in turn controls the degree of spring force and therefore braking torques. Te process begins when an anemometer signals a change in wind direction, so starting up the motor driving the gear ring on the yawing system. Te motor is subsequently stopped by a further signal when the yaw mechanism reaches the optimum up wind position and the nacelle stops. Edouard Haffner, responsible for New Market
Fig. 2. Twiflex yaw brakes provide an effective means of smoothly controlling a wind turbine nacelle as it rotates up wind.
Without forced lubrication between the mating teeth this effect could ultimately result in fretting and expensive gear damage. Te braking torque level for rotor brakes is a crucial consideration that must be calculated during initial stages of a brake design. Te maximum permissible braking torque on the rotor shaft is usually imposed by the blades, or their anchorage to the gearbox input shaft. On the other hand, high-speed shaft braking is usually related to the maximum permissible gear-tooth loading.
A minimum level of braking torque also exists,
below which the variable nature of the frictional forces under different operating conditions could place the turbine rotors at risk. It is therefore important to allow an adequate window of safety, or service factor, to ensure that the brakes will always operate effectively
Development in Europe for Altra comments: “Varying wind strengths cause varying motor loading and so impact on the accuracy of the nacelle stop relative to the change in wind direction. Motor load can be effectively controlled regardless of wind strength by installing a permanently applied, electromechanically released brake on the gear-ring face and varying its drag from the signal actuated by the rise or fall in motor current.” Typically, there are four to eight yaw motors per turbine. Te brakes usually mount to the back end of the drive motors and are commonly positioned on the underside of the yaw gear ring. Tis ensures accurate nacelle positioning and best operating efficiency. Te design eliminates potential damage from erratic movement with gear backlash, and the brake is an effective clamp to lock the mechanism in position.
Simplicity of manufacture Wind-turbine engineers agree that a mechanical disc brake is the best solution in terms of reliability, simplicity of manufacture, ease of servicing and initial cost. Disc brakes are renowned for their excellent performance in hostile environments and require little physical space relative to the braking force they provide.
Large horizontal axis wind turbines ‘pitch’ or angle
their rotor blades for best efficiency. Te rotor blades are also pitched or feathered to minimise rotation in high winds and for turbine maintenance.
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