Engine & Turbine Technology
Mr Haffner continues: “Pitch drives can be driven electrically or hydraulically; electric is most common, which lends itself to a cleaner, more compact design. In addition, the electric drive is more accurate and can be easily programmed to meet a variety of application variables. In either case, a power-off holding brake built into the drive serves as an added safety feature, as
Fig. 3. Warner ERS wind turbine pitch brakes are compact packages designed to fit the limited space of the wind turbine nose cone.
well as for dynamic braking in emergency pitch conditions.” Te general layout of an electric pitch drive includes an electric motor (AC, DC, or servo), a position sensor (encoder or resolver), and a power- off holding brake. Control logic releases the brake, drives the motor, senses the position, stops the pitch operation, and engages the brake to hold the blades in a predetermined position. Te motor drives a large ring gear integral to each blade, typically with a gear ratio in the 1,000:1 range. Tere are several considerations in selecting the
correct brake for a pitch drive. Te pitch drive itself must be a compact package because there is limited space to mount the assembly in the turbine’s nose cone, so when selecting a brake for the pitch drive, allowance must be made for sufficient torque in a compact package.
Fig.5. Twiflex yaw brakes are usually installed as drag brakes.
A properly designed disc and calliper brake can meet torque and thermal specifications. However this style of brake tends to be quite large in diameter and can be difficult to mount in a limited space. A flange mounted electrically-released/spring- engaged standard motor brake can meet the space requirement, but normally falls short in the torque and thermal specifications. More robust brakes have been designed to meet the higher standards needed in this type of application. Warner Electric has developed a series of high-
torque, electrically released, spring engaged, static holding brakes ideal for these applications. Te ERS series brakes are typically smaller in diameter than the motor assembly, add minimal length to the overall package and are rated for up to 30,000 dynamic stops – far exceeding the operational norm. An additional benefit from an electric brake is
its short reaction time, 0.20 sec or less, making it a superior choice for wind pitch drive systems. Te reliable design of this brake style, easily dissipates the heat generated far and above that required by the normal duty cycle.
Fig. 4. Twiflex rotor brakes control overspeed, and provide parking and emergency braking.
Typically, the brake must not be larger in diameter than the motor and position sensor, and must not add excessive length to the drive system. Design life must also be factored in to component selection. A large-scale turbine can have an effective design life of 20 years, so individual components and packaged systems must meet or exceed this standard. Te number of estimated emergency pitch stops in a 20-year life is generally defined to be between 500 and 1000. Due to the large inertia these stops can create, thermal dissipation and peak energy input criteria must be accounted for.
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Vital factors We can see, then, that whether the specific requirement is for rotor brakes, yaw brakes or blade pitch brakes, there are many vital factors to consider, and design decisions should be made as early as possible in the design process to ensure the best possible finished system. If in doubt, the best advice is always to consult an
expert in the field, and companies such as those in the Altra Industrial Motion group have the widest possible experience to draw on so that brakes specified for wind turbine applications provide many years of reliable service – whatever the weather can throw at them. l
For more information, visit
www.altramotion.com
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