What are the different types of wind turbine brakes?

Slowing and halting an 80-m rotor involves converting its kinetic energy into heat. Of course, there are several design decisions here. Rotor brakes control overspeed, and provide parking and emergency braking. These brakes can mount on the rotor (low-speed shaft) on the generator (high-speed shaft), and both shafts in some cases.

Low-speed-shaft braking is relatively straightforward in that a large disc brake with a large friction lining area is easy to accommodate. The drawback is that the brake must generate a high-braking torque.

Generally, the most cost-effective position is on the high-speed shaft between a gearbox and generator. The increased ratios of wind-turbine gearboxes produce a large reduction in output torque. In many cases, a major parameter regarding brake selection is choosing a friction-liner area of sufficient size to ensure adequate heat dissipation.

The energy to dissipate is the same regardless of brake location, so the total lining area must be the same. The brake-pad area must be sufficient to control the temperature rise.

These requirements are more difficult to meet on the high-speed shaft because speed and space are limiting factors with regard to the maximum disc diameter and brake selections. Nevertheless, braking on the high- speed shaft has been used on many turbines up to 750 kW. As the industry develops higher capacity turbines, the trend is leaning towards rotor-shaft braking.

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 can generate a rocking motion within the backlash of the input and output gears. Without forced lubrication between the mating teeth, this effect could result in expensive gear damage.

Pitch drive brakes: A series of high-torque, electrically released, spring engaged, static holding brakes can withstand the conditions on the pitch drive of large turbines. This brake is typically smaller in diameter than the motor assembly and adds minimal length to the overall package. One model, rated at 135 nm, has a 6.5-in. diameter and is only slightly over 2-in. long. Typical design life calls for 500 to 1,000 stops. Some brakes exceed this range. Another plus for electric brakes: A short reaction time, 0.20 sec or less.

A braking torque level for rotor brakes is one characteristic to calculate during initial stages of brake selection. The maximum permissible braking torque on a 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.

There is also a minimum level of braking torque, below which the variable nature of the frictional forces could put a turbine rotor 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 and under all climatic conditions. To achieve an adequate service factor it is helpful to consider how braking performance can vary with the same predetermined level of braking torque.

For example, suppose a turbine has a 1-MW rating, and aerodynamic (load) torque of 100,000 nm.

Applying a brake during an emergency at 20% over- speed, the rise in disc temperature and stopping time will vary depending on the chosen service factors. A commonly applied factor of 2.00 suggests:

Tb/TL = 2

Where

Tb = brake torque, and TL = load torque.

Hence, the calculated braking torque Tb = 200,000 nm.

Yaw brakes for wind turbines

A full array of caliper designs is available from Twiflex, Ltd. to meet the yaw braking-force requirements of any size wind turbine. All brake models are durable, hydraulically activated, and direct applied. Models T20 and T40 deliver up to 40 kN braking force, feature two-bolt side mounting, and are intended for light to medium-duty applications. Model VCH provides 60 kN, features four-bolt center mounting, and works well in medium-sized turbines. Model VKH generates 118 kN, and is a base mounted caliper for larger, heavy-duty turbines.

There typically are four to five yaw motors per wind turbine. The brakes mount to the back end of the drive motors and are commonly positioned on the underside of the yaw gear ring. Twiflex is a member of the Altra Industrial Motion family of power transmission companies.

Rotor brakes for wind turbines

Twiflex brakes are fully assembled, provide high levels of reliability, easy electronic monitoring and maintenance, and come with organic or metallic linings. Friction liners are sized to ensure adequate heat dissipation during an emergency stop and with an even pressure distribution across pad surfaces.

The brakes come in a range of braking forces from 100 N to 1 MN to meet the torque requirements of the most common turbines. Rotor-brake models include the GMR (15 to 35 kN), the VCS (20 to 60 kN), and the VKSD (50 to 119 kN). VCS and VKSD brakes are available as standard and floating models. Floating, single-sided brakes are mounted on sliding bushings to save space.

Twiflex spring-applied, hydraulically-released, caliper brakes are typically mounted to a turbine’s ma

in rotor shaft, between gearbox and generator, and used primarily as safety brakes during emergency stops under high wind conditions. All units are engineered to handle the large output torque generated by the high ratios found in wind-turbine gearboxes. The brake models are in operation today on hundreds of wind turbines around the world. Twiflex is a member of the Altra Industrial Motion family of power transmission companies.