Is Used To Control The Yawing Function Of A Windmill?
Wind turbines are an important source of renewable energy around the world. They convert the kinetic energy from wind into mechanical power to generate electricity. Wind turbines consist of blades attached to a rotor, a nacelle housing the drive train, and a tall tower supporting the nacelle and rotor. The rotor needs to be oriented into the wind to maximize power generation. This orientation is controlled by the yaw system.
Yawing refers to the rotation of the nacelle and rotor around the vertical axis of the tower to face the turbine into the wind. Proper yaw alignment is crucial for optimal power production from the turbine. The yaw system enables the wind turbine to track changes in wind direction and keep the rotor facing straight into the oncoming wind. This allows the blades to efficiently capture kinetic energy from the wind.
What is Yawing?
Yawing refers to the ability of wind turbines to turn and face the wind direction. Wind turbines are equipped with a yaw drive and yaw motor system that rotates the nacelle and rotor to align perpendicular to the wind. This alignment is critical for optimal power generation from the turbine.
As wind direction changes, it is important for the turbine to turn and face into the wind. Otherwise, the rotor will not be optimally oriented, reducing the energy capture. Yawing allows continuous alignment for maximum wind exposure and power production. According to research, a misaligned wind turbine can lose as much as 20% of its potential power output (https://www.quora.com/What-is-yawing-in-wind-turbine).
Yawing systems are designed to respond to even subtle shifts in wind direction. Small wind vanes and wind sensors on the turbine detect changes, triggering the yaw mechanism to gradually rotate and align the rotor. This constant correction is essential to harnessing as much energy as possible from the available wind.
Yaw Drive Mechanisms
There are several main mechanisms used for controlling the yaw of wind turbines:
Geared yaw drive – This uses a gearbox to transfer rotational force from electric motors to the nacelle to rotate it. The gearbox provides speed reduction and torque multiplication. Geared yaw drives are the most common type used today.
Direct drive – Direct drive yaw systems connect the motor shaft directly to the nacelle without any gearing. This eliminates the gearbox but requires larger low-speed motors. Direct drives are simpler and require less maintenance but are more expensive.
Hydraulic yaw drive – Hydraulic drives use hydraulic motors and hydraulic pumps to turn the nacelle. They have high torque capacity in a compact design. However, hydraulic systems require fluids that need maintenance and have more complex controls.[1]
Of these, geared and direct drive electric yaw systems are most prevalent today, while hydraulic drives are rarely used in modern wind turbines.
[1] https://en.wikipedia.org/wiki/Yaw_drive
Yaw Motors
Yaw motors are used to turn the nacelle and rotor to face the wind. There are several types of motors used for yawing in wind turbines:
AC motors: These use a three-phase AC induction or synchronous motor. They are the most commonly used type of yaw motor due to their robustness and reliability. The motor is connected to a gearbox and drives the yaw gear ring to rotate the nacelle.1
DC motors: Direct current (DC) motors can also be used for yawing. They provide precise speed and torque control. However, they require more maintenance and are less tolerant of electrical disturbances than AC motors.2
Hydraulic motors: Some wind turbines use hydraulic motors for yawing. They have high torque density but require hydraulic piping and a pump system. Hydraulic yaw motors are more complex but can hold position firmly without power.
Yaw Brakes
Yaw brakes are a critical component of wind turbine yaw systems. They are used to hold the nacelle in position and prevent unwanted rotation. There are a few main types of yaw brake systems used on modern wind turbines:
Disc brakes – These work similarly to the disc brakes on a car. Pads grip onto a rotating disc attached to the yaw bearing to provide friction and hold the nacelle in place. Disc brakes tend to be lower maintenance than some other designs.
Caliper brakes – Caliper brakes use hydraulic pistons to press pads against a yaw bearing ring. They provide strong braking force for holding position. Maintenance needs can be higher than disc brakes.
Active yaw brakes – Some systems utilize electric motors and gearboxes as active braking systems instead of passive pad brakes. These allow for precise position control.
Yaw brakes usually operate in an engaged position to hold the nacelle, then disengage when the yaw motors turn the turbine into the wind. Braking force, materials, and design are important factors impacting performance and maintenance. High-quality yaw brakes are critical for keeping turbines properly oriented.
Yaw Bearings
Yaw bearings are a critical component of wind turbine yaw systems. They allow the nacelle to rotate smoothly as the turbine aligns itself with the wind. The most common types of yaw bearings used in wind turbines are:
Slewing Bearings
Slewing bearings, also known as slew rings, are commonly used in wind turbines. They consist of an inner and outer ring with balls or rollers that allow low-friction rotation between the two rings. Slewing bearings are robust and can handle the large loads and slow rotational speeds required for yaw motion.
Tapered Roller Bearings
Tapered roller bearings use conical rollers between inner and outer rings to carry both axial and radial loads. The tapered design allows them to handle the combination of forces from yaw motion. They may require frequent lubrication and maintenance.
Spherical Roller Bearings
Spherical roller bearings allow free rotation in all directions. They can accommodate misalignment between the inner and outer rings. The design evenly distributes loads, making them suitable for handling yaw system stresses.
Arrangements
Yaw bearings are often arranged in pairs to share the load. They may be combined in face-to-face duplex pairs or back-to-back tandem pairs. This provides redundancy in case a single bearing fails. The load sharing between bearings depends on the specific arrangement and mounting.
Yaw Drives
The yaw drive is responsible for rotating the nacelle and rotor to face the wind direction. This is a critical component to maximize power generation. The yaw drive consists of electric motors, gearboxes, brakes, couplings, and bearings.
The most common type of yaw drive uses gearboxes and electric motors. The gearbox provides the high torque needed to turn the large nacelle. Common gearbox types include planetary, helical, and bevel gears. The gearbox connects to an electric yaw motor, typically in the 250-500 kW power range for large wind turbines. The motor rotates slowly with high torque output to drive the gears (Wikipedia, 2022).
Couplings connect the various drive components. They allow slight misalignment and dampen vibrations between components. Common types include flexible rubber or elastomeric couplings, gear couplings, and spline couplings. The couplings must handle high torque loads and rotate at low speeds.
Proper lubrification is critical for the gearboxes, motors, brakes, and bearings. Special high viscosity gear oils are required to handle the challenging loads and slow speeds (Windpower Engineering, 2019).
Control Systems
The control system is critical for managing the yaw orientation and positioning of wind turbines. There are two main types of yaw control systems used in wind turbines:
Active yaw control uses motors, drives, and brakes to continuously orient the nacelle and blades into the wind. This maximizes power production by keeping the rotor perpendicular to the wind. Active yaw control relies on inputs from wind vanes or sonic anemometers to detect wind direction, as well as yaw position sensors on the nacelle to track orientation. The control system processes these inputs to determine when and how much to rotate the nacelle. Hydraulic drives or electric motors are used to actuate the rotation. Active yaw control is more complex but provides optimal alignment.
Passive yaw control relies on the force of the wind itself to naturally orient the nacelle downwind. Tail fins or dampers are used to regulate the rotation speed. Passive control is simpler and less expensive, but can compromise power capture in shifting winds. However, it may be adequate for smaller turbines.
Overall, the yaw control system is essential for maximizing power production by keeping the rotor oriented into the wind as conditions change. Active control provides the best tracking but requires more sophisticated components. Passive control is simpler but less responsive.
Monitoring
Monitoring the yaw system is critical to ensuring optimal performance and reliability of wind turbines. This is accomplished through various sensors that provide real-time data on the yaw system’s status and operation.
One of the main sensors used is a yaw position sensor, typically using an incremental encoder, to precisely track the nacelle’s orientation at all times [1]. This allows the turbine control system to know the exact direction the rotor is facing. Additionally, yaw limit switches are installed to indicate when the nacelle reaches its maximum yaw angle in either direction.
Vibration sensors may also be utilized on key yaw system components to identify any abnormal vibrations that could indicate emerging issues. Tilt sensors help monitor proper vertical alignment of the nacelle. Temperature sensors can detect overheating in motors, brakes, bearings and other parts.
Monitoring systems log and analyze yaw performance data over time to identify trends and deviations. This helps schedule predictive maintenance at optimal intervals. Regular manual inspections are also conducted to visually inspect the physical condition of yaw drives, motors, brakes and bearings.
Maintenance
Regular maintenance of the yaw system is critical for maximizing uptime and minimizing failures in wind turbines. The main yaw components that require routine inspection and maintenance include the yaw motors, yaw brakes, yaw bearings, and yaw drives.
Yaw motors should be inspected according to the manufacturer’s recommendations, typically every 6 months to 1 year. Mechanics check for signs of oil leakage, loose electrical connections, worn brushes, and other deterioration. Lubrication levels are verified and motors may be sent for off-site repair or rewinding if needed.
Yaw brakes undergo biannual or annual inspections to test braking effectiveness and check for wear, corrosion, and leakage issues. Brake pads, rotors, and hydraulic fluids may need replacement after heavy use. Keeping brakes properly adjusted and maintained prevents unintended rotation and turbine damage.
Bearings enable the nacelle to rotate on the tower and should be re-lubricated regularly, such as every 6 months. Technicians check for abnormal noise, vibration, overheating, and degraded lubricant. Badly worn bearings risk seizure and failure. Complete bearing replacement may be required every 5-10 years.
The yaw drives connecting the gearbox and motors to the bearings and nacelle also need periodic inspection and lubrication. Loose bolts, oil leaks, unusual wear, and gear damage are faults to identify and address. Keeping yaw drive components properly aligned and tensioned reduces stress and extends longevity.
Overall, following scheduled maintenance procedures for all yaw components is essential to minimize downtime, prevent catastrophic failures, and maximize power production over the turbine lifetime. Trained wind techs are needed to properly inspect, adjust, repair, and replace yaw parts based on use and wear.
