How Does A Gearless Wind Turbine Work?

A gearless wind turbine, also known as a direct drive wind turbine, is a type of wind turbine design that does not use a gearbox to connect the rotor to the generator. Instead, the rotor shaft is directly coupled to the generator shaft.

In a traditional geared wind turbine, a gearbox is used to increase the rotational speed from the low-speed rotor to the high-speed generator. The gearbox allows the generator to operate at optimal rotational speeds by converting the input slow rotation to much faster rotation. However, gearboxes add complexity, weight, noise, and require regular maintenance and lubrication.

By eliminating the gearbox, a gearless wind turbine avoids these drawbacks. The generator itself is larger and optimized to operate at low rotational speeds coming directly from the rotor. Gearless turbines are simpler, lighter, and quieter. They require less maintenance due to having fewer moving parts. The direct drive connection provides a more reliable overall system.

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Generator Types

There are a few different types of generators used in wind turbines:

Permanent Magnet Generators

Permanent magnet generators use permanent magnets to generate alternating current electricity. They typically have low maintenance requirements since there is no need to supply current to the rotor. Permanent magnet generators are lightweight and efficient.

Induction Generators

Induction generators, sometimes called asynchronous generators, produce electric current through electromagnetic induction. They are robust, low-cost, and require little maintenance. Squirrel cage induction generators are commonly used in wind turbines.

Exciteless Synchronous Generators

Exciteless synchronous generators have field windings on the stator rather than the rotor. The magnetic field is created through permanent magnets on the rotor instead of an external DC current source. These generators are synchronous but do not require a separate field current supply.

Direct Drive Mechanism

One of the defining features of a gearless wind turbine is the direct drive mechanism. As the name suggests, this means that the rotor is directly coupled to the generator, without any intermediary gearbox. The low-speed shaft coming from the rotor is connected straight into the generator.

This direct drive configuration eliminates the need for a gearbox to increase the rotational speed from the rotor to the generator. Gearboxes allow a multi-stage gear-up of the speed, but they also introduce mechanical complexity, additional weight, and potential reliability issues. By removing the gearbox, a gearless wind turbine simplifies the drivetrain and improves reliability.

However, the direct drive also means the generator must be designed for lower rotational speeds than a geared turbine. That is why gearless wind turbines utilize specialized low-speed, high torque generator designs such as permanent magnet synchronous generators. The direct drive train is a defining difference of gearless wind turbines compared to traditional geared wind turbines.

Yaw System

The yaw system is responsible for rotating the wind turbine so that it always faces into the wind. This is important for maximizing power generation, as a turbine produces the most electricity when directly facing the wind.

Most large wind turbines have a yaw drive consisting of electric motors and gearboxes. The motors power the gearboxes to slowly turn the nacelle (the turbine housing that contains the generator, gearbox, etc.) and rotor to track the wind. There are typically multiple motor/gearbox yaw drives spaced around the nacelle for redundancy.

The yaw drives engage slewing bearings which allow the smooth rotation of the nacelle on top of the tower. The bearings are housed in the yaw system at the tower top. Sensors on the turbine measure wind direction, and the control system activates the yaw drives accordingly to face the rotor into the wind.

Having an effective yaw system is critical for wind turbines to optimize power output. The ability to track the wind allows them to maximize energy production and minimize turbine downtime.

Pitch System

The pitch system is a critical component of gearless wind turbines that allows them to regulate power output and control the rotational speed of the blades. Unlike traditional geared turbines that control rotor speed through a gearbox, gearless turbines rely on pitching or turning the blades to regulate the power.

Blade pitch control works by changing the angle of attack of the blades in response to wind conditions. If the wind speed increases, the pitch mechanism turns the blades slightly out of the wind to reduce the lift and limit power. This prevents the turbine from generating more power than what the generator and grid can handle. Pitching the blades decreases their aerodynamic efficiency and allows the rotor to maintain optimal tip-speed-ratio.

Conversely, the blades are pitched to capture more wind energy and increase power production when wind speeds are lower. Fine adjustments to the blade pitch, usually a few degrees at a time, are made continuously to maximize power output across all wind speeds. Pitch control combined with variable generator torque gives gearless turbines a high level of control over power production.

The pitch system includes pitch bearings, drives and motors connected to each rotor blade. Pitch motors precisely turn the blades in response to controller commands. Pitch systems add complexity but are essential for safety, preventing overspeeding and damage. Gearless turbine designs require advanced pitch control for optimal performance.

Permanent Magnet Generator

Inside the nacelle resides the permanent magnet generator, which is the component that converts the rotational kinetic energy of the turbine blades into electrical energy. Permanent magnet generators differ from electrically excited synchronous generators that require external power for magnetization.

Permanent magnet generators have permanent magnets constructed from materials like neodymium-iron-boron mounted on the rotor. The generator stator is wound with copper wire coils. When the rotor spins, it rotates the permanent magnets around the stator, inducing a voltage in the stator coils through electromagnetic induction. This voltage drives the electrical current output.

Compared to other options, permanent magnet generators have several advantages for wind turbines:

They are lightweight and eliminate the need for separate excitation equipment.
They have higher efficiency and generate more power per volume than electrically excited generators.
The permanent magnets enable variable speed operation to capture more wind energy.

They have lower maintenance requirements without slip rings and brushes.

By leveraging permanent magnets, these generators provide a robust and optimal solution for converting wind energy into usable electric power.

Main Bearing

The main bearing in a direct drive wind turbine is a large slewing ring bearing that supports the weight of the rotor. It allows the rotor to turn smoothly with minimal friction. This bearing must be very robust to accommodate the extreme bending loads exerted by the rotor blades as they spin.

Slewing ring bearings have an inner and outer race, with balls or rollers that roll between them to allow rotational motion. The outer race attaches to the stationary nacelle structure, while the inner race attaches to the spinning rotor. Having a large diameter bearing helps distribute the loads over a bigger area.

The main bearing requires regular maintenance and lubrication to prevent wear and failure. Technicians inspect the bearing for signs of pitting, cracking, looseness, or debris contamination. The goal is detecting and addressing any issues before a catastrophic failure can occur.

Advanced main bearings in direct drive turbines may incorporate sensors to monitor loads, temperature, and vibration. This helps predict maintenance needs and prevents unexpected downtime. With such a critical component, the main bearing must provide many years of reliable service.

Maintenance

One of the major advantages of a gearless wind turbine is that it requires less maintenance than a geared turbine. By eliminating the gearbox, which is known to be the component most prone to breakdowns in a wind turbine, the maintenance needs are greatly reduced.

That said, gearless turbines are not completely maintenance-free. The main areas that require regular maintenance are:

The rotor blades need to be inspected for any cracks, erosion or damage. The blades may also require cleaning if they get covered with dirt or insect debris over time.
The yaw system, which rotates the turbine to face the wind, has bearings and electric motors that need lubrication and inspection.

The pitch system, which angles the blades to control rotor speed, also requires regular lubrication and inspection.
The permanent magnet generator needs inspection to check for any worn components. The cooling system for the generator also needs maintenance.
The main bearing requires re-greasing at regular intervals.

While the maintenance needs are reduced compared to a geared turbine, they are not eliminated entirely. Proper maintenance is still required, especially on the major drivetrain components, to ensure reliable operation over the turbine lifetime of 20+ years.

Disadvantages

Gearless wind turbines come with some disadvantages compared to geared turbines. The two main drawbacks are higher capital costs and grid synchronization challenges.

Gearless turbines require larger and more expensive generators than geared turbines. The direct drive generator has to be physically larger to produce the same power output at lower rotational speeds. These large direct drive generators increase the nacelle weight and the overall turbine cost.

Gearless turbines also face technical difficulties synchronizing their output frequency to the grid frequency. Without a gearbox, fluctuations in wind speed directly impact the generator rotational speed and output frequency. Advanced power electronics are needed to match the variable frequency power to the fixed grid frequency. Rapid grid synchronization remains an engineering challenge for direct drive turbines.

Conclusion

In summary, gearless wind turbines utilize a direct drive mechanism to connect the rotor shaft directly to the generator, eliminating the need for a gearbox. This direct drive approach relies on a multi-pole generator, usually a permanent magnet synchronous generator, that can produce electricity at the low rotational speeds of the turbine rotor. Without a gearbox, gearless turbines require fewer moving parts and less maintenance. However, the larger magnets required can increase costs. Pitch and yaw systems allow the turbines to adjust their position to maximize energy production.

Going forward, gearless wind turbines will continue to gain popularity due to their reliability and lower maintenance costs. Advancements in permanent magnet technology will help reduce generator size and weight. Gearless turbines are well suited for offshore applications where serviceability is more difficult. As wind power expands globally, the simpler and robust design of gearless turbines makes them an attractive choice.