Written by: Pablo Carazo Morales – WTG Technical Lead at Iberdrola.
Over the past 20 years, wind turbine manufacturers have experimented with various drivetrain architectures, evolving their designs based on technological progress and operational insights. The drivetrain encompasses all mechanical and electrical components between the rotor (including hub and blades) and the generator. Its primary role is to match the rotor’s rotational speed to the generator’s requirements—when necessary.
Despite a few decades of development, the industry has not settled on a single drivetrain solution. Instead, multiple configurations coexist, each suited to different operational contexts and offering distinct advantages and trade-offs.
1. Drivetrain Categories
Drivetrains are typically classified by the generator’s rotational speed:
- High-Speed (HS): Utilizes multi-stage gearboxes (3–4 stages) to boost rotor speed to over 1,000 rpm.
- Medium-Speed (MS): Employs more compact gearboxes (1–2 stages), achieving speeds between 100 and 600 rpm.
- Direct Drive (DD): Removes the gearbox entirely, connecting the rotor directly to a low-speed generator (typically 6–15 rpm).
2. Dominant Drivetrain Architectures
Different main drivetrain topologies have emerged as industry standards depending on generator technology:
- DFIG (Doubly-Fed Induction Generator) + Partial Converter A high-speed configuration with asynchronous generator used widely in onshore by Nordex Group, Siemens Gamesa, GE Vernova and most Chinese OEMs (Envision Energy, MingYang Electric, Sany Renewable Energy Co., Ltd). Known for its cost-efficiency, though it requires slip rings and regular maintenance.
Another asynchronous generator option is SCIG (Squirrel Cage Induction Generator) + Full Converter without slip rings, but almost out of the market (currently only Vestas adopts this in its successful 4 MW platforms, such as the V150-4.X)
- Medium-Speed PMSG (Permanent Magnet Synchronous Generator) + Full Converter Often referred to as a hybrid drive, especially by Chinese OEMs. Used in Vestas’ EnVentus platform.
- Direct Drive PMSG + Full Converter Gearbox-free, low-speed solution used by ENERCON (onshore), Siemens Gamesa (offshore), and GE (offshore).
3. Market Trends & Shifts
Historically, high-speed drivetrains dominated in onshore market. However, a noticeable shift toward medium-speed systems is underway, driven by both Western (Vestas) and Chinese manufacturers (Goldwind). This transition is influenced by several factors:
- Maintenance: DFIG systems require slip rings, which are prone to wear and demand frequent servicing.
- Grid Compliance: Full converters offer better performance under strict grid codes, which is critical in markets like Australia.
- Scalability: As turbines grow in size and capacity, high-speed gearboxes become more complex and costly.
The Main Advantages of Using Medium-Speed Drivetrains (Onshore):
- Eliminates the high-speed gearbox stage, reducing operational expenses.
- Uses PMSGs, which require less maintenance than DFIGs.
- Enhances grid compliance and fault ride-through capabilities.
However, there is a significant drawback, the higher upfront investment (CAPEX) due to the cost of PMSGs and full converters.
4. Offshore Wind: A Mixed Landscape
Offshore applications balance between direct drive systems (favored by Siemens Gamesa) and medium-speed configurations (used by Vestas, Mingyang, etc.). High-speed gearboxes are now rare offshore due to reliability concerns.
Below we can see the GW installed by each OEM and the forecast for the next decade, led by the aforementioned SGRE, Vestas, and Mingyang.
The Main Advantages of Using Medium-Speed Drivetrains (Offshore):
a. Lower Use of Rare Earth Materials (up to 75% reduction) – These rare earth magnets enable high power density and efficiency, which are critical for offshore applications where maintenance is costly, and reliability is paramount. Direct drive systems require large quantities of rare earth materials (primarily neodymium and dysprosium) to achieve the necessary torque and performance. The key Impacts are the following:
– Supply Chain Vulnerability: Most rare earths are mined and processed in China, creating geopolitical and supply risks for European manufacturers.
– Environmental Concerns: Rare earth mining and refining can be environmentally damaging, involving toxic waste and high energy consumption.
– Cost Volatility: Prices of rare earths are subject to market fluctuations and export controls, which can affect turbine costs and project economics.
– Strategic Dependence: The reliance on rare earths raises questions about Europe and US industrial resilience and energy security, especially as offshore wind scales up.
b. Smaller & Lighter Generators – In floating offshore wind, weight is a critical constraint. Unlike bottom-fixed turbines, floating platforms must maintain stability and minimize motion in harsh marine environments. Every kilogram added to the RNA (Rotor Nacelle Assembly) increases structural demands on the floater, mooring system, and dynamic cables — driving up cost and complexity.
– Direct drive wind turbines, like Siemens Gamesa’s SG15-236, use large permanent magnet generators (PMGs) without gearboxes. While they offer high reliability and lower maintenance, the generator itself is extremely heavy and voluminous, especially at multi-megawatt scale. This poses a challenge for floating applications, where mass and center of gravity must be tightly controlled.
– Medium-speed wind turbines, such as those used by Vestas in offshore platforms, incorporate a two-stage gearbox and a smaller generator. Despite the added mechanical complexity, this architecture allows for a lighter and more compact nacelle, which is a major advantage for floating wind.
To put it in figures, we are talking about specific models such as SGRE’s SG15-236, where the generator alone weighs almost 250 tons, while its main competitor, Vestas, has a much lighter generator in its V236-15MW, which even compensates for having a gearbox, resulting in a lighter RNA.
Challenge: Offshore environments demand exceptional reliability. The integration of gearboxes into offshore wind turbines introduces significant challenges in terms of maintenance complexity and operational costs. Gearboxes, being mechanically intricate components, are prone to wear and failure over time, which necessitates regular inspection and potential replacement.
In offshore settings, such interventions are logistically demanding and financially burdensome. They often require the deployment of specialized jack-up vessels—high-cost marine platforms capable of stabilizing operations at sea—as well as highly trained personnel equipped to work under harsh maritime conditions.
These requirements not only elevate the direct costs of maintenance but also increase downtime, impacting energy production and overall project profitability. Consequently, minimizing mechanical complexity, such as by adopting direct-drive systems, is often favored to enhance reliability and reduce lifecycle costs in offshore wind applications.
5. Conclusion
Medium-speed drivetrains are gaining traction across both onshore and offshore wind sectors. Leading OEMs like Vestas, Goldwind, and Mingyang are driving this shift. Meanwhile, others such as Siemens Gamesa, Nordex, and GE continue to rely on high-speed or direct drive technologies, depending on market demands and strategic focus.
About the author:
Pablo Carazo Morales is the WTG Lead Engineer at Iberdrola, serving as a global technical authority for onshore and offshore wind turbines. With over a decade of experience, he has contributed to billion-euro offshore projects including East Anglia 3, Baltic Eagle, and Vineyard Wind. Passionate about advancing wind energy, Pablo specializes in WTG technology assessment, design, testing, manufacturing, and O&M strategy. He is driven by a deep commitment to renewable energy, continuous learning, and solving complex engineering challenges that accelerate the energy transition.