Wind turbine generator -ABB

PM or permanent-magnet generators use the magnetic field generated by magnets mounted on a rotor. Variations on this design put magnets on the stator and let the coils rotate. There are advantages to each.

The wind industry prefers magnets made of relatively expensive rare-earth elements for the field strength they generate. They are worth the expense because the PM generator then needs no external power source to initiate a magnetic field, an advantage for wind farms in remote locations. Selfexcitation also means a bank of batteries or capacitors for other functions can be smaller.

Other plusses for PM generators are that the high energy density eliminates some weight associated with copper windings, along with problems of degrading insulation and shorting. Another great advantage of PM generators is that a large-diameter design allows dispensing with the potentially troublesome gearbox.

On the downside, rare-earth magnets do not tolerate high temperatures. They can permanently lose magnetic field strength, which demands more from a generator’s cooling equipment. In addition, the cost of rare-earth-permanent magnets is a concern because key raw materials, mostly from China, are not available in significant quantities in the U.S. However, with the rise in PM costs, sources other China will likely come online in the next few years. That will keep prices in what has been a recent downward trend.

In addition, because gearboxes are expensive to maintain, wind turbine designers have been experimenting and commissioning turbines with drivetrains that have no gearboxes. This makes PM generators essential. Still, the PM generator in multi-megawatt machines call for a certain circumferential speed to function properly. This means the generator may be 5 to 6-m in diameter, and this size loses its weight advantage.

The solution may be a hybrid design in which a one or two-stage gearbox increases the rotational speed to provide a required output. This hybrid design also allows a lowest weight drivetrain, lighter than a direct drive design for a given power production.

Future generator designs may get around the rare-earth cost penalty and weight handicaps by using superconductive materials that work at temperatures a few degrees above absolute zero. These would also allow generators in 10 to 15-MW range without the weight penalty of conventional designs. At least two superconducting generator concepts are in development but don’t expect to see prototypes for at least three years.

Windpower Engineering & Development

Jukka-Pekka Mäkinen, The Switch President and CEO

The Switch came to the market five years ago on a mission to bring better drive train technology to wind power generation and allows more energy per turbine. The drivers today remain the same: better quality power, more energy, and more robust, compact design for PMGs.

Despite the technology transition, we’re living in a world now marked by constant turmoil. Although many things are moving forward, there is always some force pushing true success in the renewable energy industry down. This makes it impossible to predict volumes as before.

There is a demand for higher quality products with lower prices. The times are forcing turbine manufacturers to focus on their core competences and select value-adding partners who can carry responsibility for their products and services.

The players aiming to survive and thrive in the renewable energy industry must develop organizations with an ability to work in an networked manner. Vertical integration worked fine when there were shortages of components and the industry was still emerging. But those times are past. Vertically integrated companies find themselves facing challenges due to volatile market conditions when it comes to technology, production, and inflexible organizations. They’re feeling the pain of trying to do it all themselves.

The way forward for these turbine manufacturers requires a new way of working – even a new business model – that embraces cooperation and collaboration, which leads to greater effectiveness. The perfect business model during our unpredictable times is based on specialists that know how to network and add value for better end results.

Five reasons for optimism
In spite of market uncertainty, we see reasons to believe in a bright future. For example:

• Money is available from private equity, which hasn’t been there before. This is because the wind-power industry is finally mature enough and ROI becoming more attractive due to the short payback times of wind-power installations.
• Positive signs continue for offshore, especially in certain regions or countries such as Germany, France, Denmark, the UK – and eventually China and the U.S.
• Public opinion still favors renewable energy. Most governments still have it on their agendas despite the global economic crisis.
• Real advancement in nuclear power has stalled, creating gaps between the nuclear output planned and rising energy demands. The decisions in Germany are now being followed by other countries – and are putting renewable energy back into plans with greater interest.
• In good wind locations, wind power is the cheapest way of all to produce energy. Not only is it the safest, most secure energy generation investment with the shortest payback time, it is also the fastest to build.

China’s steps ahead
China has finally placed quality ahead of quantity and wants to evaluate the performance of the turbines they are installing. Growth in the overall market has slowed, but the value of higher performance equipment has been realized. Some Chinese power producers now specify PMG and fixed price contracts in their requirements.

China’s internationalization may not have materialized as expected a few years ago. The image of poor Chinese quality slowed the process. Nevertheless, the country’s turbine manufacturers and power producers have set their sights on internationalization. This will not be the “takeover” scenario of earlier feared by many, but rather a gradual and natural internationalization process, including localization of operations with new job creation.

Time: the biggest threat to growth
Time is one of the biggest threats to a rosier growth outlook. For example, how long will it take the Chinese government to regain trust and move forward? How long will it take for turbine manufacturers to get their offshore turbines developed and installed at sea for qualification? How long will it take before turbine manufacturers realize that vertical integration is an outdated model that will actually choke their future?

The way manufacturers handle their time pressure will be critical to their success. Aligning with partners long term can alleviate some of this pressure and lead to even greater added-value innovations for the industry at large.

Opportunities for the industry in 2012
We also see that permanent-magnet  technology has good application opportunities in areas such as marine. High-speed motors are also in wider use and replacing the geared systems used in compressors and pumps.

The graph plots investments in clean and fossil-based generating capacity from 2004 to 2010 ($billion). Public opinion remains favorable towards renewable energy and positive signs continue for offshore. Despite the volatile economic situation, there is growing proof that New Energy competes effectively as a source of power generation. The installed capacity of clean energy from 2004 to 2010 has nearly matched that of fossil-based generating capacity. Source: Bloomberg New Energy Finance, November 2011

We are confident about thinking once again like a winning start-up company – and are open to new partnerships and technologies that make products even better together. For instance, the horizontal supply chain collaboration between Moventas and The Switch led to the innovative FusionDrive wind turbine drive-train concept.

The company’s Model Factory concept lets clients move into new production areas such as, near-shore parks. This creates local jobs at locations convenient to final wind-farm sites.

The company is in a position to take more control in customers’ production facilities, ramping up and down in a flexible manner, and to enter regional production cooperation with them.

A recommendation for the future is to outsource risk to proven added-value partners, divide responsibility among specialists in a networked team for value-added business collaboration, and innovate new-generation turbines for the future.

The Switch
Theswitch.com

Windpower Engineering & Development

Motor efficiency can be determined with greater accuracy in MotorSolve v2.6 due to improved loss predictions.

The latest release of MotorSolve, electric-machine design software, is now available. Engineers from a wide range of industries use Infolytica software to design and analyze applications such as electromechanical devices, non-destructive testing (NDT), induction heating, power electronics, sensors, and industrial transformers.

The software sports new features, including synchronous-reluctance motor templates and improved loss predictions. Stator winding modeling has been enhanced to allow 3D viewing and more accurate end effect calculations.

The software allows accounting for mechanical factors such as friction, windage, and stray losses. Companies looking to design motors without permanent magnets, due to rising costs, can use the new synchronous reluctance templates which have been added to the package.

Other improvements in the latest release:

• Slot liner modeling
• Assymetric overhangs
• Vertical or horizontal layering of windings
• Magnetic impact of shaft components
• Improved temperature settings

MotorSolve v2.6 is available for PC’s running Microsoft Windows XP, Vista and 7. Maintained clients can visit support.infolytica.com to download this update.

Infolytica Corporation
www.infolytica.com

Windpower Engineering & Development

Tecogen of Waltham, MA, is a manufacturer of natural gas fueled engine-driven cogeneration systems. Since 2005, Danotek has supplied Tecogen with more than one hundred PM generators for the company’s cogeneration modules, specifically the flagship InVerde Ultra 100 CHP module.

Danotek says their PM generators are thermally optimized high power density systems that reduce operating costs by their performance and reliability. High efficiencies at rated and partial loads are critical in enabling Tecogen’s CHP systems achieve overall efficiencies exceeding 90%, and the lightweight generator’s unique reluctance design produces negligible cogging torque making its performance ideal for cogeneration and distributed generation applications. High performance is supported by robust operations, especially the PM generator’s low operating temperatures, high quality class H insulation, and lack of brushes and slip-rings that  enhance reliability and durability.

“Tecogen is pleased to continue the successful relationship we’ve had with Danotek,” said Mr. Robert Panora, Tecogen’s President and Chief Operating Officer. “Our InVerde Ultra 100 CHP systems deliver high efficiency and reliability that maximize our customers’ energy-related cost savings. With their excellent performance and low maintenance needs, PM generators developed by Danotek are vitally important components within our CHP systems.”

All forty PM generators are scheduled to be delivered from Danotek’s Canton, MI, facility in 2012 with deliveries commencing mid-February. They will be installed in colleges, schools, hospitals, nursing homes, large residential facilities, hotels, and similar facilities that have a demand for electrical and thermal energy.

“We highly value our close and highly cooperative relationship with Tecogen” said Danotek’s Senior Program Manager, Greg Bac, “This latest multi-unit purchase order is a further testament to our capabilities to deliver equipment and support services that add value to Tecogen. We are very excited about the products Tecogen is developing for the marketplace and being a part of providing customers with clean and efficient energy solutions.”

Danotek

Windpower Engineering & Development

An electrical discharge has found its way to ground through the bearing. The result is a “fluting” pattern on the race. There are many “band-aids” to remedy this situation. You must detect it before fixing it. The easily detected vibration appears in the chart to the right.

Most of the recent end-of-warranty inspections in the US market seem focused on gearbox health. This is rightfully so because the component has a rather high failure rate and associated costs. But what about the generator? How do you inspect the generator which is also a high-ticket item to repair? Moreover, what do you look for? You cannot exactly “bore scope” a generator as you can most of the gearbox. This is where vibration condition monitoring detects the three most common generator problems: misalignment, electrical discharges, and lubrication.

Misalignment between gearbox and generator is the first common problem. Paul Berberian of Easy Laser, a company that manufacturers wind turbine alignment tools, explains, “Misalignment is easy to detect and correct, but there are no standards per se because everything moves around so much. The gearbox moves back axially and differently from tower to tower. Secondly, temperature variations make for a wide margin of allowable misalignment.” In addition, environmental temperatures vary upwards of 100 degrees from season to season, and component temperatures vary due to non-wind days, seasons, and periods. The thermal growth and physical movement make for a loose alignment tolerance. Therefore, seeing misalignment in vibration readings is common, along with prematurely wearing high-speed-shaft bearings. Generator input bearings then become collateral damage.

There are three different types of directional misalignment. A moderately misaligned gearbox and generator will add stress on associated components. It is common to see post-alignment vibration readings significantly drop on the bearings on both sides of the coupling. But the location still must be monitored to see if permanent damage was caused by the corrected condition. Yes, it is possible to cause significant damage with an easy fix of an alignment check.

Motors and generators suffer from the same ailments, such as stray currents. These discharges find the path of least resistant to ground, often through bearings.

A vibration spectrum (velocity readings) will show a high-amplitude peak at the generator’s running speed. The higher this amplitude at 1x generator/hss (high speed shaft) running speed, the higher the level of misalignment. Vibration signatures show before (tall) and after an alignment in the accompanying plot. The “before” peak at generator running speed is quite high, likely affecting efficiency and jeopardizing reliability.

Electrical discharge in the generator is another common issue. The phenomenon occurs as stray electrical currents find a path to ground and do so through the generator bearing. Detecting electrical discharge in a generator provides an example of the electrical line frequency in a vibration spectrum.

The accompanying photo (below) also shows what happens when an electrical discharge is not addressed. It electrically flutes the bearing race, as shown by the clearly visible bearing outer race defects (BPFO: Ball Pass Frequency Outer race). If left to discharge, the bearing likely seizes.

By: David Clark/Condition Monitoring Consultant

Lubrication is the final common generator issue. Over lubrication and under lubrication both contribute to bearing failure. The chart, When generator bearings become discharge paths, is from another generator bearing in which the cage defect (FTFI labels) for this bearing shows multiple harmonics in vibration, albeit at low levels. Now would be the time to check the maintenance interval and lubrication before amplitude and damage increase. A follow-up reading to monitor changes would determine if the damage increases over time. BPFI indicates inner race defects for this generator bearing. The amplitudes are approaching an alert level in vibration. It’s time to lubricate and monitor for an improvement or decline.

All three common generator-related issues – misalignment, electrical discharge, and lubrication – are detectable in vibration measurements. In fact, wind-turbine manufacturers would be wise to monitor for these common failures prior to or during commissioning so the unit will not have under-warranty issues related to these failure modes. Obviously, owners would be wise to monitor for these common failure modes for the next 18 years out-of-warranty when they will have to foot the bill. Even wiser would be to inspect the generator as well as the gearbox during end-of-warranty inspections. WPE

(for full charts see November print issue of WPE)

Windpower Engineering & Development

A Danotek generator of this sort is in test on a Minnesota wind farm.

A developer of permanent magnet generators has passed a major milestone in the company’s growth following initial successful up-tower operation of its PM generators by a leading wind-turbine OEM. The milestone – the first up-tower validation of Danotek’s PM generator technology – followed extensive in-house testing at the Danotek facility and installation in a Minnesota-based multi-megawatt wind turbine.

The installation and testing of Danotek’s PM generators is being conducted in conjunction with the Eolos Wind Research Project, a wind research consortium formed by the University of Minnesota, Illinois Institute of Technology in Chicago, and the University of Maine, to support wind energy technology research, development, and career education focused on increasing wind turbine performance and reliability, and helping train the next generation of wind power industry engineers.

The satisfactory initial operation of the PM generator is a prelude to a comprehensive field test procedure that will continue into early 2012. Upon passing the three to four month up-tower test program, work on delivering an initial volume of PM generators is expected to commence.

“Successful facility and up-tower testing is a major achievement that validates Danotek’s approach to the design of PM generators,” says Don Naab, Danotek’s President and CEO. “It represents the culmination of two years of advanced development, design, and test work by our team of industry-leading technical specialists. We’re proud to be supporting the Eolos project and are confident our PM generators will make a major contribution to increasing both the performance and reliability of the consortium’s wind turbine generator.”

Compared with traditional generator technologies, PM generators provide higher efficiencies at part-loads, a significant advantage for wind turbines that often operate at loads below their rated output due to variable wind resources. PM generators developed by Danotek offer many additional advantages to wind turbine OEMs including higher efficiency at rated load, greater durability arising from maintaining low magnet temperatures, and extremely low cogging torque that lets a wind turbine commence operations at lower wind speeds.

Danotek
www.danotek.com

Windpower Engineering & Development

GE says its experience with superconducting equipment from its healthcare MRIs is applicable to superconducting generators for wind turbines. High torque at low rotational speeds may let wind turbines product up to 15 MW without a gearbox. See where the direct-drive superconducting generator is located in the wind turbine nacelle.

The technology development arm of a large electrical firm says it has begun work on the first phase of a two-year, $3 million wind project from the U.S. Department of Energy. GE Global Research (ge-energy.com/wind) says it will begin work on a next-generation generator for wind turbines that could support applications in the 10 to 15-MW range.

Conventional wind turbines often use a gearbox to increase slow rotor speeds to a higher rpm required by conventional generators. While such drivetrains are effective, they become expensive as they scale to larger wind platforms due to their additional weight and maintenance needs. It is possible to get additional power from larger drivetrains, only with an increase in the cost of electricity.

“New technologies will be needed to support larger-scale wind platforms,” says Keith Longtin, Wind Technology Leader, GE Global Research. He says the company will apply its experience with superconducting magnets used in healthcare MRI equipment. “Field windings are where we want to use the superconducting materials and cryogenics. So to leverage MRI experience, we will go with the topology of a rotating armature, sort of the opposite of a conventional generator.”

Longtin adds that superconducting technology may allow significant improvements to the generator and eliminate the gearbox. For example, magnetic fields would be larger from superconducting coils, even larger than those from rare-earth magnets. Hence, greater outputs from a similar size. The key is in reducing generator size and weight while dealing with lower shaft speeds and high torque. For size comparisons, Longtin says, “Our offshore turbine is rated for 4.1 MW, has a diameter of about 6 m, and weighs about 85 metric tons. We think with superconducting technology we can get 10 to 15 MW from about a 5-m diameter and the same weight. So that’s about three times the output.”

GE says the superconducting machine will use commercially available cryogenic coolers (for temperatures below 77°K) to improve the reliability of the complete machine. “We will investigate use of superconducting materials such as niobium-titanium, niobium-tin, MgB2, YBCO, and other second generation materials along with liquid nitrogen, helium, and neon to get the generator to superconducting temperatures, and techniques for staying there,” says team member Kiruba Haran, Manager of the Electric Machines Lab at GE Global Research.

The proposed superconducting machine aims to have more then twice the torque density of competing technologies and will further reduce dependence on rare-earth materials prevalent in permanent-magnet generators that are finding favor in recent turbines. The greater potential power from superconducting generators, coupled with better energy-conversion efficiency leads to more favorable economies of scale. For example, fewer towers would be needed for a given wind-farm output, which will help reduce the cost of energy produced by wind turbines.

The generator wind project will have two phases. Phase I will focus on developing a conceptual design and evaluating the economic factors associated with it. Phase II will explore potential commercialization of the technology. The Oak Ridge National Lab will be a partner on the generator wind project, helping investigate and mitigate high-risk technology challenges. Oak Ridge has facilities for more fundamental research, so they will run reliability testing on cryogenics. “It’s relatively easy to make something once and get the power needed, but can it be done in a reliable and cost effective manner over and over?” asks Haran. “We must do component tests to find the answer along with vibration and environmental tests for life data.” WPE

Windpower Engineering & Development

In the past, 95% of rare-earch material used in applications such as wind-turbine generators has been supplied to the world by China, now an unreliable source.

Increasing demand and a shrinking supply of rare-earth elements for magnets creates an opportunity for a research team at Oak Ridge National Laboratory and the University of Minnesota. The goal is to create a recipe for a replacement that doesn’t use scarce ingredients. The prospect of not having enough rare earth elements such as neodymium and dysprosium for magnets looms large for industries that need them.

“Worldwide demand for rare earths is expected to exceed supply by some 40,000 tons annually by the end of the decade,” says Larry Allard, a researcher in ORNL’s Materials Science and Technology Division. “In the past, 95% of the material has been supplied to the world by China, but in recent years China has begun limiting exports. By 2015 it is expected to become a net importer.”

Most people never give it a second thought, but magnets are used in everything from the motors that power hybrid vehicles and electric windows to windmills, computers, and hundreds of items that touch our lives every day. The traction-drive components of a Toyota Prius, for example, use about 2 lb of magnet materials while a 3-MW wind turbine uses 550 lb. From an economics and national security perspective, the shortage would be catastrophic.
ORNL’s David Parker commented that there’s nothing “sacred” about rare-earth elements. “Their main advantage is that due to their large nuclear charge, spin-orbit coupling is very strong and serves to fix the magnetization direction of the unpaired electrons. Other heavy elements may play the same role.”

Researchers Allard, Edgar Lara-Curzio and Mike Brady of ORNL, and Jian-Ping Wang at the University of Minnesota are focused on developing magnets made from abundant and inexpensive materials. Of specific interest is an iron-nitride compound with a specific phase that potentially exhibits the highest saturation magnetization ever reported for a material.

“This is a critical parameter related to the highest degree to which a material can be magnetized,” said Allard, who said this particular iteration of the iron-nitrogen compound has values up to 18% higher than the best commercial alloy, iron cobalt. The problem is that this material is metastable and exhibits relatively low coercivity, which means it demagnetizes easily. The best permanent magnets, such as those made of neodymium-iron-boride, score high in such areas.

The team will devise a method of producing this pure phase, iron-nitride compound and use specialized modeling methods to better understand the role of alloying additions that might stabilize the material so it retains its magnetic properties. The researchers hope to better understand the magnetic behavior of the “alpha double prime” phase by correlating microstructure at the atomic level to processing and magnetic behavior.

Once researchers characterize the elusive phase, their goal will be to make bulk quantities of the material and move toward their ultimate goal of replacing neodymium-iron-boride magnets for wind, automotive, and other energy technologies. This work with the University of Minnesota builds on previous work with Wang in which ORNL researchers were able to characterize iron nitride films with demonstrated potential. Allard noted that the Spallation Neutron Source made it possible to perform polarized neutron reflectometry, a test performed by Valeria Lauter to determine magnetic property.

In a separate effort, ORNL’s David Parker hopes to computationally screen dozens of materials and then mix elements that emerge as promising candidates in a way to create a compound that will behave like rare-earth elements. This material must also be scalable, retain its magnetic properties under many conditions, and meet cost-performance criteria. Parker noted that the compounds identified often have useful properties consisting of elements with greatly differing melting points, stabilities, and other traits, but can be difficult to manufacture.

“We have a suite of conventional and novel processing approaches to try to make the computationally predicted compounds, including a range of powder consolidation and gas reaction approaches,” said Parker.

ORNL
ornl.gov

Windpower Engineering & Development

Direct-drives wind turbines are just one application for permanent-magnet generators. Others may be better.

Dr. Daniel M. Saban, PE, SMIEE/Chief Technology Officer/Danotek Motion Technologies/www.Danotek.com

Wind-turbine manufacturers are looking for a drivetrain that delivers high efficiencies at part load, increased availability, and with a simplified grid-tie that protects the generator from grid-side disturbances. The designs should also be applicable to direct-drive, medium speed, and high-speed systems. Many wind turbine OEMs already know the answer lies in a rapidly maturing line of permanent magnet (PM) generators, those with great potential to enhance the financial performance of onshore and offshore projects.

A common viewpoint is that PM generators are applicable only to offshore direct-drive wind turbines, where their use removes the potentially troublesome gearbox. However, PM generators offer so many compelling advantages they are now being designed into drivetrains with gearboxes to operate across a wide range of output speeds.

Double-fed induction generators

Double-fed induction generators (DFIGs) have given the wind industry many years of excellent service. But the industry is changing. The number of undeveloped Class I (ideal) wind sites diminishes with each passing year while project developers are demanding improvements in plant reliability and availability. New market entrants bring increased competitive pressures between the wind-turbine-generator OEMs. Similarly, the cost of power converters has decreased significantly so that the comparative gain of a partial-rated converter is evaporating. In such an evolving situation, PM generators deliver a wide range of performance advantages to wind-farm developers and owner-operators that can make a significant and positive difference in project economics.

For example, PM generators are incredibly flexible. Their design can be customized to optimize performance of the complete wind-turbine drivetrain. Speed, efficiency, weight, and cost are four fundamental, inter-dependent factors that can be varied to meet the wind turbine OEM’s objectives. The PM generator designer can trade magnet, stator copper, and stator-lamination properties and weights to hit required goals without need for the turbine OEM to accept a suboptimal, off-the-shelf, standard generator. The marginally higher capital costs are paid back many times during the operating life of the wind turbine. The table highlights a few PM generator system advantages over a DFIG system.

PM generators and rare earth magnets

A modular drivetrain arrangement: One potential use of a PM generator would be in the traditional modular drive train as a replacement for the double fed induction design. The model of an integrated drivetrain arrangement (semi-combined) suggests a possible design.

The magnets on a PM generator’s rotor do, of course, use rare earth metals. Rapidly increasing magnet pricing over the past 12 months caused more than passing consternation within generator manufacturers and wind-turbine OEMs. Fortunately the peak magnet prices experienced this summer are already in decline with reductions of ~30% evident at the time of this writing. Note that magnets used within PM generators typically contain ~30% Nd (neodymium) by weight, with most of the remaining weight made up of low-cost ferrous materials.

The price of neodymium peaked near the end of July 2011 and has been dropping since. Other factors, such as the additional sources, are likely to bring its cost down further.

Current economic uncertainties have curtailed demand for rare earth metals, but of greater significance is the emergence of new rare earth supplies in the US, Asia, and Australia, and the partial relaxation of export restrictions within China. All indications are that magnet prices will continue to decline, the major unknown being the level at which magnet magnet pricing will stabilize[1],[2].

High speed to direct drives 

Integrated drivetrain arrangement: The schematic for an combined drivetrain combines a main bearing with the gearbox housing for a more compact design.

PM generators are flexible. They let wind-turbine designers consider drivetrains with a wide range of topologies from conventional modular gearbox and generator arrangements to fully integrated (or hybrid) systems assembled within a single enclosure, and even direct drive systems that work without a gearbox. Each topology has merits.

Modular drivetrains

A conventional high-speed drivetrain with fully independent, or modular, gearbox and PM generator appears in Modular drivetrain arrangement.

This topology provides the turbine OEM with maximum flexibility in selecting suppliers of the gearbox and generator because they are both fully independent. It does however produce a least-compact arrangement.

Semi-integrated drivetrains

By integrating some drivetrain components, the overall length and weight can be significantly reduced. This is accomplished by combining the main bearing and gearbox for a medium or high speed shaft design, or by close coupling the gearbox and generator as shown in Integrated drivetrain arrangement.

The model of an integrated drivetrain arrangement (semi-combined) suggests a possible design.

Close-coupled drivetrain

In a close-coupled gearbox and generator, the generator is overhung from the non-drive end of the gearbox. This arrangement, often found on medium-speed drivetrains with single or two-stage gearboxes, minimizes drivetrain length. The relatively simple physical connection between the gearbox and generator housing retains the turbine OEM’s flexibility to select independent gearbox and generator suppliers.

Improvements in gearbox technology, design, and durability now let medium-speed wind-turbine drivetrains with single or two-stage gearboxes achieve reliabilities once thought only possible with direct drives. PM generators designed to operate at speeds from about 125 to 500 rpm are significantly smaller and lighter than direct-drive equivalents.

For example, a 3-MW PM generator operating at 400 rpm has a diameter less than 2.5 m and weighs little more than 10 tons. A direct drive of similar capacity has a diameter of over 5 m. Today, medium-speed systems appear to be a preferred solution of wind turbine OEMs seeking the best balance between reliability, efficiency, size, weight, and cost.

Fully-integrated (or hybrid) drivetrains

The most compact medium-speed drivetrain topology features a fully integrated gearbox and PM generator with gears mounted inside the generator rotor. This arrangement couples suppliers for the gearbox and PM generator, and removes the flexibility of the wind-turbine OEM to independently select its preferred gearbox and PM generator suppliers. The degree of integration, however, typically involves more complex remedial work in the event of unscheduled maintenance.

Direct drives

By removing need for a gearbox and coupling the PM generator directly to the turbine rotor’s shaft, direct-drive systems provide the simplest drivetrain, which ought to deliver the highest reliability. However, this is difficult to achieve in practice.

Direct-drive topology

With increasing wind-turbine outputs, a performance advantage can be negated by the extremely large and heavy PM generator required to handle rated torque at relatively slow (10 to 15 rpm) rotational speeds. To illustrate, consider two 3 MW PM generators. The direct drive PM generator’s diameter is more than twice that of a medium speed (400 rpm) generator, and its weight is five times that of a more compact medium-speed machine. As outputs increase, size and weight differentials grow. A PM generator in the 8 MW class can have a diameter exceeding 7m and a weight exceeding 90 tons. As generator weight increases (approximately proportional to the square of its diameter) slower speeds add considerable cost to the generator and to the turbine’s structure including the tower.

A close coupled, medium-speed arrangement offers perhaps the best compromise: reduced drivetrain length and weight, high reliability and efficiency, and an opportunity to mix-and-match major component from a range of suppliers.

Only a small fraction of the wind turbines in operation today use PM generators. But with the superior efficiencies, higher reliabilities, and lower costs arising from greater market penetration, PM generators are set to become a major part of tomorrow’s wind turbine drivetrains. WPE

The model for a direct drive PM generator shows a one with a much larger diameter, over twice that of the close-coupled design.

What’s available now 

In a growing field of PM generator system providers, Danotek is the only U.S. based supplier currently developing and manufacturing highly efficient energy conversion systems in the 600 kW to 8 MW range for the wind energy and industrial markets. The company’s approach has attracted investments from energy venture capitalists including Khosla Ventures, CMEA Capital, and GE Energy Financial Services. Patents are pending on many aspects of the company’s PM generator and power electronics products for a wide range of new and existing applications, including wind energy, power generation, and variable-speed propulsion systems and accessory drives for electric and hybrid-electric vehicle markets. A few differentiators include PM generator designs with extremely low cogging and torque ripple, and a wide operating speed range that enables a lower speed cut-in for the turbine that increases energy capture, especially at sites with frequent low wind speeds. The company’s PM generator designs can be customized to accommodate either liquid or air cooling with minimal design changes. This lets the turbine OEMs design nacelles without major changes in the overall drivetrain assembly and thus reduce cost. The advantages of optimizing each PM generator to meet the needs of turbine OEMs gives partners the opportunity to influence the generator arrangement and customize it to meet the goals of the turbine program.

1. Financial Times, June 19th, 2011: “Rare earth prices soar as China stocks up.”

2. Wall Street Journal, September 22, 2011: “Rare-Earths Demand Eases, Sapping Prices”

Windpower Engineering & Development

The software Opera was selected as a design automation for its speed and efficiency, and its ability to cross-couple with other software used in the overall equipment design.

When designing new generators for wind turbines, engineers typically use the package’s 2D simulations to rapidly find ‘sweet spots’ in the design space. Siemens` wind turbine generators are large machines, with many poles, but the fast execution of Opera 2D simulations, says the company, means designers are free to thoroughly explore new architectural concepts.

Opera’s scripting capability lets designers quickly set up thousands of simulations. Because an individual 2D Opera simulation runs quickly, taking perhaps 10 minutes, and the design team has a computer that can run 80 simulations in parallel, thousands of potential design solutions can be explored and compared within a few hours. The ability to quickly assess so many design variants makes Opera useful part of the design process, enabling a level of optimization that would otherwise not be possible. After this 2D exploration, the team can characterize a best potential design solutions using 3D simulation.

Getting to a best design solution is always a balance between performance and cost. For wind turbines in particular, there are competing design goals such as generator efficiency and power output quality versus the need for compact physical size and the minimization of expensive materials, such as rare-earth permanent magnets. The benefits of using the analysis software for wind turbines are design excellence, combined with speed of development, and to minimize the need to manufacture costly prototypes.

Cobham Technical Services
www.cobham.com/technicalservices

Windpower Engineering & Development