Generator manufacturer adds a 7-MW offshore PM unit

 

A manufacturer of permanent magnet (PM) generators and power converters for the wind energy market, says it will be expanding its portfolio with the addition of large PM generators for low- to medium-speed applications primarily intended to meet the growing market for offshore wind turbines with rated outputs within the range of 6 to 8 MW.

The 7-MW PM generator class builds on the foundations laid by the company’s existing range of medium and high-speed generators for onshore installations. By scaling up existing designs that are already in advanced development or early production, Danotek says it can offer wind turbine OEMs, prototype systems optimized to their requirements within 12 months. Both direct drive and medium-speed drivetrains are applicable to offshore installations, the preferred solution being a compromise to the often divergent goals of maximizing reliability (by simplifying gearbox design), minimizing weight, increasing efficiency, and maintaining a low capital cost. Direct drive, low-speed PM generators are physically much larger but allow a less complex drivetrain by removing the need for a gearbox. A single or two-stage gearbox with a medium-speed PM generator is said to deliver a highly reliable drivetrain without incurring the significant additional weight and costs associated with large diameter, direct-drive generators. Danotek’s new product family will be able to scale direct drive and medium-speed drivetrains.

The offshore wind market has been steadily growing globally. From today’s installed base of 4.1 GW, offshore wind is forecast to grow at a CAGR of more than 60% to exceed 70 GW by 2017 (Pike Research, 2011, Global Wind Energy Outlook Report). Growth in offshore wind is primarily driven by increasing demand in Europe and Asia. One of the challenges facing developers of offshore wind projects is the reliability of the wind turbine drivetrain. Traditional double-fed induction generators (DFIG) have the disadvantage of requiring extensive, and potentially expensive, routine servicing of the brush gear and slip-rings. PM generators completely avoid this maintenance headache.

In June 2011, a consortium led by the National Renewable Energy Laboratory (NREL) that includes Danotek, was one of six groups selected for an award by the Department of Energy (DOE) to advance next-generation designs for wind turbine drivetrains. The project will focus on reducing the cost of wind energy by optimization of the wind turbine drivetrain, with technology developed being scalable to drivetrains of 10 MW or more. Commenting on the programs’ synergies, says Danotek CEO Don Naab said “We anticipate leveraging knowledge gained from the DOE program to enhance and accelerate our 7 MW offshore generator program.”

Danotek Motion Technologies
www.danotekmotion.com

Customizable generators come with standard base

Recent wind power generators from ABB combine a standard-base construction with customized interface connections to lower costs for turbine manufacturers and shorten delivery times. “It all began when our market intelligence revealed big changes taking place in the wind turbine industry,” says ABB R&D Manager for Wind Power Generators Raimo Sakki. “Previously, such generators had proprietary components, custom-designed to fit an individual  manufacturer’s turbine. Turbine manufacturers were showing interest in standard generators – provided they were designed for wind turbines.”

So ABB made the decision to complement its proprietary generators with a new range of standard units based on its own platform. Market surveys helped detail requirements. While the new generators would be standardized as much as possible to maximize the benefits of large-scale production, they also needed flexibility to accommodate different manufacturers’ interfaces. ABB chose a modular approach, building the required interface flexibility around a core made from a relatively small number of basic components.

“The most difficult task was defining the basic design requirements,” says Sakki. “With that done, we could get on with development work which was made easier because the new products are based on the proven technology of our earlier designs.” The company began building the first prototype, but soon faced a new challenge. “Potential clients began giving us their requirements. As a result we ended up ‘aiming at a moving target’ and making several different prototypes.”

The first product family built on ABB’s standard platform is the new 1.5 to 2.0 MW slip-ring generator series. Launched in June 2010, the new generators have been developed to fit most doubly-fed (DF) turbines. They feature an enhanced rotor design with patented carbon-fiber winding-end support rings. This feature enhances overspeed tolerance and improves cooling of the rotor winding and connections, resulting in better overall reliability. The company says the new platform is easily expandable and serves as a basis for permanent magnet (PM) and induction generators.

In September 2010, the company launched another product series, the 2.5 to 3.5 MW high-speed PM generators, which shares the same basic platform as the DF series. In fact, they are mechanically interchangeable (i.e., wind-turbine manufacturers may use the same drivetrain design for both types of generators). This makes it easy for turbine manufacturers to expand their existing DF offering to also include full converter PM turbines. A customer using a DF system who would like to test a PM generator can order a unit with identical fixings and interfaces that can be simply ‘slotted in’ to replace the DF unit.

“One major benefit of the standard-platform approach is that development cycles speed up,” says Sakki. “That means faster prototype-delivery times for clients. Previously, a custom designed generator would typically take nine months to develop. The standard platform now cuts this time by about half. The engineering work alone has been reduced from four months to just four weeks.”

From the outset, the new generators were developed for a global approach to design, sourcing, and manufacturing. They can be manufactured at all ABB’s wind power generator plants, for instance, and sold to clients anywhere around the world.

“For example, a buyer in China can talk to our people in China, and the engineering work can be done in the local office there. The bulk of the work is therefore done as close as possible to the customer.”

ABB
www.abb.com

How do wind turbine generators work?

The wind industry uses induction and permanent- magnet designs. There are many variations of these two, but in general terms, the induction generator must be spun at about 1,000 rpm or more to produce useful power. It produces current by first generating an electric field by passing alternating current (ac) through a coil. A series of these coils are mounted on a rotating structure (a rotor) that is turned by the wind through the drive train. Surrounding the rotor is a stationary series of coils, a stator. When the electric fields on the rotor pass coils on the stator, the field induces a current in the stator coils which is conducted away as output. Induction generators are not self-exciting. That means they require an external power supply to produce a magnetic flux or field.

PM or permanent magnet generators use the high- field strength 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 expensive rare-earth elements. They are worth the expense because the PM generator needs no external power source to initiate a magnetic field, an advantage for wind farms in remote locations. The self-excitation 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. The design also reduces electrical losses.

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 are not available in significant quantities in the U.S. Should competition for PM materials increase, lead times and costs will increase.

Because gearboxes are expensive to maintain, wind turbine designers have been experimenting and commissioning turbines with drivetrains that have no gearboxes, which make PM generators essential. Still, the PM generator in such a design calls for a certain circumferential speed to function properly. This means the generator may be 5 to 6-m in diameter.

Report tells of global wind-turbine generators

A 134-page report gives an in-depth analysis of the Global Wind Turbine Generator market and provides forecasts up to 2020. This alternative energy research is said to give historical and forecast statistics for the period 2001 to 2020 for market size of the generators in $million and unit volumes. This research is also said to give detailed analysis of market structures of the technology and regulatory policies that govern it. Detailed information on key companies, their market share and a competitive benchmarking is included in the report, code ASDR-9511. Other topics include:

• Detailed market opportunities and challenges for present and potential markets
• Technological introduction to the types of generators
• Detail analysis of producers share, and geographic distribution of market
• Competitive benchmarking and market shares for major companies

The researchers say the report will let users:

• Identify key growth and investment opportunities in the global wind turbine generator market
• Identify key partners and business development avenues
• Position a company to gain the maximum advantage of the industry’s growth potential by developing strategies based on the latest operational and regulatory events
• Facilitate decision-making based on strong historic and forecast data

ASDReports
Asdreports.com

Low speed wind power generator could take weight out of the nacelle

The weight of wind turbine generators is a significant issue because weight translates to costs. The structural weight of a direct-drive generator, for example, can exceed 80% of the total weight on the tower. The structure is needed to overcome the force of magnetic attraction between stationary and moving parts. The attraction force, a result of the normal component of Maxwell stress, can be 10 times the torque producing shear stress. The function of the generator structure is to maintain an airgap between the rotor and stator.

Ideally, a directly-driven generator should produce moderate to high shear stress while negating the effect of the magnetic attraction. A new topology has potential to meet that challenge, without resorting to exotic structural or magnetic materials such as superconductors. The concept takes the active materials in the machine – copper, magnets and steel – and changes their relative positions to minimize the normal-force effects. The result is a structure that need only support the weight of the active components, leading to a reduction of about 55% compared to conventional permanent magnet (PM) machines. Results show the weight reduction while maintaining high efficiency at all loads. Experimental results from a 20kW, 100 rpm prototype verify the expected performance.

Design details

The new design is air-cored, meaning there is no iron in the stator so there is little attraction between rotor and stator. In two-sided axial-flux air-cored machines, the two rotors attract each other. Because the airgap-flux density, B, is lower than for an iron-cored machine, the shear stress, σ, is lower as shown by

σ ∝ BJ (1)

where J = current density, A/mm2. The current density in the generator coils is limited by copper losses and cooling requirements. To produce the same torque, T, the outer radius, ro, of an axial-flux machine must increase to accommodate the lower shear stress because

where kr = ratio of inner to outer radii.

The increase in machine radius, when moving from iron-cored to air-cored designs can cancel the expected reductions in structural weight. One wind turbine design from Goliath Wind OU, Estonia, proposes an ironless radial-flux generator which has no airgap closing force. This is accomplished with an ironless outer stator. Its generator has a large radius, R, held in place by a lightweight spoked structure. The ironless stator produces a large airgap, so its flux density and shear stresses are small. A large air-gap radius is needed because of the low shear stress. Equation 3 is the radial-flux equivalent of Equation 2,

where L = axial length. A previous simple study showed that air-cored machines are potentially lighter for a range of power ratings.

Developing a C-core machine

A logical development of such axial-flux disc machines is to increase the rotor-shaft radius. Because airgap normal forces act near the junction of shaft and discs, the discs can be made thinner and therefore lighter. Taking this further leaves a C-shaped cross section, where the limbs carry magnets and the stator winding is held independently between them. A further step lets flux cross the web of the C and makes the rotor out of modules each carrying a pair of magnets. Rotating the C-core modules 90° produces a radial flux machine. Increasing the axial length allow increasing the radial-flux generator’s torque rating without increasing the outer diameter.

This topology has advantages over existing ironless designs. A radial-flux ironless permanent magnet machine has a large effective airgap. This C core machine, however, has a smaller airgap length making possible higher flux densities and shear stresses. A corollary is that less permanent-magnet material is needed to produce the same flux density, so the design will be cheaper. This machine also has the advantage of two main flux paths, longitudinal and transversal, not just one. Because the amount of magnetically active steel depends on its non-saturation, this should be a lighter design than an axial-flux, two-disc machine.

Active and inactive material

The new topology is structurally superior to an iron-cored machine. In a conventional radial-flux machine, large airgap normal forces can act at distances of several meters from points where these forces can be reacted against. This implies that the rotor and stator structures must be stiff, large, and heavy. In contrast the new machine has no forces on the stator. Although the two limbs of the C-core are attracted to each other, normal stresses are reacted at points within the C-core – close to their point of application. This topology means that the steel in the C-core fulfills both active and inactive roles.

To test the ideas, a 20kW, 100rpm generator was designed at the University of Edinburgh and built by Fountain Design Ltd. (fountaindesign.co.uk) It is instructive to describe the build sequence and highlight topology’s manufacturing advantages. The rotor of the prototype C-core machine was made of 32 modules, each carrying a pair of permanent magnets similarly oriented. In this small machine, the C-core module was assembled from three trapezoidal pieces of mild steel, with magnets sliding onto the inner and outer pieces. For larger machines, magnetic material could be glued in place and magnetized later.

This would ease the problems of handling large magnets. Modularity allows the cheap and efficient production of large volumes. An assembled module is quite benign and safe to handle, because there is relatively little leakage flux outside the confines of the C core. The modules can be brought together and fixed to a common rotor structure. The prototype uses an aluminum disc.

Bringing together a rotor and stator can be a difficult and dangerous task in a conventional permanent-magnet machine because of the large magnetic attractions. Doing so is liable to pull either the rotor or stator off center and produce an unbalanced magnetic pull, thereby closing the airgap clearance even further.

A feature of the C core machine is that because there is no iron in the stator, there is no force of attraction between rotor and stator. This makes a straightforward task of threading the stator winding into the rotor. The stator in the prototype is made of 24 pseudo arc shaped concentrated coils, clamped between two rings. The coil’s discrete nature means they will be easy to replace. This will be a significant advantage in larger machines, as electrical faults are one of the more common causes of failure in direct-drive wind-turbine generators.

The prototype

The 20kW prototype machine generated a perfectly sinusoidal no-load voltage waveform of 26.7 Hz frequency at 100 rpm. A power-versus-efficiency chart gives the mechanical-to-electrical efficiency of the prototype generator over a range of speeds for a range of loads, typical for this size machine. Results show that the design matches the performance of conventional PM synchronous generators.

Thinking bigger

Conventional PM machines tend to have optimal aspect ratios (axial length to airgap diameter). In terms of electromagnetically active material, less of it is needed with a small aspect ratio and large airgap radius, R. This is because the active weight is almost proportional to the airgap surface area (2πRL) whereas the torque, according to Equation (3), is proportional to R2L. Increasing the radius therefore increases the specific torque (with respect to the active weight).

There are two limits to how small the aspect ratio can be. There is a practical limit to how big the airgap diameter can be so that the generator can be transported and fit into a nacelle. The second limit is the structural material: the structural weight of a radial-flux machine is proportional to the square of the airgap radius (for a constant axial length and for a deflection fixed in relation to the airgap clearance). The C-core machine follows the same scaling laws for active weight, but has a different law for the structural material.

In C-core modules, limbs deflect into the airgap by:

where y = deflection, w = uniformly distributed load and a product of the normal component of Maxwell stress and the width of the limb. l = length of the cantilevered beam, and A = the second moment of area.

For a fixed axial length and a trapezoidal cross section, the weight of structural material, wstr (needed to limit y to a fixed proportion of the airgap clearance) is related to the airgap radius by:

This means the specific torque with respect to structural weight also rises with increased airgap radius.

The Generator weight and airgap charts show the generator weight based on the C core concept for 100 kW to 2 MW wind turbines. The electromagnetic design used the same basic pole pitch layout as the 20kW machine, with the number of poles and coils varied proportionately to the airgap radius.
The airgap clearance above and below the coil was taken as 0.1% of the airgap diameter. The magnetic flux was modeled using a simple lumped parameter magnetic circuit and the iron was not allowed to saturate.

For structural modeling, the maximum deflection of a C-core limb was restricted to 10% of the airgap clearance. Generator mass versus power show results for five different axial active lengths. As expected, designs with an active length of 0.4m are lightest because they reduce the active and structural weight. At larger ratings, these axially short machines may not be practical because of their large airgap diameters. Even the axially longest have great promise because these designs are not yet optimized for minimum weight.

Novel magnetic-field manipulation leads to unusual generator

One way to wring cost out of a wind turbine is to eliminate the gearbox and at the same time, increase the power density of the generator which would allow reducing its size and weight. Advanced Magnet Lab, Palm Bay, Fla., says it has a generator design that may allow doing so. It uses the company’s 2002 physics discovery that produces transverse magnetic fields without harmonics or fields that are more uniform than those based on the modulation of solenoid windings.

The AML development called Double-Helix includes CNC manufacture methods that need no special tools or fixtures to provide precise placement of conductors on curved surfaces for single and multi-layer geometries.

In the simplest Double-Helix design, a conductor is wound around a coil at about a 45° to perpendicular. A second layer would be wound at the opposite 45° so conductors cross about 90°. This gives a rotor or stator for a motor or generator with two poles. With a computer precisely controlling the location of the wire, and depending on the diameter of the core, which can be a cylinder, the two layers can cross at 90° many times giving a pure sinusodial field distribution of many poles. The accompanying image of red-wire coils shows a six-pole magnet which could work as either a rotor or stator. AML Chief Scientist Dr. Rainer Meinke invented the Double-Helix theory, software, and manufacturing.

At a recent conference, the company  introduced its design for a10-MW and larger superconducting generator for wind turbines that would need no gearbox. In addition, it would weigh only about 65 tons versus the 500 tons needed for a conventional design with gearbox. AML President Mark Senti did say the company has a generator design with most components already proven, and is ready to build and demonstrate a subscale prototype by 2011.

Senti says the company “packages” its technology based on an integrated process of 3D coil design using the company’s CoilCAD software. “It allows finding a best magnet-based solution for application requirements instead of forcing conventional coil configurations, such as saddle or racetrack coils, to generate the required fields.” Pick here for an animation of a Double-Helix Coil.

The Double-Helix (DH) design leads to other possibilities. For instance, suggests Senti, take a copper cylinder and using a small diameter mill or a laser, cut in the cylinder along a path normally reserved for a conductor. For physical stability during machining, the thin copper cylinder is firmly mounted to a substrate. Now, says Senti, the cylinder becomes the coil and the cut paths (air gaps) insulators, or they can be filled with an insulator. The result is that the conductor cross sections (cylinder wall thickness by distance between cuts) can be a optimized to  to significantly increase the magnetic field strength. “The current densities possible are two to five times what anyone has achieved, they approach superconducting power densities. The result is a stronger magnet that produces higher fields at reduced heat loads,” he adds.

The firm calls this variation Direct Double Helix (DDH) and says it works in resistive-conducting and superconducting applications. The magnetic fields are also more uniform with low harmonics, lower than those produced by other methods. In addition, rotors made with DDH in small-diameter motors could hit higher speeds than conventional design of a similar size because of they would be lighter.

Although the designs use conventional materials, more recent nanomaterials such a Graphene with its high thermal conductivity can be considered for magnetic applications because of the physical properties of Double-Helix and manufacturing techniques of DDH.

Additional applications include medical treatments and diagnoses in which DH and DDH allow greater precision for magnetic-field control while significantly reducing a system’s size and cost. For example, charged-particle-beam lines using protons (Proton Therapy) eliminate the need for radiation because a narrow proton beam can hit a tumor and miss most all normal tissue.

Generators and services for wind turbines

Electrical design firm Potencia developed a 5 kW wind turbine in 1975 but has evolved as a designer and supplier of generators for the wind-turbine industry. The company has since developed 500 kW induction generators, and 750 kW doubly-fed wound-rotor generators in the 90′s. More recently, the Mexico-based company has developed permanent-magnet generators for multi-megawatt turbines for Clipper Windpower, Carpinteria, Calif.
Potencia says it has manufactured generators classified as induction, wound rotor, salient pole, and permanent magnet, and that its custom generators can meet any specification. The firm adds it is aware of the need for high-reliability in equipment and so guarantees a lowest temperature-rise and highest efficiency for its units. The generators are manufactured with U.S. materials and dollar-based competitive prices with shipping to any place in North American.

The company also refurbishes several generator designs. The tables list a few specs for a new Leroy Somer, wound-rotor generator in a 660kW unit, and then performance figures for before and after it was refurbished.

A few specs for a Leroy Somer generator

Performance figures for before and after rebuilding the generator