Posted on 01 November 2019

Nd-Fe-B Magnets are Going Offshore in Wind Mill Generators

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Wind Energy in Europe: Status, Hints and Potentials

Using permanent magnets in wind mill generators may be of advantage regarding total weight and in special designs even the possibility to incorporate reliable easy gearboxes to reduce maintenance, effort and costs.

By Dr. Denis Rollik and Bernd Schleede Vacuumschmelze GmbH & Co. KG, Hanau, Germany


The use of Wind Energy has a long history. Besides the possibilities of driving mechanical gears, such as grain mills or water pumps, the combination with a generator to produce electrical energy has been in focus from the very beginning. With today’s electronic possibilities, it’s no problem at all to run a wind mill generator in such a way, that the power is directly fed into the power-line network. Based on today’s technology the selection of the location to erect the wind mill is more or less independent from the location of the energy consumer. Therefore the general trend is to increase the size and power of the individual wind mill, select a near or off-shore location to erect the tower / mast and focus on extremely reliable and maintenance free operation. Especially when operating off-shore the maintenance issue becomes an overruling factor, since maintenance or even replacement of major components such as gears or generators are virtually impossible. In Europe the possible locations of erection is decreasing, due to the fact that the regions, where the wind is sufficient to operate a wind mill effectively, are limited. One of the key arguments against wind mills are the noise emission and optical appearance which mutilate the landscape. At least these arguments are of minor impact for off-shore installations.

In general the average size of a wind mill is continuously growing to sizes of up to 5 MW or even 7 MW as recently announced by GE. For all sizes these generators may be of asynchronous or synchronous type. In case of a synchronous generator the excitation is either electrical or with NdFeB permanent magnets.

The total capacity of installed wind mills in Europe is predicted to increase dramatically within the next 15 years:

     2003 installed        28 GW On-shore + 1 GW Off-shore
     2010 predicted      65 GW On-shore + 10 GW Off-shore
     2020 predicted      70 GW On-shore + 70 GW Off-shore
                                                                  EWEA figures (set in 2003)

Assuming almost all off shore installations to be in the range of 1 MW to 5 MW of power and considering per today only 1 GW of off shore installation is in place, EWEA expects almost the full load of 70 GW off shore to be erected within the next 15 years. By adding only 30 GW of the on shore potential, we end up with about 100 GW of power all in all. Assuming an average power level of 3.5 MW per wind mill we are looking forward to about 30,000 installations of high power wind mills (> 1 MW) during the upcoming 15 years in Europe. Not counting a possible demand in South East Asia and the USA, no doubt this industry may be one of the fast growing industry world wide.

A share between the different generator types is still uncertain. Even only with an estimate of 15% of installations using NdFeB permanent magnets a total market in Europe may be foreseeable of about 30 million € per year for the next 15 years.

In some cases permanent magnets have been mounted on rotors to build a synchronous generator.

While the asynchronous high power generator ( up to 5 MW type ) wind mill needs a very heavy gear box with the weakness that at “high load @ high speed” failure may occur, a synchronous type may only use a simple lighter reliable gear. Thus in the case of off-shore erection the risk of additional maintenance can not be neglected.

Comparison of technologies for 4 -5 MW wind mill generators

Direct drive

Currently there is only one manufacturer on the market with a direct driven 4.5 MW windmill: ENERCON ® E 112 with a 114 m diameter of the blades and a height of 124 m of the rotor axis. The big diameter ring is directly attached to the hub and has a typical speed of 6 to 15 rpm. The unique design of the ring generator has a diameter of about 8 m and is electrically driven. In principle this electric excitation could be directly replaced by NdFeB permanent magnets. Such a design change could save up to 70 tons of weight (copper and steel). The application of permanent NdFeB magnets of such design has been proven in several prototypes e.g. VENSYS ® of 1.5 MW without any problems.

For off-shore application the additional weight is a major handicap for this type of design. The high weight of the nacelle calls for a more stable / stiff and heavier tower especially with high gust of wind, which is typical for off-shore application. This may cause severe problems apart from leading to higher costs in construction and maintenance.

Standard gears 1 :100 to 1:200

The traditional construction of big wind mills is using a gearbox to increase the speed of rotation level by a factor 100 to 200 from typical 12 rpm to 2000 rpm. The asynchronous standard generator – even for 5 MW – is rather small at such a high level of rotational speed. The majority of today’s wind mills are equipped with such combinations of gearboxes and generators. The maintenance of the gearbox contributes to the majority of the maintenance-costs. High gust of wind may be a risk especially when operated off-shore.

With planetary gear 1 : 10

As a “hybrid” between the two a.m. concepts a solution has popped up recently based on a single and reliable one stage planetary gear and thus a generator running at 50 to 200 rpm. This so called “MULTIBRID” concept has the lowest weight of all three solutions and has been successfully tested in installations of 1 MW, 3 MW and 5 MW. All three sizes of generators are produced by using NdFeB permanent magnets ( typically VACODYM® 655 ) sub-assemblies. A solution with electric excitation is not being considered.


All three principle designs are competing against each other in the market. In all three cases, solutions with or without permanent magnets are feasible. Considering the total costs per device of approximately 4 million € and the typical costs of NdFeB permanent magnets assemblies being a share of approximately 0.1 million €, the decision in favour or against NdFeB permanent magnets can only be : Do the future applications of wind mills call for weight reduction and reduction of maintenance, which in off-shore application may be the only reasonable solution? In this case with the latest high end NdFeB permanent magnets such as VACODYM® 655 or VACODYM® 863 grades including latest coating VACCOAT® 10047 or VACCOAT® 20011 and assembly technology there is a very good chance for VACUUMSCHMELZE GmbH & Co. KG, Hanau , Germany, to participate in the upcoming market.

Reqirement profile for NdFeB magnets and assemblies

There are several requirements for permanent magnets and subassemblies which are mandatory to be fulfilled, especially as soon as the devices are placed off-shore: humidity; salty atmosphere, high gust winds, and vibration.

Material grade

As mentioned before, the total weight is an important criteria for such machines, thus the remanence level Br should be as high as possible at a specific temperature and magnetic load line (typically VACODYM® 655 or VACODYM® 863). Material grades to fulfil all requirements are available with most of the established and especially licensed NdFeB magnet manufacturer.

Corrosion stability

Because of the harsh environment the generator producers are calling for a corrosion resistant NdFeB grade and an appropriate coating. A typical test requirement is: 240h @ 130°C and 95% relative humidity to withstand and to prove a mass reduction of max. 2 mg / cm² for an uncoated magnet after 10 days of exposition. In addition a salt spray resistance according to ASTM B 117 or DIN 50021 of the coating VACCOAT® 10047 or VACCOAT® 20011 is mandatory.These requirements are well known and have been proven to both VACODYM 6xx and VACODYM 8xx series with our Al- Spray coating.

Typical values for VACODYM® 6xx with VACCOAT® 10047 according to ASTM B 17 or DIN 50021 standards are:
     > 96 h             coating thickness        > 5 µm
     > 240h            coating thickness        > 15 µm
     > 500h            coating thickness        > 20 µm
     > 1000h          coating thickness        > 30 µm

Bonding procedure

As most magnets are mounted to sub-assemblies the handling and the gluing of big magnet blocks need to be performed in an absolute reliable condition.

A pure Nickel coating of the magnet may not always be the best solution, since the glue and its adhesion on a steel back-iron deteriorates due to the lack of surface roughness of Nickel.

One further item needs to be considered very carefully, the fact, that the mismatch of thermal expansion coefficient of NdFeB magnets and steel can be up to 10 x 10-6 / K, thus causing internal mechanical stresses within the glue. Therefore we may in some case suggest to coat only 5 sides of a magnet block leaving one side uncoated to be glued directly onto the steel iron back-joke.

All this includes the selection of preferred glues, correct preparation of the surfaces, controlled curing of the glue and meaningful testing procedures for the bonding strength. The harsh environment together with the temperature shifts and all forces during assembly and operation is a challenging task for appropriate glue selection.

Mounting to Rotors and Bandaging

The last step a magnet manufacturer is typically involved in is the mounting of sub-assemblies to the rotor. This usually requires a lot of experience and a very special and unique tooling to handle high magnetic forces.

A bandage of carbon fibres / glass fibres or stainless steel hood may be applied to the individual modules prior to mounting. Alternatively the fully assembled rotor may be bandaged with a carbon or glass fibre-wrapping. In this case the mounting of the rotor into the stator is a very tricky and impressive procedure as extreme magnetic forces are to be kept under control.


The current tendency to bigger wind mills offers – at least in Europe -- a rather big market potential for high-end NdFeB magnets. On top of the pure magnet material, special know-how regarding coating, gluing, bandaging and mounting of sub-assemblies of very heavy devices is required.

Using permanent magnets in wind mill generators may be of advantage regarding total weight and in special designs even the possibility to incorporate reliable easy gearboxes to reduce maintenance, effort and costs. Especially in off-shore applications these factors will draw more attention to, as foundation costs erection time are directly linked to the overall weight of a wind mill. High gust winds need to be considered and maintenance cost need to be minimised by using high end technology.



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