Abstract: Permanent square neodymium magnets wind turbines use high magnetic performance sintered neodymium iron boron permanent magnets. The coercive force is high enough to prevent the magnets from losing magnetism at high temperatures. The life of the square neodymium magnets depends on the base material and surface anti-corrosion treatment. Corrosion prevention of NdFeB square neodymium magnets should start from manufacturing.
Keywords: permanent square neodymium magnets wind turbine, high performance sintered NdFeB, magnet reliability.
I. Introduction
The direct-drive permanent magnet wind turbine uses the fan impeller to directly drive the generator to rotate, eliminating the speed-increasing gearbox required by the traditional AC excitation doubly-fed asynchronous wind turbine, and avoiding gearbox failure and maintenance during operation. At the same time, the permanent magnet wind turbine adopts permanent magnet excitation, has no field winding, and does not have collector rings and brushes on the rotor; therefore, it has a simple structure and reliable operation.
From 1993, Germany’s Enercon GmbH developed the first large-scale direct-drive permanent magnet wind turbine, to 2008, Xinjiang Goldwind Technology Co., Ltd. produced hundreds of 1.5 MW direct-drive permanent square neodymium magnets. The development of wind turbines and permanent magnet wind turbines is in the ascendant. The overall level of China’s permanent square neodymium magnets wind turbines is at the forefront of the world.
The working environment of wind turbines is very harsh. They must be able to withstand high temperatures, severe cold, wind and sand, humidity and even salt spray. The design life of wind turbines is generally twenty years. At present, both small wind turbines and megawatt-level permanent magnet wind turbines use sintered NdFeB permanent square neodymium magnets. Therefore, the selection of the magnetic parameters of NdFeB permanent magnets and the requirements for the corrosion resistance of the square neodymium magnets are very important.
2. Typical magnetic properties of sintered NdFeB used in permanent magnet wind turbines
NdFeB permanent square neodymium magnets is called the third generation rare earth permanent magnet and is the permanent magnet material with the highest magnetic performance so far. The main phase of sintered NdFeB alloy is the intermetallic compound Nd2Fe14B, and its saturation magnetic polarization intensity (Js) is 1.6T. Since the sintered NdFeB permanent square neodymium magnets alloy is composed of the main phase Nd2Fe14B and the grain boundary phase, and the orientation of the Nd2Fe14B grains is limited by process conditions, the current maximum remanence of the magnet can reach 1.5T.
The German vacuum smelting company (Vacuumschmelze GmbH) produces NdFeB magnets with a maximum magnetic energy product (BH) max of 57MGOe. Domestic NdFeB manufacturers can produce N50 grade magnets with a maximum magnetic energy product of 53 MGOe (Editor’s note: This article was published in 2010. With the development of technology, N54 grade square neodymium magnets are currently on the market, with a maximum magnetic energy product of 53MGOe. 55MGOe).
Increasing the proportion of the main phase of the alloy, increasing the orientation of the grains and the density of the magnet can increase the maximum magnetic energy product of the magnet; however, it will not exceed the theoretical value of the maximum magnetic energy product of single crystal Nd2Fe14B, 64MGOe.
The demagnetization curve of NdFeB at room temperature is approximately a straight line. Therefore, when designing a permanent square neodymium magnets motor, high-grade NdFeB (that is, the material has a high (BH) max) is often selected to obtain a high air gap magnetic density. When the motor is running, due to the existence of the alternating demagnetizing field and the demagnetizing effect caused by the instantaneous large current when the load changes suddenly, it is required to select NdFeB square neodymium magnets with sufficiently high coercive force.
Adding elements such as dysprosium (terbium) to the alloy increases the intrinsic coercive force (jHc) of NdFeB, but the remanence (Br) of the square neodymium magnets will decrease accordingly. Therefore, high-performance NdFeB magnets for wind turbines take into account both its coercivity and remanence.
3. Temperature stability of NdFeB permanent square neodymium magnets
Wind turbines work in the wilderness and are subject to severe heat and cold; at the same time, motor losses also cause the motor temperature to rise. The sintered NdFeB square neodymium magnets given in the above table can work at 120℃. The Curie temperature of NdFeB permanent magnet alloy is about 310°C. When the temperature of the square neodymium magnets exceeds the Curie point, it changes from ferromagnetic to paramagnetic.
Below the Curie temperature, the remanence of NdFeB decreases as the temperature increases, and its remanence temperature coefficient α (Br) is -0.095~-0.105%/℃. The coercive force of NdFeB also decreases as the temperature increases, and its coercive force temperature coefficient β (jHc) is -0.54~-0.64%/℃. Select an appropriate coercive force so that the magnet still has a sufficiently high coercive force at the maximum operating temperature of the motor design; otherwise, demagnetization will occur.
The remanence and coercive force of NdFeB permanent square neodymium magnets materials are complementary. Adding heavy rare earth elements dysprosium (Dy) and terbium (Tb) to the alloy can significantly increase the coercive force of the magnet. As the coercive force increases, the remanence and maximum magnetic energy product decrease accordingly. Obviously, the choice of high coercivity magnets for wind turbines must be at the expense of residual magnetism and maximum magnetic energy product.
4. Consistency of magnetic properties of wind power NdFeB magnets
NdFeB magnets are manufactured using a special powder metallurgy process, in which the main manufacturing process is completed under a protective atmosphere or vacuum. The NdFeB green body is pressed in a very strong (~1.5T) magnetic field. The size of NdFeB magnets is limited by these special process conditions.
A large permanent magnet wind turbine usually uses thousands of NdFeB magnets, and each magnetic pole of the rotor is composed of many pieces of magnets. The consistency of the rotor magnetic poles requires the consistency of the magnetic steel, including the consistency of dimensional tolerances and magnetic properties. The so-called consistency of magnetic properties includes that the deviation of magnetic properties between different individuals must be small, and it also includes that the magnetic properties of a single magnet must be uniform.
Il magnetismo dell'acciaio magnetico è di due tipi: magnetismo apparente e magnetismo intrinseco. Il cosiddetto magnetismo apparente dell'acciaio magnetico può misurare il flusso magnetico a circuito aperto e l'intensità del campo magnetico superficiale. Il magnetismo apparente dell'acciaio magnetico è legato alla forma e allo stato di magnetizzazione dell'acciaio magnetico. Le caratteristiche intrinseche dell'acciaio magnetico vengono testate misurando la curva di smagnetizzazione del campione. La curva di smagnetizzazione fa parte dell'anello di isteresi, che riflette le caratteristiche di rimagnetizzazione del materiale del magnete permanente. Per misurare la curva di smagnetizzazione dei campioni di acciaio magnetico, è necessario che il campione sia magnetizzato a saturazione prima della misurazione.
To test whether the magnetism of a single magnet is uniform, it is necessary to cut the magnet into several small pieces and measure their demagnetization curves. During the production process, to test whether the magnetism of a furnace of magnetic steel is consistent, it is necessary to extract magnetic steel from different parts of the sintering furnace as samples and measure the demagnetization curve of the samples. Because the measuring equipment is very expensive, and it is almost impossible to ensure that every piece of magnetic steel being measured is intact. Therefore, full product inspection cannot be done. The consistency of NdFeB magnetic properties must be guaranteed by production equipment and process control.
5. Resistenza alla corrosione di NdFeB
NdFeB alloy contains active rare earth elements and is easy to oxidize and rust. In applications, unless the NdFeB is encapsulated and isolated from air and water, surface anti-corrosion treatment of NdFeB is required. Common anti-corrosion coatings include electronickel plating square neodymium magnets, electrophoretic zinc plating and electrophoretic epoxy resin. Surface phosphating treatment can prevent NdFeB corrosion in a short period of time in a relatively dry environment.
Rare earth intermetallic compounds can react with hydrogen under certain pressure and temperature. After NdFeB absorbs hydrogen, it releases heat and fragments. Hydrogen fragmentation in the production process of NdFeB utilizes this characteristic of NdFeB. From a usage perspective, the hydrogen fragmentation of NdFeB is harmful. Strictly speaking, the corrosion of NdFeB starts from its processing process. Degreasing after cutting and grinding, pickling before electroplating square neodymium magnets, and the electroplating process all have an impact on the surface layer of NdFeB. Improper processing may result in unqualified coating quality (such as pinholes) and weak bonding between the NdFeB surface layer and the coating.
It is worth noting that although the magnetic properties of the same brand of NdFeB square neodymium magnets produced by different manufacturers are basically the same, there will be differences in the composition of the alloys, especially the difference in the microstructure of the square neodymium magnets.
Magnetic steel with excellent performance and good corrosion resistance has the characteristics of fine and uniform grains and high magnet density. In the two metallographic photos of sintered NdFeB magnets below, the magnet shown on the left has fine and uniform grains, while the magnet shown on the right has larger and uneven grains.
6. Reliability test of NdFeB square neodymium magnets
The design life of the wind power generator is twenty years, which means that the magnets are required to be used for twenty years without significant attenuation of their magnetic properties and no significant corrosion of the magnets. The following test and detection methods can be used by manufacturers and users of wind electromagnetic steel to evaluate and inspect square neodymium magnets.
Test di perdita di peso: Utilizzare come campione un pezzo nero rettangolare di 10 mm × 10 mm × 12 mm (l'altezza di 12 mm è la direzione di magnetizzazione), collocarlo in un ambiente con 2 atmosfere standard, umidità 100% e 120°C. Dopo 48 ore, estrarlo e rimuovere lo strato di ossido. Una volta rimosso, la perdita di peso è inferiore a 0,2 mg/cm2.
Test di smagnetizzazione termica: 120℃×4hr, perdita di flusso magnetico a circuito aperto inferiore a 3%.
Test di shock caldo e freddo: Dopo tre cicli di alte e basse temperature da -40°C a 120°C, la perdita di flusso magnetico a circuito aperto è inferiore a 3%.
Il test in nebbia salina e il test di temperatura e umidità sono metodi per valutare la galvanica e altri rivestimenti anticorrosione.
Other physical properties, such as thermal expansion coefficient, thermal conductivity, resistivity and mechanical strength, etc., all have varying degrees of impact on the usability and reliability of magnetic steel.
