Application of High Performance Sintered NdFeB Magnets in Wind Power Generators

Abstract: The permanent magnet wind turbine adopts high-performance sintered NdFeB permanent magnets, and the high enough coercive force can prevent the magnets from demagnetization at high temperature. The life of the magnet depends on the substrate material and surface anti-corrosion treatment. The anti-corrosion of NdFeB magnets should start from the manufacture.

Key words: permanent magnet wind power generator 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, which cancels the speed-up gearbox required by the traditional AC excitation double-fed asynchronous wind generator, and avoids the failure and maintenance of the gearbox during operation. At the same time, the permanent magnet wind generator adopts permanent magnet excitation, no excitation winding, and no collector ring and brush on the rotor; therefore, the structure is simple and the operation is reliable. From 1993 when Enercon GmbH in Germany 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 magnet The development of wind power generators and permanent magnet wind power generators is in the ascendant. The overall level of China’s permanent magnet wind turbines has been at the forefront of the world.

The working environment of wind turbines is very harsh, and it must be able to withstand the test of high temperature, severe cold, sandstorm, humidity and even salt spray. The design life of wind turbines is generally twenty years. At present, whether it is a small wind power generator or a megawatt-level permanent magnet wind power generator, sintered NdFeB permanent magnets are used. Therefore, the selection of magnetic parameters of NdFeB permanent magnets and the requirements for the corrosion resistance of magnets are very important.

2. Typical magnetic properties of sintered NdFeB used in permanent magnet wind turbines

NdFeB permanent magnet is called the third generation rare earth permanent magnet, and it 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 (Js) is 1.6T. Since the sintered NdFeB permanent magnet alloy is composed of the main phase Nd2Fe14B and the grain boundary phase, and the degree of Nd2Fe14B grain orientation is limited by the process conditions, the current maximum remanence of the magnet can reach 1.5T. The German vacuum melting company (Vacuumschmelze GmbH) produced NdFeB magnets with a maximum energy product (BH) max of 57MGOe. Domestic NdFeB manufacturers can produce N50 brand magnets, the highest magnetic energy product is 53MGOe 55MGOe). Increasing the proportion of the main phase of the alloy, increasing the orientation degree of the grains and the density of the magnet can increase the maximum energy product of the magnet; but it will not exceed the theoretical value of the maximum energy product of single crystal Nd2Fe14B 64MGOe. Sintered NdFeB Magnets in Wind Power Generators

The demagnetization curve of NdFeB at room temperature is similar to a straight line. Therefore, when designing a permanent magnet motor, high-grade NdFeB (that is, high (BH) max of the material) is often selected to obtain a high air-gap flux density. When the motor is running, due to the existence of the alternating demagnetization field and the demagnetization effect caused by the instantaneous large current when the load suddenly changes, it is required to select NdFeB magnets with sufficiently high coercive force.

Adding dysprosium (terbium) and other elements to the alloy increases the intrinsic coercive force (jHc) of NdFeB, but the remanence (Br) of the magnet will decrease accordingly. Therefore, the high-performance NdFeB magnets used in wind turbines take into account its coercivity and remanence. Sintered NdFeB Magnets in Wind Power Generators

Application of High Performance Sintered NdFeB Magnets in Wind Power Generators

Application of High Performance Sintered NdFeB Magnets in Wind Power Generators

3. Temperature stability of NdFeB permanent magnets

Wind turbines work in the wilderness, withstood the test of heat and cold; at the same time, the loss of the motor also leads to the temperature rise of the motor. The sintered NdFeB magnets given in the above table can work at 120°C. The Curie temperature of NdFeB permanent magnet alloy is about 310°C. When the temperature of the magnet exceeds the Curie point, it will change from ferromagnetism to paramagnetism. Below the Curie temperature, the remanence of NdFeB decreases with the increase of temperature, and its remanence temperature coefficient α (Br) is -0.095~-0.105%/℃. The coercive force of NdFeB also decreases with the increase of temperature, and the temperature coefficient of coercive force β (jHc) is -0.54~-0.64%/℃. Select the appropriate coercive force, the magnet still has a sufficiently high coercive force at the maximum operating temperature of the motor design; otherwise, demagnetization will occur. Sintered NdFeB Magnets in Wind Power Generators

The remanence and coercivity of NdFeB permanent magnet 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 coercivity increases, the remanence and maximum energy product decrease accordingly. Obviously, the selection of high coercive force magnets for wind turbines must be at the expense of remanence and maximum energy product.

4. Consistency of magnetic properties of NdFeB magnets for wind power

NdFeB magnets are manufactured by special powder metallurgy process, and the main manufacturing process is completed under protective atmosphere or vacuum. NdFeB green bodies are pressed in a 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 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 should be small, and that the magnetic properties of a single magnet should be uniform.

There are two types of magnetism: apparent magnetism and intrinsic magnetism. The so-called apparent magnetism of the magnetic steel can measure its open-circuit magnetic flux and its surface magnetic field strength. The apparent magnetism of the magnet is related to the shape and magnetization state of the magnet. Intrinsic characteristics of magnetic steel are tested by measuring the demagnetization curve of the sample. The demagnetization curve is part of the hysteresis loop, which reflects the reverse magnetization characteristics of the permanent magnet material. Measure the demagnetization curve of a magnetic steel sample, provided that the sample is saturated and magnetized before measurement. Sintered NdFeB Magnets in Wind Power Generators

To detect 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 check whether the magnetism of a furnace of magnets is consistent, it is necessary to take magnets 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 the integrity of each piece of magnetic steel to be measured. Therefore, all products cannot be inspected. The consistency of NdFeB magnetic properties must be guaranteed by production equipment and process control.

5. Corrosion resistance of NdFeB

NdFeB alloy contains active rare earth elements, which are easy to oxidize and rust. In application, unless NdFeB is encapsulated and isolated from air and water, it is necessary to do surface anti-corrosion treatment for NdFeB. Common anti-corrosion coatings are nickel plating, zinc plating and electrophoretic epoxy resin. Surface phosphating can prevent NdFeB from rusting in a short 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 takes advantage of this characteristic. From the point of view of use, the hydrogen fragmentation of NdFeB is harmful. Strictly speaking, the corrosion of NdFeB starts from its processing. Degreasing after cutting and grinding, pickling before electroplating, and electroplating process all affect the surface of NdFeB. Improper treatment process may result in unqualified coating quality (such as pinholes), weak combination of NdFeB surface layer and coating, etc.

It is worth noting that although the magnetic properties of the same brand of NdFeB magnets produced by different manufacturers are basically the same, there will be differences in the composition of the alloy, especially the difference in the microstructure of the magnets may be very large. The magnetic steel with excellent performance and good corrosion resistance has the characteristics of fine and uniform crystal grains and high density of magnets. In the following two metallographic photos of sintered NdFeB magnets, the magnet grains shown in the left picture are fine and uniform, and the magnet grains shown in the right picture are larger and uneven.

6. Reliability test of NdFeB magnets

The design life of the wind power generator is 20 years, that is to say, the magnet steel can be used for 20 years, its magnetic properties have no obvious attenuation, and the magnet steel has no obvious corrosion. The following test and detection methods can be used as a method for manufacturers and users of wind magnetic steel to evaluate and inspect magnets.

Weight loss test: use a 10mm×10mm×12mm cuboid black piece as a sample (12mm height is the direction of magnetization), place it in an environment of 2 standard atmospheric pressure, 100% humidity, and 120°C, take it out after 48 hours and remove the oxide layer Removed, its weight loss is less than 0.2 mg/cm2.
Thermal demagnetization test: 120℃×4hr, the open circuit flux loss is less than 3%.
Thermal shock test: After three cycles of high and low temperature from -40°C to 120°C, the loss of open circuit magnetic flux is less than 3%.
Salt spray test and temperature and humidity test are methods for evaluating electroplating and other anti-corrosion coatings. Sintered NdFeB Magnets in Wind Power Generators
Other physical properties, such as thermal expansion coefficient, thermal conductivity, electrical resistivity and mechanical strength, have varying degrees of influence on the usability and reliability of magnets.

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