Wind energy is one of the important applications of neodymium magnets. Currently, the global wind power installed capacity has exceeded 200 million kilowatt hours. However, due to factors such as overcapacity and a sharp increase in rare earth raw materials, wind power equipment has an overcapacity of 260%. The working environment of wind turbines is very harsh. They must be able to withstand high temperatures, severe cold, sand, humidity, and even salt spray. The design life of wind turbines is usually 20 years. Currently, sintered neodymium iron boron permanent magnets are used in small-scale wind turbines and megawatt-level permanent magnet wind turbines. Therefore, it is very important to choose the magnetic parameters of neodymium permanent magnets and their corrosion resistance requirements. The two main generators in the world today are doubly-fed generators and direct drive generators. In a doubly-fed generator unit, the generator cost accounts for 7% of the total unit cost, while in a direct drive generator unit, the generator cost accounts for 31.5%.
Neodymium magnets are known as third-generation rare earth permanent magnets and currently have the highest magnetic properties. The main phase of sintered neodymium iron boron alloy is the intermetallic compound Nd2Fe14B, with a saturation magnetization of 1.6T. When designing permanent magnet motors, advanced NdFeB is usually chosen to obtain high air gap magnetic density. When the motor is running, due to the demagnetization effect of the alternating demagnetization field and the momentary large current caused by the load suddenly changing, high coercivity neodymium magnets are required.
Wind turbines can work in the wilderness and withstand high temperatures and cold. At the same time, the motor losses also cause the motor temperature to rise. The sintered neodymium iron boron magnet can work at 120°C. The Curie temperature of neodymium iron boron permanent magnet alloy is about 310°C. When the temperature of the magnet exceeds the Curie point, it changes from ferromagnetic to paramagnetic. At the Curie temperature, the residual magnetism of neodymium iron boron decreases with increasing temperature, and the coercive force decreases with increasing temperature. The coercive force temperature coefficient β is -0.54-0.64%/℃. If the appropriate coercive force is selected, the magnet will still have a sufficiently high coercive force at the maximum operating temperature of the motor design; otherwise, it will lose excitation.
Large permanent magnet wind turbines usually use thousands of neodymium magnets. Each pole of the rotor contains many magnets. The consistency of the rotor magnetic poles requires the consistency of the magnets, including the size tolerance and magnetic performance consistency. The so-called magnetic performance consistency includes: the magnetic performance deviation between different individuals should be small, and the magnetic performance of a single magnet should be uniform.
Neodymium iron boron alloy contains active rare earth elements, which are easy to oxidize and rust. In practical applications, unless neodymium iron boron is encapsulated and isolated from air and water, it should be regarded as a surface corrosion protection agent. Common anti-corrosion coatings are nickel plating, zinc plating, and electrophoretic epoxy resin. Surface phosphating treatment can prevent neodymium iron boron from rusting in a relatively dry environment for a short time. Rare earth intermetallic compounds can react with hydrogen under certain pressure and temperature. When neodymium iron boron absorbs hydrogen, it heats up and decomposes. The hydrogen cracking in the production process of neodymium iron boron utilizes this characteristic. The design life of wind turbines is 20 years, which means that the electromagnetic steel needs to be used for 20 years. The magnetic performance of neodymium magnets does not show significant deterioration, and the electromagnetic steel does not show significant corrosion.
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