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Typical Magnetic Properties of Wind Turbine Magnets

Wind turbine magnets are the important parts of wind turbines. This article will discuss and introduce the typical magnetic properties of wind turbine magnets.

As we all know, wind power is a kind of clean energy. As society progresses, the demand for wind power generation is also increasing. The installed capacity of wind turbines is increasing and wind turbine magnets play a crucial role in enhancing the performance of wind turbines.

Wind turbine magnets mainly refer to NdFeB permanent magnets. Because of their highest magnetic properties, they are known as the third generation of rare earth permanent magnets. The  Leading magnet supplier has manufactured high-performance NdFeB magnets with a maximum magnetic energy product (BH)max of 57MGOe. There are several ways to increase the maximum BH of the magnets, such as increasing the ratio of the main phase of the alloy, improving the grain orientation, and enhancing the density of the magnets. However, for a single-crystalline Nd2Fe14B magnet, it will not exceed the theoretical maximum BH value of 64 MGOe

wind turbine magnets

Image Source: ABI Electronics

Corrosion Resistance of Wind Turbine Magnets

The wind turbine often operates in a harsh environment and the NdFeB alloy contains lots of iron, which is easy to oxidation and rust. In practical applications, unless the NdFeB magnets are isolated from air and water, they are necessary to perform surface anti-corrosion treatment on the magnets. Common anti-corrosion and anti-rust coatings include nickel plating, zinc plating, and electrophoretic epoxy. Surface phosphate treatment is another option, which provides short-term protection in a relatively dry environment.

Under particular pressure and temperature circumstances, hydrogen can react with rare earth compounds. After absorbing hydrogen, NdFeB will release heat and generate crumbles. This property of NdFeB is utilized in its production process through a method called hydrogen decrepitation. However, hydrogen decrepitation of NdFeB will damage the performance of wind turbine magnets.

Technically speaking, wind turbine magnets’ corrosion starts from their processing. Procedures like degreasing after the cutting and grinding process, acid washing for pre-plating treatment, and the plating process all have an effect on the NdFeB’s surface layer. Improper processing may result in subpar plating quality (e.g. pinholes), poor adhesion between the NdFeB surface layer and the plating layer, and more.

It’s better to source wind turbine magnets from one manufacturer. The reason is that produced by different manufacturers, even the same-grade magnets may have variations in the composition of the alloy, particularly the magnets’ microstructures. Fine and uniform grains and density are crucial to the magnets with excellent performance and good corrosion resistance.

Uniform Magnetic Properties of Wind Turbine Magnets

High-performance NdFeB magnets are manufactured under an inert-gas protective condition. During the process, semi-finished magnets are pressed under high pressure. The size of the NdFeB magnets is then constrained by the manufacturing equipment and production conditions.

Intechhopen

Image Source: Intechopen

A typical modern land-based wind turbine has over 170 feet long blades, which requires lots of  NdFeB magnets. For the performance of the huge wind turbine, wind turbine magnets require smaller differences in magnetic properties among different magnets in different parts. For a single magnet, the consistency of magnetic properties is also taken into account. that means stricter standards would apply to wind turbine magnets, such as dimensional tolerances and other magnetic properties. 

There are two components to a magnet’s magnetic force: intrinsic and apparent magnetism. The surface magnetic field strength or open-circuit magnetic flux can be used to calculate its apparent magnetism. The shape and magnetization direction of a magnet are related to its apparent magnetism. A magnet’s intrinsic characteristics will be evaluated by the demagnetization curve.  Prior to measurement, a magnet sample must be thoroughly saturated with magnetism in order to get the correct demagnetization curve.

Usually, a single magnet will be split into several tiny pieces, and measure each part’s demagnetization curve in order to confirm that the magnet’s magnetic characteristics are uniform.

Different magnets must be sampled during the production process, and the samples’ demagnetization curves must be measured, in order to guarantee consistency. Complete product inspection is not practicable due to the high cost of measuring equipment. Precise process control and quality equipment are the keys to the consistent magnetic properties of magnets.

Temperature Stability of Wind Turbine Magnets

When wind turbines work, the temperature will rise. Moreover, the motor power losses can also cause the motor temperature to rise. The working temperature of NdFeB magnets is not above 120℃. So the stability of wind turbine magnets under high temperatures should be carefully evaluated.

WIND TURBINE MAGNETS AND HEAT

Image Source: Stanfordmagnets

The NdFeB permanent magnets, which are applied to the wind turbines,  have a Curie temperature of 310°C. Above the Curie point, demagnetization will occur. Below the Curie temperature, the rising temperature will decrease the remanent magnetization of NdFeB and influence the wind turbine’s performance. Moreover, NdFeB’s coercivity falls as temperature rises, and its temperature coefficient of coercivity β (jHc) is -0.54~-0.64%/℃. That is, if the temperature rises 1 degree, the coercivity of magnets will decrease 0.54~0.64%.  So, when choosing the wind turbine magnets, high coercivity is needed at the motor’s maximum operating temperature.

Reliability Testing of Wind Turbine Magnets

The designed lifespan of a wind turbine generator is 20 years. Therefore, the wind turbine magnets should function well for 20 years without significant magnetic property loss or obvious corrosion.

The following tests and detection methods can be used by wind power magnet manufacturers and users to evaluate and inspect the magnets:

Weight loss test: Using a 10mm x 10mm x 12mm rectangular sample (12mm height for magnetizing direction), it is placed in an environment of 2 standard atmospheric pressures, 100% humidity, 120°C for 48h, then removed and deoxidized. The weight loss should be less than 0.2 mg/cm2.

Cold and heat shock test: After enduring from -40°C (low temperature) to 120°C(high temperature) for 3 cycles, the open circuit flux loss of a magnet is less than 3%.

Methods for evaluating plating and coatings include salt spray tests and temperature and humidity tests.

salt spray test for wind turbine magnets

Image Source: Wikipedia

The serviceability and dependability of the magnets can also be impacted to varying degrees by other physical characteristics such as mechanical strength, thermal conductivity, electrical resistivity, and coefficient of thermal expansion.

Note: Some data and contents are from OFweek and the web. 

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Jonah Jin
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