How to Reduce Eddy Current Losses of Rare Earth Magnet?

The consumption of motor is the greatest among the rotating machinery. The eddy current losses are one of the worst enemies to high-speed rare earth permanent magnet motor. The eddy current will give rise to the temperature of the rare earth permanent magnet, then the magnet will be demagnetized, therefore influence the work efficiency of the motor. As the metallic material, rare earth magnet has lower electrical resistance, and it also become the direct source of eddy current losses.

What is Skin Effect?

The internal of metal with good conductivity will generate the induced electromotive force under the external changing magnetic field, and formed the eddy current inside the metal. The Joule heating effect of the induced eddy current lead to the above-mentioned eddy current losses. Actually, the eddy current concentrates mainly on the surface layer of the conductor. This phenomenon is known as the skin effect, and the basic reason is that the changing electromagnetic field generates an opposite vortex electric field inside the metallic conductor. The skin depth δ can be described as below relation：

Where μ is the permeability of conductor; ρ is the electrical resistance; ω is the angular frequency of alternating magnetic field, ω=2πf.

The higher rotation speed, high permeability and low electrical resistance will cause lower skin depth and high losses.

How to Measure the Electrical Resistance of Rare Earth Magnet?

How to Reduce Eddy Current Losses of Rare Earth Magnet?

The eddy current loss of rare earth magnet can be reduced by many ways, among them, the material science researchers have proposed to prepare the high-resistivity magnet, such as:

a. Bonded magnet.

b. High-resistivity sintered magnet.

Due to the existence of binder, the electrical resistance of the bonded magnet would be 10² to 10⁴ times more than simple sintered magnet, but the motor power and maximum operating temperate will be limited. For sintered magnet, submicron fluoride will be served as additive to enhance the grain boundary electrical resistance. Fluoride will help magnet to form a high resistivity layer on its grain boundary, thus improve the electrical resistivity of the whole material.

Magnet segmentation technology is a very practical way to make high-resistivity magnet without changing the composition of the magnet.

What is Magnet Segmentation Technology?

Magnet made by magnet segmentation technology can be called laminated magnet, layered magnet, split magnet and segmented magnet. For magnet segmentation technology, the whole magnet will be segmented into several pieces, then processed into specific shape and dimension after glued together by special isolation glue.

According to many simulation cases and actual products, the magnet segmentation technique has excellent performance to prevent the eddy current. With the increase of segment, the eddy current losses of permanent magnet significantly decreased. Magnet segmentation technique has been already applied in the Neodymium magnet and Samarium Cobalt magnet.