Eddy current loss of motor magnets is highly detrimental to motors. These losses transform electrical energy into heat, thereby increasing the temperature of neodymium magnets, which may cause demagnetization. In this article, we will introduce eddy current losses and how to mitigate their impact on neodymium magnets.

This is the power core of electric racing cars – the permanent magnet synchronous motor. It comes with multiple advantages including high efficiency, high power density, high response speed, and being lightweight. Under optimal working conditions, the efficiency of the permanent magnet synchronous motor can exceed 95%, which is far better than that of traditional internal combustion engines.In contrast, even the high-performance internal combustion engines used in professional Formula 1 racing cars usually have an operating efficiency of only about 50%.

The permanent magnet motors installed in some top tier electric racing cars can achieve a maximum rotational speed of nearly 20,000 RPM and possess a speed adjusting capability of 150 RPM/ms. This outstanding performance offers stable and robust power support to the vehicles during start up, when ascending slopes, and under circumstances of frequent acceleration and deceleration.

Of course, such high performance will inevitably come with certain costs. When the motor is running at high speeds, it will generate huge eddy current losses.

Eddy current losses of neodymium magnets in permanent magnet motors

Eddy current loss is a phenomenon within electromagnetism. When a conductive material is placed in a changing magnetic field, an induced current is generated inside it. This current often forms closed loops that are shaped like whirlpools, which is why it’s called eddy current. As the eddy current flows through the material, it encounters resistance and gives off heat. This energy loss is what we refer to as eddy current loss.

P=I²R

Eddy current losses in materials can be calculated in accordance with Joule’s Law. Within this formula, the P denotes power loss, with the unit of watt (W). The I represents the eddy current, in units of ampere (A). And the R stands for the resistance of the material, with the unit of ohm (Ω).

Typically, when we talk about the energy losses in permanent magnet motors, we mainly focus on the iron losses in the stator and rotor as well as the copper losses in the coils.

However, neodymium magnets, being non-insulating materials, can also generate eddy current losses in high-frequency changing magnetic fields.

Take the sintered neodymium magnets used in high-speed motors for example. Their resistivity is relatively low, and their temperature resistance is not very good either. If we add the situation of not-so-good heat dissipation in the rotor area, when the motor runs at high speed, the neodymium magnets are very likely to experience severe heating.

Once the temperature of the neodymium magnet exceeds its operating temperature, its magnetic properties will decline. Prolonged exposure to high temperatures will cause irreversible magnetic losses to the magnet. If the Curie temperature is exceeded, the neodymium magnet will even be completely demagnetized. By then, the permanent magnet motor will also become scrapped as a result.

Learn more: Curie Temperature And Working Temperature Of Neodymium Magnets | HQ Magnet

To prevent the occurrence of such a bad situation, we must reduce the eddy currents inside the neodymium magnets.

Skin Effect

Prior to this, there is a concept that requires our understanding, namely the skin effect. It pertains to the non – uniform distribution of current density across the cross – section of a conductive material when alternating current flows through a wire. With an increase in the frequency of current variation, the current becomes increasingly concentrated on the wire’s surface, and the current within the wire diminishes accordingly. 

Because of the skin effect, these eddy currents concentrate on the surface of the magnet, resulting in a high – temperature zone on the surface.

Reduce the eddy current loss of motor magnets through structural design

1.Laminated magnets.

Laminated magnets consist of multiple magnetic sheets, and these sheets are glued together using insulating adhesive. Once the glue has fully cured, we can then machine it into whatever form we need. After the magnetization process is finished, a laminated magnet is successfully produced.

As the magnet is cut into several small pieces and the connections are insulated by glue, the induced current can’t form a complete eddy current loop within the laminated magnet. Instead, only tiny eddy currents are generated inside each small piece.

This structure prevents the eddy currents from concentrating together and causing local high-temperature demagnetization. As can be seen in the graph, as the number of layers of the magnet segmentation increases, the eddy currents are distributed more uniformly across the magnet’s surface and bonding interfaces, and the maximum value of the eddy current density is also reduced further.

Of course, this doesn’t mean that the more times of segmentation, the better. Excessive segmentation may lead to a decline in the performance of the magnet. Each laminated magnet needs to determine how to design its structure finally according to its own volume and the working conditions of the motor.

2.Snakeline magnet

Besides laminated magnets, there is now a new alternative – snakeline magnets. After experiments and calculations, by cutting suitable snakeline grooves inside the magnets, this magnet structure can impede the formation of eddy current loops. It’s just like setting many “curves” and “obstacles” on the originally unobstructed “current paths”, reducing the generation of large-scale and concentrated eddy currents.

Compared with laminated magnets, this structure avoids complicated processing steps and the use of adhesives, and is simpler and more efficient.

Learn more about serpentine magnets：Snakeline Magnet | HQ Magnet

Reduce the eddy current loss of motor magnets by changing materials

1.Use bonded neodymium magnets

Bonded magnets are magnets made by mixing permanent magnet powder with binders and then using processes such as pressing and injection molding. Bonded neodymium magnets have their resistivity significantly increased due to the use of binders with high resistance like nylon and epoxy resin. Generally, their resistivity is 10²-10⁴ times that of sintered neodymium magnets. In an alternating electric field, the eddy currents generated by bonded magnets are relatively small, so the eddy current losses can be effectively reduced.

However, the disadvantages of bonded neodymium magnets are also quite obvious. Its maximum energy product is only 6-12 KOe, and its temperature resistance is not as good as that of sintered neodymium magnets. The maximum energy product of sintered neodymium magnets is 33-55 KOe, which has a huge advantage in magnetic performance. In some application scenarios with high requirements for power and temperature, bonded magnets may not be able to meet the needs.

2.Coat with high-resistivity material coatings

Using an epoxy resin coating to cover sintered neodymium magnets is also a feasible way to increase the resistivity. Due to the skin effect, eddy currents concentrate on the surface of the magnets, and this insulating coating can effectively reduce eddy current losses.

3. Add high-resistivity powders

According to Joule’s law. Adding high – resistivity powders like Al₂O₃ constitutes one of the effective means to enhance the resistivity of sintered magnets. Powders of this kind, including Al₂O₃, possess relatively high resistivity. When incorporated into sintered magnets, they can augment the overall resistivity of the magnets, thereby reducing eddy – current losses.

Nevertheless, this method will, to a certain degree, cause the decrease of performance indices such as remanence and coercivity of sintered magnets. Researchers are required to adjust parameters including the proportion and granularity of the added powders, so as to minimize the adverse effects on magnetic properties while increasing the resistivity for reducing eddy – current losses.

The above is the entire content of the article. If you are interested in our magnets or have other questions, please feel free to contact us.🤗

Request for Quote:RFQ | HQ Magnet

Watch the video:How to reduce eddy current loss in motor magnets? Laminated and Snakeline magnets🧲

Our YouTube channel:HQ Magnet – Neodymium Magnets Factory Wholesale – YouTube