Yunlu New Energy Technology: Application of Nanocrystalline Magnetic Rings in Bearing Corrosion Problems of 800V High-voltage Platforms

Source from Gasgoo

In 2021, the industry began to raise the issue of electric corrosion of electric drive bearings. With the trend of 800V electric drive systems, this issue has become the industry’s focus.

What are the causes of bearing electrical corrosion? On the 400V platform, it is mainly due to magnetic imbalance and asymmetry. The bearing cuts the magnetic induction lines during rotation to generate shaft voltage, and electrostatic induction generates shaft voltage. The 800VSiC high-voltage platform will instantly generate higher du/dt and di/dt when switching quickly, and a common-mode voltage will be generated during the propagation process; when the motor speed is low or the bearing temperature is high during long-term operation, the bearing lubrication and Insufficient or reduced insulation performance will break down the bearing oil film, destroy its insulation, and cause pitting corrosion in the bearing.

Regarding solutions to bearing electrical corrosion, on December 14, 2023, at the 4th Automotive Electric Drive and Key Technology Conference, Zhang Ge, R&D General Manager of Qingdao Yunlu New Energy Technology Co., Ltd. proposed: Reduce or eliminate bearing electrical corrosion The main method is to insulate the bearing, rotating shaft or bearing chamber, guide the shaft current to the motor shell in a directional manner and suppress the increase of the shaft voltage. The main methods are "attenuation", "drainage" and "blocking".

Zhang Ge said that the nanocrystal magnetic ring uses the attenuation principle to consume the harmonics on the three-phase side to reduce the shaft voltage. At the same time, Zhang Ge systematically elaborated on the nanocrystal characteristics requirements, shape selection, core loss calculation, production requirements, magnetic core fixation methods, and magnetic ring reliability evaluation of the nanocrystal magnetic ring.

Zhang Ge | R&D General Manager of Qingdao Yunlu New Energy Technology Co., Ltd.

The following is the summary of the speech:

Analysis on the Causes of Bearing Electrical Corrosion

There are several solutions available

To reduce or eliminate bearing electrical corrosion, the main means are to insulate the bearing, rotary pump or bearing chamber, guide the shaft current to the motor shell and suppress the increase of shaft voltage. The main methods are as follows: blocking, diversion and reduction. Barrier methods avoid current cross-talk by insulating bearings and related components, which can be achieved by making ceramic bearings or adding coatings to the bearings. The grooming method uses carbon brushes or grounding rings to release the voltage in the bearing through grounding. Finally, the reduction method uses a filter magnetic ring to eliminate harmonics, thereby reducing the bearing voltage.

Each individual solution has its pros and cons, and there are limitations to relying on any one method alone to solve the problem of bearing corrosion. A more reliable and effective solution is to use a combination of "reduction", "diversion" and "blocking".

Applications of Nanocrystalline Magnetic Rings

The nanocrystal magnetic ring is used to consume most of the harmonics on the three-phase side to reduce the shaft voltage. Why are nanocrystals used on the three-phase AC side?

1) The magnetic permeability of nanocrystals is generally higher than that of ferrite in a wide frequency range, and they have higher impedance under the same volume. 2) The saturation magnetic density of nanocrystals is higher than that of ferrite. Choosing the appropriate magnetic permeability can achieve stronger bias resistance; 3) The Curie temperature of nanocrystals is 560°C, which is much higher than the Curie temperature of ferrite. On the DC side, we usually do not consider the temperature factor because its temperature rise is low. However, on the three-phase AC side, due to the influence of harmonics, the core heats up seriously. To reduce the volume of the magnetic ring, we want the temperature rise to be as high as possible. The current temperature resistance point of nanocrystals is about 560°C, while the temperature resistance of ferrite is usually 150°C or lower.

However, considering that the temperature resistance of the plastic containing the nanocrystalline magnetic core is limited to below 180°C, the main bottleneck we face is not the magnetic ring itself, but the temperature resistance of the plastic. Major manufacturers are working hard to increase the maximum temperature of three-phase magnetic rings to about 180°C to reduce product volume.

Next, let’s discuss the characteristic requirements of nanocrystals. The harmonics on the three-phase AC side are very large, causing the magnetic core to easily saturate. This requires nanocrystals to have certain anti-saturation capabilities and broadband characteristics. In addition, the thinner the strip, the better the high-frequency properties of the nanocrystals and the lower the losses. At present, the 14um ultra-thin nanocrystalline magnetic core reaches higher impedance at 500kHz and 30MHz, and is more suitable for applications on the three-phase AC side.

The anti-saturation capability of the magnetic core can be improved by reducing the magnetic permeability, which can be achieved by adjusting the composition of the strip and the heat treatment process. At present, the commonly used magnetic permeability of three-phase magnetic rings is about 60,000-80,000, but when the shaft current is too large, the core temperature will rise, which may cause the plastic shell to burn and melt. Therefore, it is necessary to improve the anti-saturation capability of the magnetic core and reduce the magnetic permeability. Yunlu has been able to reduce the magnetic permeability to less than 10,000 and is researching low-cost mass-production technology.

Regarding the 14 micron ultra-thin tape and 18 micron conventional tape, the thinner the tape, the better the high-frequency impedance characteristics are. The development of 14-micron strips originally originated from the heavy ion accelerator project built by the country in Huizhou, Guangzhou, which has very high requirements for high-frequency impedance. In the field of new energy vehicles, we also found the need to develop in the direction of high-frequency impedance, so we applied this technology to the electric drive three-phase magnetic ring. Test results show that under the same size, the impedance of 14-micron tape can be increased by 30% compared with 18-micron tape, and the volume can be reduced by 20%-30% under the same performance.

Regarding the shape of nanocrystals. Nanocrystals are wound from ribbons and are therefore sensitive to stress. To maintain stable performance, stress needs to be minimized during the manufacturing process.

Currently, the ring shape is the least stressed during the manufacturing process, followed by the racetrack shape, and finally the rectangular shape. In the case of the same volume, length, and cross-section, the difference between the three shapes of magnetic rings is about 5%. However, despite its superior performance, the ring is not commonly used in the industry due to its insufficient space utilization. The runway shape is widely used due to its better performance in small spaces.

In addition to shape, the length of the magnetic ring is also a key factor affecting performance. In the case of the same volume, the shorter the magnetic circuit length, the smaller the overall impedance, and the higher the performance. To achieve this goal, we design the magnetic circuit length of the product to be as short as possible.

Among the above factors, the temperature rise problem is still the main factor limiting the performance of the magnetic ring. To solve this problem, we consider using simulation technology to predict temperature rise. Currently, the decomposition method is commonly used in the industry to calculate core loss, but this method may not be accurate in complex electric drive models.

To improve accuracy, we have launched a project in cooperation with Tsinghua University. We plan to establish a loss calculation model or method suitable for electric drive operating conditions through large amounts of data collection and experiments, so that we can more accurately predict temperature rise through simulation.

In terms of production, magnetic rings are wound from strips. Initially, we produce the world's widest strips, which are then cut and rolled as needed. Currently, we are studying automated production. Since the usage in the automotive industry is relatively small, sometimes manual production with auxiliary tooling may be more economical. In order to ensure the performance and characteristic stability of the magnetic core, the industry generally adopts curing method. Although curing is detrimental to core performance, it ensures the cleanliness of the nanocrystals and the stability of their properties.

In response to the needs of the new energy vehicles and optical storage component industries, we have established a 3,000 square meter strip and magnetic core production line. The current market competition is very fierce, and both cost and space are required to reach the limit. Therefore, we have put forward higher requirements for parts and components. We established the pilot center to meet the current market needs and be able to quickly prototype and develop products that meet customer requirements.

In original models, the problem of bearing corrosion was often not considered and no corresponding space was reserved. The initial method adopted by a certain car company was to create a small space for connection between the motor and the electric drive. There are currently three main fixing methods, among which the method of mounting on the electric drive board is less used because it is not conducive to the standardized control of the electric drive board. Since each car model and even different platforms have different filtering requirements, the three-phase magnetic rings are currently non-standard designs. To standardize the electric drive board, the magnetic ring is mainly fixed between the electric drive shell or the electric drive board and the motor.

At present, there are two main ways to fix the magnetic core: glue fixation and potting. Relatively speaking, dispensing is more recommended. Its process is simple, low cost, and the stress squeeze on the nanocrystals is small, resulting in a small degree of attenuation before and after assembly. However, in some oil mist environments, nanocrystals need to be sealed, and potting is required. Welding is also used in the industry, but there are risks. High vibrations and alternating hot and cold conditions can cause welds to crack. Once cracked, causing the seal to be broken, oil may enter the shell and mix with nanocrystalline debris, bringing the debris into the motor environment, causing insulation problems. Considering the oily environment, potting is the more common method.

However, a major difficulty currently facing potting is stress, which may cause core degradation. To this end, Yunlu has conducted a lot of research. Initially, many companies used normal pressure potting without vacuuming, and the surface and performance tests seemed normal, and the core performance even showed no attenuation. However during long-term impact and high-temperature aging tests, problems began to appear. There will be bubbles sealed under normal pressure potting. These bubbles will collide under impact and high temperature, resulting in changes in the performance of the magnetic core and the expansion of the plastic case.

Vacuum potting is widely used in other industries, but in the case of nanocrystals, simple vacuum processing can cause huge stresses on the core. If the glue flows into the middle of the magnetic core, it will cause a huge change in its performance. After research, we developed a stepped vacuum injection method.