The drive motor of new energy vehicles has the characteristics of high power, high voltage, high speed, and high frequency, accompanied by the occurrence of bearing electrical corrosion. There are four main types of shaft voltage, and the damage to bearings is not completely the same, so we will not elaborate on them one by one here. How to limit the occurrence of bearing electrical corrosion varies among different enterprises, but it can be roughly divided into three categories: choke restriction, insulation blocking, and bypass diversion.

01 Choke restriction

By placing a choke ring or choke coil at the output end of the frequency converter, if designed properly, the choke ring can consume the surge and extinguishing peak voltage when the frequency converter is turned on and off, and also reduce the harmonic voltage caused by the high-frequency inductance of the conductor, effectively improving EMI performance. However, it should be noted that it cannot eliminate the common mode voltage of the motor, and it is precisely the common mode voltage breakdown that causes severe bearing electrical corrosion in automotive drive motors. In addition, this method usually requires a relatively large installation space.

02 Insulation blocking

Cut off the path of shaft current by insulation measures for bearings or related components. It is worth noting that if ceramic ball bearings are used, the problem of electrical corrosion of the bearing can be basically solved. But if insulation coating is used, the high-frequency impedance of the insulation layer is very low at high frequencies, and it may not be able to withstand voltage surges. This breakdown situation occurs in other industries. Of course, even if the insulated bearing itself is protected, the shaft and the load end behind it are hard connected. When there is electricity on the shaft, it will be output to the load end through the connection point and released at the weak point on the load end.

03 Bypass diversion

Adding a low resistance bypass allows charge to be discharged through this increased bypass, resulting in a decrease in shaft voltage. This reduces the current passing through the bearing and minimizes the risk of voltage breakdown, effectively protecting the bearing. The mainstream diversion solutions on the market include four types: conductive carbon rods, conductive oil seals, conductive grease, and grounding conductive rings. Below are the advantages and disadvantages of four counseling solutions.

Crimp conductive (conductive carbon rod, conductive metal sheet)

Use a carbon rod with one end in contact with the shaft and the other end grounded to conduct electricity underground to prevent the generation of shaft voltage.

Advantages:

Low cost

Disadvantages:

Both the carbon rod and the shaft are hard solid materials, and due to the incomplete smoothness of the contact surface, their contact is always a bouncing contact during high-speed relative motion. Under direct current or low frequency, due to the action of the compression spring, its opening time is very short and will not affect conduction. But the shaft voltage is a high-frequency voltage, and at the moment when the carbon rod bounces open, there is a high possibility that multiple frequency shaft voltages are not discharged and discharge occurs at the bearing, corroding the bearing.

Secondly, carbon rods have reliability and durability issues. Carbon rods are pressed as a whole material and may experience wear and tear. It belongs to consumables in the industrial motor industry and needs to be replaced every 4-6 months. In addition, in an oil cold environment, the penetration of lubricating oil can make the carbon rod soft and the carbon powder easier to wear off, exacerbating and shortening its service life.

In addition, due to the bouncing contact between the carbon rod and the shaft mentioned above, carbon rod breakage also occurs from time to time.

Tightly matched conductive (conductive PTFA, conductive rubber, conductive carbon cloth)

Install conductive tight fitting oil seal material on the shaft, and the charges on the casing and rotor are conducted to the casing through the conductive oil seal. The casing is connected to the ground.

Advantage: No additional installation required. The effect is particularly good in the early stage of installation, especially at high speeds where the shaft rotation drives the deformation of the conductive oil seal, making the conductive material mouth tighter and the conductivity better.

Disadvantage: The conductive oil seal and bearing are tightly fitted and pressed in, requiring strong clamping force. This can result in a loss of power and cause the motor to heat up. The outer edge of the conductive oil seal is made of plastic and rubber parts. As cars are moving parts, the shaft itself also has a certain degree of jumping. After a long time of grinding, the tight fit becomes loose, and there will be a gap between the shaft and the conductive part, making the conductivity intermittent. In an oil cold environment, after oil enters the gap, it forms an isolation layer, and electricity cannot be conducted out. So the conductive oil seal performs well in the initial state, but there are durability issues.

Conductive grease

Same size as regular bearings, except that some conductive particles are added to the insulating grease to unload the shaft voltage and avoid capacitive breakdown discharge.

Advantages: Simple design; Small optimization changes, cost-effective; The test axis voltage has decreased significantly.

Disadvantage: Conductive grease is actually lubricating grease mixed with some conductive materials. Lubricating grease has excellent wetting properties, and a layer of insulating oil film will form on the surface of conductive particles, forming a discrete distribution of discontinuous conductors. The conductivity of grease becomes a combination of insulating grease capacitance breakdown and conductive material resistance conduction. Although the voltage drops, the sustained breakdown discharge can even cause bearing damage faster than non-conductive grease. In addition, the heat generated by breakdown discharge can cause rapid hardening and oxidation of oil, affecting the conductivity effect.

Interference conductive (fiber conductive ring)

Design fiber interference fit to ensure that fibers can make contact with the shaft throughout the entire life cycle of the car, and current is discharged through the fibers.

Advantages: Maintenance free throughout the entire lifecycle, and shaft runout has almost no impact on the conductive ring.

But attention should be paid to the following aspects:

The size of static resistance

The resistance of non insulated bearings in static state is generally around 1 Ω. After installing the conductive ring, the resistance of the shaft to ground should be lower than 1 Ω in order to protect the bearings during car starting and acceleration/deceleration.

chemical stability

In oil cooled motors, the conductive ring needs to be in direct contact with the ATF oil. ATF oil contains various additives and has a complex chemical composition, which requires high chemical stability of the products in contact with it and chemical compatibility with ATF oil. Otherwise, chemical reactions may occur, affecting the shaft grounding effect and even leading to lubrication, cooling, and insulation failure of the entire electric drive system. So the chemical stability of shaft grounding products used under oil cooling conditions is an important indicator that needs to be considered. Therefore, it is not recommended to use low stability organic compounds such as adhesives in the shaft grounding product scheme.

Ensure good contact

Ensure that the fiber tips of the conductive ring are always in contact with the shaft. There is a layer of oil film on the shaft of the oil cooled motor, which forms an insulation layer between the shaft and the conductive ring. The fibers of the conductive ring need to be hard and firm enough to pierce through this layer of oil film.

Solution: Grounding conductive ring, with one end in contact with the shaft and the other end installed on the housing (grounded), can effectively prevent the generation of shaft voltage by introducing electricity underground.

1. Special materials that combine conductivity and flexibility, ensuring a contact resistance far less than 1 Ω, and preventing fracture during high-speed wear;

2. The special structure ensures that the fibers will not fall off under any circumstances and that no organic components are used, without any compatibility issues;

3. Special design ensures both longevity in air-cooled/water-cooled conditions and conductivity in oily environments.

2024 August 4th Week VAFEM Product Recommendation:

Agricultural Wheel Hub Unit:

This type of product provides a complete maintenance-free sealed bearing unit with important advantages for farm applications. These quick-mount bearing units use long-life grease and seals.

Features and advantages of agricultural machinery hub unit:

Extended length, field-tested performance, stronger sealing performance, life-long grease lubrication, basic maintenance-free, simple installation and replacement procedures, special application design, low grease consumption, reduced maintenance and ownership costs, and ease of environmental Influence, improve production efficiency.

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