Time : May 18, 2023
In recent years, variable voltage and variable frequency control AC traction motors have been installed on newly manufactured electric vehicles. These AC traction motors have the characteristics of light weight, compact structure, and high-speed operation. However, in order to adapt to the characteristics of high-speed rotation of bearings and temperature rise of various components of traction motors, it is necessary to solve the “electrical corrosion” hazard of existing steel bearings to improve the heat resistance and durability of bearings and lubricating grease.
Note: Electrical erosion refers to the damage caused by local melting caused by electric current passing through the very thin oil film in the rolling contact area of the bearing during rotation.
Figure 1. Axial current corrosion phenomenon via network
1.Basic requirements for insulated bearings for traction motors
At present, the solution to electrical corrosion is to set up a bypass circuit to allow current to flow to the outside of the bearing, or to insulate the surrounding area of the bearing to cut off the current. The specific methods are:
From the perspective of price and reliability, insulated bearings are the main method.
There are roughly three types of insulated bearings used for traction motors, namely: resin coated bearings; Hybrid ceramic bearings; Ceramic sprayed bearings.
① Resin coated bearings
The most commonly used type of bearing is PPS (polyphenylene sulfide) insulated bearing, whose outer surface is coated with PPS insulation resin to prevent bearing corrosion. The advantage is excellent electrical insulation performance and low price. The disadvantage is that this type of bearing is not suitable for machinery with rapidly increasing bearing temperature or occasions with strict requirements for interference changes, so it is often used in motors with relatively loose conditions.
② Hybrid ceramic bearings
The only rolling element parts in bearings are made of ceramic materials, also known as hybrid ceramic ball bearings. They use engineering ceramics as bearing materials and have good mechanical and thermal properties, such as sufficient strength, stiffness, hardness, fracture resistance, impact resistance, high temperature resistance, oxidation resistance, and a series of better properties than metal materials. Ceramic materials suitable for bearings mainly include silicon nitride (Si3N4), zirconia (ZrO2) Aluminum oxide (Al2O3). Among these three ceramic materials, silicon nitride has superior comprehensive performance and has become the preferred material for ceramic bearings. Its characteristic is that the failure mode, like that of bearing steel, occurs in the form of pre existing peeling. However, zirconia and alumina both exhibit a form of fragmentation failure.
Figure 2. High temperature bearing via network of zirconia silicon nitride hybrid ceramic
③ Ceramic spray bearing
It is to spray a layer of ceramic insulation coating on the inner or outer ring of the bearing. The ceramic insulating coating is prepared by plasma spraying, and then sealed with sealing agent. Bearings with coated outer rings, mainly used for medium size bearings; Bearings with coated inner rings are mainly used for larger bearings or occasions with higher insulation requirements. The dimensional tolerance and accuracy of the bearings comply with relevant ISO standards and are completely consistent with the prototype. Its performance, size, and accuracy are the same as ordinary bearings and can be directly replaced.
In addition to the general requirements for bearings, insulated bearings for traction motors also have additional requirements for insulation performance. Mainly including:
Overall, ceramic bearings are more in line with the practical application of traction motors.
Advantages of combined ceramic bearings for traction motors
Under direct current, ceramic bearings without sealing rings or dust covers are an insulator with a resistance value of several gigabits of ohms; Under the action of high-frequency current, the ceramic rolling element of ceramic bearings acts as insulation between the inner and outer rings. Due to the very low capacitance of hybrid ceramic bearings, small raceway contact, relatively large rolling element diameter, and relatively low dielectric constant of silicon nitride materials, even in a high-frequency environment with a current frequency of 1 MHz, its resistance value is above 1 k Ω. Therefore, ceramic bearings are very effective in preventing electrical corrosion damage to bearings, especially under high-frequency current conditions.
When steel bearings pass through current, the local high temperature generated by current discharge can cause the additives of lubricating grease to react with the base oil, causing the base oil to burn or carbonize. The lubricating grease will become hard and black, and the grease will rapidly decay. Although the lubricating grease of ceramic bearings may deteriorate like ordinary bearings, it will not cause burns. By comparing the rotation time until burnout, the grease life of ceramic bearings is 3-4 times that of steel bearings.
When the viscosity of lubricating oil is high and the formation of oil film is easy, the lifespan of ceramic bearings and steel bearings is almost the same; When the viscosity of lubricating oil is low, the lifespan of bearings is generally very short. Because at this point, the rolling element and the inner and outer rings are of the same material, which can easily adhere to each other. However, using ceramic rolling elements of different materials is beneficial for improving their lifespan due to the lack of activity.
More about XBRG Thrust ball bearings:
Thrust ball bearings are separable bearings. The single-acting thrust ball bearing consists of a shaft washer, a housing washer and a ball set with cage. The double-acting type consists of a shaft washer (centre washer), two housing washers and two ball sets with cages. Thrust ball bearings can be applied for high axial loads and low to medium speeds. However, they cannot take radial loads.
http://xinzhou.bearingshow.net/product-cat/thrust-ball-bearings/
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