作为一名优秀的翻译请你用正式的表达用英语翻译以下PPT内容注意语言的规范性和正确性和简洁性并按照原文格式进行排版。轴承电腐蚀不是个新问题 护承核TM轴承电腐蚀 解决方案 共模电压 u 三相绕组的中性点处共模电压为Vcm=Vu+Vv+Vw3； u 不同于工频交流电机变频器的矩形脉冲电压输出使得变频驱动电机各相相电压Vu、Vv、Vw 为矩形脉冲在任意时刻都不可能对称因此任意时刻共模电压Vcm不会

Bearing electrical corrosion is not a new problem. Protecting TM bearing from electrical corrosion is the solution.

Common mode voltage:

• The common mode voltage at the neutral point of a three-phase winding is Vcm=(Vu+Vv+Vw)/3.
• Unlike AC motors powered by mains frequency, the rectangular pulse voltage output of the inverter in variable frequency drives (VFDs) results in non-symmetrical voltages Vu, Vv, and Vw at any given moment. Therefore, the common mode voltage Vcm is never zero.
• The high dv/dt and di/dt characteristics of high-frequency common mode voltage induce shaft voltage and shaft current on the motor shaft.

Types of shaft voltage:

• Electrostatic machining voltage: generated by the friction of the rotor movement through the air, it has low energy.
• Common mode voltage division (see right chart): for motors with a power range of 11-400KW, the majority of the shaft voltage comes from common mode voltage division.
• High-frequency loop: stray inductance of motor winding, stray capacitance of insulation and air gap form high-frequency loop voltage. For motors above 100KW, high-frequency loop voltage gradually becomes the main threat.
• Magnetic flux unbalance voltage: shaft voltage caused by design or manufacturing defects. This is a low-frequency shaft voltage that must be considered for motors in the megawatt range, but has less impact on smaller motors.

Methods for detecting shaft voltage:

• Test equipment: oscilloscope (bandwidth ≥ 100M).
• Test mode: sampling mode.
• Ensure that the motor/drive system is grounded at a single point and has good grounding.
• Connect a low-friction, conductive contact to the shaft and connect it to the oscilloscope probe. Connect the oscilloscope probe ground wire to the motor casing.
• Use low-friction test probes.

Typical forms of shaft voltage:

• Stopped: thick oil film.
• Startup: thin oil film.
• Steady state: mixed lubrication.
• Low speed, acceleration: low frequency.
• High speed: high frequency.
• Load: high and low.

How much shaft voltage is safe: Answer: I don't know. It depends on the bearing's ability to withstand the oil film. NEMA MG1 Part 31.4.4.3 previously defined that shaft voltages above 20VP-P had a higher risk of breakdown discharge. However, current standards no longer have relevant descriptions. With the continuous development of motor technology, the speed has increased, and the viscosity of the corresponding bearing lubricating oil has decreased significantly compared to the past. This results in a thinner oil film during operation and weaker ability to withstand shaft voltage. (For some high-speed motors, even shaft voltages below 10VP-P still carry the risk of oil film breakdown discharge.) Automotive motors have more complex and variable operating conditions, resulting in more unstable oil film conditions. Therefore, it is difficult to give an absolute safe voltage.

Shaft electrical corrosion is a new issue for automobiles:

• With the development of automotive motors towards higher frequencies, higher voltages, higher speeds, and higher powers, bearing electrical corrosion issues have become more common.
• Electrical corrosion damages the bearing balls and races, generates operational noise, affects the bearing's service life, and in severe cases, can cause bearing seizure, affecting motor operation and driving safety. It has become a problem that all automotive OEMs must face.

Methods to address shaft voltage:

• For dv/dt output from the inverter: choke treatment. By simulating complex circuits, harmonic energy can be shielded. This is the best way to deal with dv/dt. Whether to add a choke magnetic ring depends on the quality of the inverter and the length of the cable between the inverter and the motor. Chokes have certain benefits for high-frequency loop currents but are useless for common mode currents.
• For high-frequency loop currents: insulation treatment. Use insulation to cut off high-frequency loop currents. However, this method is not effective for common mode currents because they are grounded. The shaft conducts the current through a hard connection to the gearbox or even the main shaft. If insulation weakens at any point, breakdown can occur.
• For common mode currents: conductive treatment. Use conductive materials to connect the motor shaft to the motor casing, creating a low-resistance circuit to divert the shaft current. This effectively reduces the amplitude of the shaft voltage. For high-frequency loop currents, a conductive connection at both ends can also form a low-resistance circuit, allowing the current to bypass the bearings.

Types of bypass:

• Bearing protection measures: advantages and disadvantages.
  • Tight-fitting conductive: conductive non-woven fabric, conductive PTFE, conductive rubber.
    • Advantages: no additional installation or space required, low cost, good initial conductivity (especially at higher speeds).
    • Disadvantages: conductivity decreases rapidly with wear and shaft deflection. It cannot penetrate the oil film, resulting in reduced conductivity in oil environments.
  • Spring pressure contact: conductive carbon rod.
    • Advantages: low cost, good conduction for low frequency or direct current. Commonly used in static equipment.
    • Disadvantages: cannot fully fit when two hard materials are in high-speed relative motion. Only intermittent contact is possible, and there is a risk of discharge at the moment of contact separation. Service life is greatly reduced under oil immersion conditions. The elasticity of the pressure spring weakens after long-term heating, requiring regular replacement. There is a risk of carbon rod bouncing, sparking, and fragmentation under high vibration conditions.
  • Interference fit: conductive fiber ring.
    • Advantages: hundreds of thousands of conductive fibers form an uninterrupted conduction path, effectively solving the skin effect of high-frequency currents. Full contact surface throughout the entire service life. It simultaneously addresses system shaft voltage and improves system EMI.
    • Disadvantages: pure carbon fiber is brittle and prone to fracture. Some manufacturers intentionally make it longer to prevent fracture, but it becomes too soft and difficult to penetrate the oil film. Pure carbon fiber has poor conductivity and cannot protect against bearing electrical corrosion during startup or speed changes.
  • Conductive grease: grease mixed with graphite or copper powder.
    • Advantages: reduces shaft voltage.
    • Disadvantages: by mixing, only the distance for conduction is reduced, but the oil film breakdown still occurs. It also provides negative impedance performance, which accelerates bearing electrical corrosion.
  • Sacrificial bearing: small conductive bearing.
    • Advantages: the oil film thickness of sacrificial bearings is thinner than that of the main bearings. Sacrificial bearings are not load-bearing, so the oil film thickness remains relatively constant. However, during startup and rapid acceleration or deceleration, the oil film on the main bearings may be thinner than that on the sacrificial bearings, resulting in electrical corrosion in the main bearings, while the sacrificial bearings cannot prevent it