Protecting Automotive Bearings from Electrical Corrosion: A Comprehensive Solution

● Protecting the Core™ Bearing from Electrical Corrosion Solution ● Content Agenda ● Bearing electrical corrosion is not a new problem ● Common-mode voltage ● At the neutral point of a three-phase winding, the common-mode voltage is Vcm=(Vu+Vv+Vw)/3; ● Unlike the sinusoidal AC voltage output of a regular AC motor, the rectangular pulse voltage output of a frequency inverter results in asymmetric phase voltages Vu, Vv, Vw, leading to a non-zero common-mode voltage Vcm at any given time. ● High-frequency common-mode voltage with high dv/dt and di/dt characteristics induces shaft voltage and shaft current in the motor shaft. ● Types of shaft voltage u Electrostatic machining voltage, generated by the static machining caused by rotor movement, with low energy u Common-mode voltage division (as shown in the right figure), where the majority of the shaft voltage in motors ranging from 11 to 400KW is derived from the common-mode division u High-frequency loop: High-frequency loop voltage is formed due to the stray inductance and stray capacitance of the motor windings, insulation, and air gaps. For motors above 100KW, high-frequency loop voltage gradually becomes the main hazard u Magnetic imbalance voltage: Shaft voltage caused by design or manufacturing defects, which is a low-frequency shaft voltage. Normally, megawatt-level motors must be taken into consideration, while small motors have less impact ● Methods of detecting shaft voltage u Testing equipment: Oscilloscope (bandwidth≧100M) u Testing mode: Sampling mode u Ensure the motor/drive system is single-point grounded and well grounded u Connect the low-friction conductive probe to the shaft and connect it to the oscilloscope probe. Connect the oscilloscope probe ground wire to the motor housing. ● How much shaft voltage is safe u Depends on the bearing oil film's ability to withstand u NEMA MG1 Part 31.4.4.3 previously defined that shaft voltage above 20VP-P has a high risk of breakdown discharge, but the current standard no longer has relevant descriptions u With the continuous development of motor technology, the rotational speed is increasing, resulting in a significant decrease in the viscosity of the corresponding bearing lubricating oil. During operation, the oil film thickness is greatly reduced, thus weakening the ability to withstand shaft voltage. (For some high-speed motors, there is still a risk of oil film breakdown discharge even with shaft voltages below 10VP-P) u Automotive motors have more complex and variable operating conditions compared to industrial motors, resulting in more unstable oil film states u Therefore, it is difficult to provide an absolute safe voltage ● Typical forms of shaft voltage ● Bearing electrical corrosion is a new problem for automobiles ● With the development of automotive motors in terms of high frequency, high voltage, high speed, and high power, the problem of bearing electrical corrosion in drive systems has become more prevalent ● Electrical corrosion damages the bearing balls and raceways, resulting in operational noise and affecting the service life of the bearings. In severe cases, bearing seizure may occur, impacting motor operation and driving safety. It has become a problem that all automotive OEMs must face ● Content Agenda ● Methods to address shaft voltage ● Types of bypass ● Patent issues are one of the fatal problems in this industry ● How to determine the effectiveness of bearing grounding protection measures From the typical forms of shaft voltage discharge mentioned earlier, it can be seen that the shaft current mainly manifests as

• Resistive discharge current during low-speed and high-vibration states
• Capacitive breakdown discharge current during unstable oil film states and stable states (e.g., during acceleration or deceleration) Therefore, we believe that there are three main factors affecting the effectiveness of protection measures:

1. The ratio of the impedance of the parallel grounding bypass to the impedance of the bearing, especially the high-frequency dynamic impedance
2. The magnitude of the reduction in shaft voltage (percentage)
3. The amplitude of the shaft voltage after reduction (Vp-p) ● Content Agenda 430V 150KW 16000RPM water-cooled motor ● Without any measures, the highest measured shaft voltage is only around 6V, indicating severe bearing electrical corrosion ● After installing VGR-52.73-30-8, the shaft voltage is significantly reduced ● Analysis of domestic supply of new energy vehicles in 2022 ● Company introduction ● Summary u With the development of automotive motors, the problem of bearing electrical corrosion has become increasingly prominent and must be addressed u Bearing electrical corrosion in automotive motors is mainly caused by frequency inverters u Among various technical solutions, the conductive fiber diversion solution stands out due to its comprehensiveness and overall advantages u Hochen conductive fiber rings effectively divert shaft current, reduce shaft voltage, and help customers solve the problem of bearing electrical corrosion. In some cases, the combination with insulated bearings or high-conductivity shaft sleeves yields even better results.