With the rapid development of the electric car industry, particularly in the context of China EV market expansion, the application of permanent magnet synchronous motors (PMSMs) has surpassed tens of millions of units. In practical applications, the issue of abnormal noise generated by motor bearings has garnered widespread attention. Common problems include surface roughness and metal corrosion at the bearing locations, which manifest as weld-scale-like corrosion patterns on the inner and outer rings. This corrosion phenomenon leads to abnormal vibrations in the motor system, thereby reducing the noise, vibration, and harshness (NVH) performance of electric cars. The electrical corrosion of bearings in PMSMs has become a critical factor limiting the performance and reliable operation of these motors in China EV applications. This paper delves into the mechanism of bearing electrical corrosion and its suppression techniques, providing a theoretical foundation for improving this issue in the electric car sector.
The surface cause of bearing electrical corrosion lies in the presence of shaft voltage, which is induced by common-mode voltage, leading to the formation of shaft current and triggering the corrosion process. By establishing a theoretical model of electrical corrosion, the distribution of shaft current in the bearing is analyzed in depth. I innovatively propose that the shaft voltage is directly related to the turn-off characteristics of the insulated gate bipolar transistor (IGBT). Additionally, measurements of motor parasitic parameters and simulation waveform analyses are conducted, with an experimental platform built to verify the relevant theories. This research underscores the importance of addressing bearing electrical corrosion in the context of electric car advancements, especially as China EV manufacturers strive for higher power density and efficiency.
1. Mechanism Analysis of Bearing Electrical Corrosion
The root cause of bearing electrical corrosion in electric car PMSMs is the excessive shaft current, which causes localized heating and melting of contact surfaces, severely damaging the bearings. The shaft current anomaly primarily arises when the shaft voltage exceeds the insulation strength of the lubricating oil film, allowing current to form a path through the bearing. The common-mode voltage of the motor plays a decisive role in this process, as it is the key driver of shaft voltage generation. In China EV applications, the widespread use of PWM-controlled inverters with IGBTs exacerbates this issue due to their fast switching characteristics.
1.1. Generation Mechanism of Common-Mode Voltage
The common-mode voltage in a PMSM is essentially the zero-sequence voltage produced by the three-phase inverter. For a star-connected motor, the common-mode voltage \( V_{com-g} \) is defined as the voltage at the motor’s neutral point relative to ground. Mathematically, it is expressed as:
$$ V_{com-g} = \frac{V_{u-g} + V_{v-g} + V_{w-g}}{3} $$
where \( V_{u-g} \), \( V_{v-g} \), and \( V_{w-g} \) are the phase voltages relative to ground. In electric car drives, the IGBT switches with rise and fall times of just a few hundred nanoseconds, resulting in high \( dv/dt \) at the output. This rapid switching induces a common-mode voltage that couples through the motor’s internal stray capacitances to the bearings, generating shaft voltage and current. The proliferation of China EV models has heightened the need to understand this mechanism to ensure reliability.
1.2. Equivalent Model of Motor Bearings
Motor bearings consist of inner and outer races with rolling elements separated by a lubricating grease. Under normal operation, the grease forms an insulating oil film that prevents direct metal contact, presenting a capacitive characteristic. However, when the motor is stationary or operating at low speeds, or if the oil film is insufficient to withstand the shaft voltage, breakdown occurs, and the bearing exhibits resistive-inductive properties. This dual behavior is critical in analyzing electrical corrosion in electric car motors.
The electrical characteristics of bearings can be summarized as follows:
- When the oil film is intact, the bearing acts as a capacitor with equivalent capacitance \( C_b \).
- When breakdown occurs, the bearing behaves as a series combination of resistance \( R_{mot-bearing} \) and inductance \( L_{mot-bearing} \).
Experimental waveforms have shown distinct differences in shaft voltage and common-mode voltage between the breakdown and non-breakdown states, highlighting the importance of the oil film’s integrity in China EV applications.
1.3. Equivalent Circuit of PMSM System
The high-voltage battery in an electric car supplies power to the motor controller, which inverts it to drive the PMSM. The common-mode voltage \( V_{com-g} \) generated at the motor’s neutral point couples to the rotor bearing inner ring through the parasitic capacitance \( C_{wr} \) between the stator and rotor. This results in shaft voltage \( V_{mot-bearing} \) and shaft current that flows through the bearing elements to the outer ring and motor housing, returning to the battery negative terminal.
For the non-breakdown state, the bearing is modeled as a capacitive network, simplifying analysis. The equivalent circuit includes components such as stator winding resistance and inductance, controller stray inductance, and parasitic capacitances. For the breakdown state, a distributed parameter high-frequency model is used, as it more accurately represents the real system structure. The transfer function for the motor bearing end in this state is given by:
$$ \frac{V_{mot-bearing}}{V_{com-g}} = \frac{s C_{wr} (L_{mot-bearing} s + R_{mot-bearing})}{1 + s C_{rg} (L_{mot-bearing} s + R_{mot-bearing}) + s C_{wr} (L_{mot-bearing} s + R_{mot-bearing})} $$
where \( C_{rg} \) is the equivalent capacitance between the rotor shaft and housing. This transfer function indicates that the \( dv/dt \) of the input common-mode voltage significantly influences the shaft voltage, providing theoretical support for subsequent analyses in electric car systems.
2. Main Factors Influencing Shaft Voltage
In electric car PMSMs, several factors affect shaft voltage, but the switching speed of IGBTs is paramount. As China EV manufacturers push for higher efficiency and power density, understanding these factors becomes crucial for mitigating bearing electrical corrosion.
2.1. Impact of IGBT Switching Speed
The common-mode voltage is directly influenced by the motor’s three-phase voltages relative to ground, which are equivalent to the output voltages of the motor controller. Therefore, the switching rate of IGBTs in the controller is a key factor. A shorter IGBT turn-off time results in a faster change in common-mode voltage slope, leading to higher shaft voltage through the equivalent circuit. This is because the IGBT turn-off rate determines the rate of change of motor phase voltages, affecting high-frequency components in the common-mode voltage. Thus, investigating different IGBT switching characteristics is essential for addressing bearing electrical corrosion in electric cars.
2.2. Simulation Analysis of IGBT Switching Speed Impact
To validate the influence of IGBT switching speed, a simulation model was built using MATLAB based on the equivalent circuit for the breakdown state. The electrical parameters used in the simulation are summarized in the tables below, which are typical for China EV PMSM systems.
| Parameter | Value |
|---|---|
| Rated Power (kW) | 44 |
| Peak Power (kW) | 150 |
| Motor Poles & Slots | 8 poles & 48 slots |
| Parameter | Value |
|---|---|
| Stator Winding Inductance \( L_{u/v/w} \) (mH) | 0.24 |
| Stator Winding Resistance \( R_{u/v/w} \) (mΩ) | 12.5 |
| Controller Stray Inductance \( L_{c-IGBT} \) (mH) | 0.248 |
| Controller Equivalent Resistance \( R_{c-IGBT} \) (mΩ) | 0.89 |
| Controller PN to Ground Equivalent Capacitance \( C_{P/NG} \) (nF) | 425 |
| Controller PN to Ground Equivalent High-Frequency Resistance \( R_{P/NG} \) (kΩ) | 155 |
| Motor Stator Neutral to Housing Stray Capacitance \( C_{NeuG} \) (nF) | 17.32 |
| Stator-Rotor Parasitic Capacitance \( C_{wr} \) (nF) | 43.8 |
| Rotor Shaft to Housing Equivalent Capacitance \( C_{rg} \) (pF) | 4.2 |
| Motor Bearing Equivalent Inductance in Breakdown \( L_{mot-bearing} \) (μH) | 0.1 |
| Motor Bearing Equivalent Resistance in Breakdown \( R_{mot-bearing} \) (Ω) | 4 |
The simulation tested the effect of varying IGBT turn-off times on shaft voltage and current. Results showed that when the turn-off time decreased from 500 ns to 200 ns, the common-mode voltage slope increased from 5.2 kV/μs to 12.8 kV/μs, and the shaft current peak rose from 0.08 A to 0.15 A. Conversely, when the turn-off time was extended to 800 ns, the shaft voltage and current approached zero. This theoretically suggests that if the IGBT turn-off speed is reduced below a certain threshold, shaft voltage and current can be minimized, lowering the risk of electrical corrosion in electric car motors. This insight is vital for China EV development, where balancing performance and reliability is key.
3. Experimental Verification
To validate the equivalent circuit model and the factors influencing shaft voltage and current, an experimental test bench was set up. This section details the bench configuration and results from testing different IGBT modules, emphasizing implications for the electric car industry.
3.1. Test Bench Setup
The test bench was designed to measure shaft current and voltage accurately. An insulating ring was embedded in the motor bearing outer ring to isolate it from the motor housing. A wire was welded to the bearing outer ring and connected to the housing to measure shaft current. A carbon brush contacted the rotor shaft to collect the potential difference between the shaft and housing, representing the shaft voltage. The common-mode voltage was measured at the stator’s three-phase neutral point relative to the housing. After setup, tests were conducted under various operating conditions relevant to electric car applications.
3.2. Results of IGBT Module Differences
On an inductive load test bench for module single-pulse and double-pulse tests, four IGBT modules were compared for their turn-on and turn-off performance. The test conditions were set at 420 V and 480 A RMS. Key findings include: IGBT turn-off speeds are faster than turn-on speeds due to diode characteristics, and different modules exhibit variations in switching speeds influenced by drive parameters and diode properties.
Specifically, modules labeled A29 and A12 were tested in an electric car drivetrain under conditions of 320 V bus voltage, 3000 rpm output speed, and zero torque. The measured shaft voltage, shaft current, and common-mode voltage waveforms were analyzed, and peak values are summarized in the table below.
| Module | Turn-on Time (ns) | Turn-off Time (ns) |
|---|---|---|
| A26 | 240 | 2200 |
| A12 | 120 | 680 |
| A29 | 120 | 360 |
| A20 | 120 | 1500 |
| IGBT Module | Shaft Voltage Peak (V) | Shaft Current Peak (A) |
|---|---|---|
| A29 | 14.18 | 0.113 |
| A12 | 12.91 | 0.102 |
The turn-off time for module A12 was 680 ns, while for A29 it was 360 ns, representing an 88% increase for A12. Correspondingly, A29 exhibited a 9.8% higher shaft voltage peak and a 10.8% higher shaft current peak compared to A12. These results align with the simulation findings, confirming that IGBT switching speed directly affects shaft voltage and current in electric car PMSMs. This has significant implications for China EV manufacturers selecting components for optimal performance and durability.
4. Conclusion and Suppression Techniques
This study provides a comprehensive analysis of bearing electrical corrosion in PMSMs for electric cars, leading to several key conclusions. The root cause is excessive shaft current due to shaft voltage surpassing the oil film’s insulation strength, driven by common-mode voltage. IGBT switching speed plays a critical role, as faster switching increases common-mode voltage \( dv/dt \), elevating shaft voltage and corrosion risk. Simulations and experiments consistently demonstrate that reducing IGBT turn-off speed can mitigate this issue, but the trend in China EV development toward higher power density and lower loss, such as with SiC and GaN semiconductors, exacerbates the challenge.
Since shaft voltage cannot be entirely eliminated, the industry focuses on “diverting” and “blocking” strategies to minimize shaft current damage to bearings. In diverting approaches, methods like increasing capacitance between the bearing and housing or using conductive carbon brushes provide alternative paths for high-frequency currents. In blocking approaches, enhancing bearing voltage resistance through materials like ceramic rolling elements is common. These techniques are essential for improving the reliability and longevity of electric car motors, particularly in the competitive China EV market.
Future work should explore advanced materials and control algorithms to further suppress common-mode voltage without compromising efficiency. As electric car technology evolves, continuous research into bearing electrical corrosion will be crucial for sustaining the growth of China EV and global automotive industries.