As a professional in the field of EV repair, I have witnessed the rapid growth of electric vehicles driven by the global energy revolution. The motor, particularly the permanent magnet synchronous motor (PMSM), is the core component that propels these vehicles, and any malfunction can lead to operational failures or even safety hazards. In this article, I will share my insights into the working principles of PMSM in electric vehicles, outline a systematic fault diagnosis process, and present a detailed case study to illustrate effective repair techniques. This comprehensive guide aims to enhance the understanding and practices in electrical car repair, emphasizing the importance of accurate diagnosis and maintenance in ensuring vehicle reliability and performance. The discussion will incorporate multiple tables and mathematical formulas to summarize key concepts, providing a robust framework for professionals engaged in EV repair.
The permanent magnet synchronous motor used in electric vehicles consists of several critical components, including high-voltage line interfaces, rear covers, clutches, stators with windings, and rotors equipped with permanent magnets. Its operation relies on the interaction between magnetic fields: when three-phase alternating current flows through the stator windings via the high-voltage interface, it generates a rotating magnetic field within the stator. The direction and speed of this field are determined by the current’s frequency and phase. This rotating field crosses the air gap and acts on the permanent magnets on the rotor. Since the rotor’s magnetic field is fixed, the rotating field attracts or repels it, causing the rotor to synchronize with the stator’s rotating field and thus enabling motion. Despite their efficiency, PMSMs have drawbacks, such as the susceptibility of permanent magnets to demagnetization under high temperatures, vibrations, or overcurrent conditions, which increases the risk of motor failure in complex operating environments. Additionally, the high cost of permanent materials often limits their use to mid-range and high-end electric vehicles, making EV repair a critical area for cost-effective solutions.
In my experience with electrical car repair, the fault diagnosis process for electric vehicle motors must be methodical to identify issues accurately. It begins with gathering fault information from the driver, including symptoms like startup failure, power interruption, abnormal temperature rises, or unusual vibrations and noises, along with contextual data such as environmental temperature, road conditions, and load status. Visual inspections follow, checking for physical deformities in the motor housing, leaks in the cooling system, or damaged wiring. Next, the power system is examined using insulation testers to measure the high-voltage battery pack’s insulation resistance, which should typically exceed 500 Ω/V, and multimeters to assess total voltage and individual cell voltage differences, with normal values staying within 50 mV. Circuit checks involve verifying main relay contact voltage drops below 50 mV, pre-charge circuit capacitance within ±10% of nominal values, and fuse continuity. On-board diagnostics are then performed via the OBD interface to retrieve fault codes from the motor control unit, analyzing deviations between actual and commanded torque outputs. Oscilloscopes capture signals from resolvers, such as phase differences between excitation and sine waves, which should be around 120°±2°, and confirm IGBT module drive voltages at least 50% of the DC bus voltage. Finally, a comprehensive analysis integrates electrical data, signal waveforms, and thermal imaging to pinpoint fault components, leading to a repair plan based on fault tree analysis (FTA), including parts lists, processes, and quality standards for a closed-loop management system. This systematic approach is essential in EV repair to address the multifaceted nature of motor failures.
To illustrate the practical application of these diagnostic methods in electrical car repair, I will discuss a case involving a domestic electric vehicle model. The vehicle, an MPV with a rear-mounted single PMSM, had been in use for over two years and accumulated approximately 15,000 km. The owner reported symptoms such as sluggish acceleration, reduced power output, abnormal noises from the motor compartment, rapid temperature increases of up to 50°C, and noticeable speed fluctuations dropping by around 1,500 rpm. The fault manifestations are summarized in the table below, which highlights the importance of detailed data collection in EV repair.
| Fault Type | Specific Manifestations | Phenomenological Features |
|---|---|---|
| Power Performance Anomaly | Delayed acceleration response; power interruption during overtaking; top speed reduced from 130 km/h to 102 km/h | Dashboard displays “power limited” warning; BMS records peak power output衰减 to 80 kW |
| Abnormal Noise | High-frequency sharp noise (8–10 kHz) from motor compartment, resembling metal friction, worsening during rapid acceleration | Noise spectrum analysis shows main frequency at 12 kHz, amplitude up to 75 dB(A); strong correlation with motor speed (r=0.92) |
| Temperature Anomaly | Motor temperature surge from 85°C to 102°C triggering thermal protection; radiator outlet temperature reaches 115°C | Thermal imaging indicates local hot spots on windings (145°C); coolant flow rate measured at 3.2 L/min against standard 6–8 L/min |
| Control System Alerts | OBD codes P0A00 (motor temperature sensor fault) and P0E00 (overload protection); control signal delays | CAN bus data: temperature signal fluctuations ±20°C; current harmonic distortion (THD) at 9.3% |
The diagnostic process began with an inspection of the cooling system, a common focus in EV repair. Using a 0–20 bar pressure pump, I tested the sealing integrity of the cooling circuit and observed pressure fluctuations of ±2.3 bar, significantly higher than the normal ±0.5 bar, indicating a faulty water pump. The coolant flow rate was measured at 3 L/min, below the standard range of 5–8 L/min, suggesting possible blockages. Further analysis with X-ray fluorescence spectroscopy (XRF) revealed high concentrations of CaCO3 and CaSO4 scale deposits, confirming that infrequent coolant changes had led to clogging, impairing circulation and causing overheating during high-speed operation. This underscores the need for regular maintenance in electrical car repair to prevent such issues.
Next, the electrical system was examined using a HYG-10kV high-voltage megohmmeter with ±2% accuracy. Measurements of the stator winding insulation resistance showed values of 9.8 MΩ for U-phase, 8.2 MΩ for V-phase, and 9.5 MΩ for W-phase, all below the standard of ≥20 MΩ. Partial discharge detection under 1.5 times the rated voltage revealed discharge levels of 106 pC, far exceeding the ≤50 pC limit, indicating insulation aging as a primary cause of speed fluctuations. Analysis of the motor control unit (MCU) fault logs identified anomalies in current loop PI parameters, with KP=0.8 and KI=0.02 exceeding standard values of 0.45 and 0.015, respectively, leading to overcurrent protection false triggers. Additionally, a Fluke 438-II power quality analyzer detected a 15° phase shift in the A-phase Hall signal, posing risks of control logic errors. These findings highlight the complexity of EV repair, where multiple electrical factors can contribute to motor faults.
Vibration spectrum analysis was conducted using a PCB 352C33 triaxial accelerometer with a range of ±50g and frequency response of 1–10 kHz. The measured vibration velocity was 12 mm/s, compared to a normal value of 7.1 mm/s. To identify bearing-related issues, I calculated the bearing fault characteristic frequency (fBPFO) using the formula:
$$ f_{BPFO} = \frac{N}{2} \left(1 – \frac{d}{D} \cos \alpha \right) f_r $$
where N is the number of rolling elements, d is the ball diameter, D is the pitch diameter, α is the contact angle, and fr is the rotational frequency. For a deep-groove ball bearing 6205 with N=9, D/d=1.12, α=0°, and measured speed of 1,000 rpm (fr=20 Hz), the calculated fBPFO was 11.2 kHz, closely matching the observed peak frequency of 12 kHz in the vibration spectrum. This confirmed severe bearing wear and inner race peeling, common issues addressed in electrical car repair to restore motor stability.
Based on the diagnosis, I implemented targeted repair measures. First, for the cooling system, I employed a combined physical and chemical approach to clean the pipelines. This involved circulating a pH 8.5 weak alkaline cleaning agent while using ultrasonic vibrations to dislodge scale, ensuring an internal surface roughness Ra ≤1.6 μm. High-pressure nitrogen pulses at 0.5 MPa were applied to clear debris from radiator fins, restoring the coolant flow cross-section to design specifications. Post-cleaning, a turbine flow meter confirmed a flow rate of 6 L/min, within the normal range. This step is vital in EV repair to prevent overheating and extend motor life.
For the worn bearings, I followed standardized replacement procedures using third-generation ceramic ball bearings selected based on shaft neck dimensions with H7/js6 tolerance. The contact angle was set to 15° to enhance axial load capacity. Installation involved a hydraulic method with an oil pressure expansion sleeve to achieve a 0.03 mm interference fit. After positioning, high-temperature lithium-based grease (NLGI grade 2) was applied, filling 30% of the free space, and oil seal dust covers were added for protection. This meticulous approach in EV repair ensures durability and reduces future failure risks.
To address the low insulation resistance, I performed a drying treatment. Compressed air with a dew point ≤-40°C was used to blow down the windings for 6 hours to remove metal dust and contaminants. The motor was then placed in a vacuum drying oven and subjected to a 24-hour gradient heating process at 90°C and 100 Pa, with temperature increases not exceeding 5°C per hour. After this, a 500 V megohmmeter measured the winding-to-ground insulation resistance, which improved to 12 MΩ. This procedure is a cornerstone of electrical car repair for maintaining electrical safety and performance.
Post-repair validation included laboratory and road tests to quantify improvements. Laboratory tests employed a no-load to load two-stage method, with results compared against standards. For instance, in no-load tests at 12,000 rpm for 2 hours, vibration severity met ISO 10816-3 standards at 5.2 m/s². Load tests showed a temperature rise of 40°C, within the ≤50°C limit. Road tests demonstrated enhanced performance, such as acceleration from 0 to 100 km/h improving from 12.5 s to 9.7 s, and CLTC range recovering to 310 km. NVH performance at high speeds (120 km/h) showed motor compartment noise reduced to 62 dB(A) and in-cabin noise to 48 dB(A), meeting requirements. The table below summarizes key performance metrics before and after repair, illustrating the effectiveness of the EV repair interventions.
| Test Item | Before Repair | After Repair | Standard Requirement | Judgment |
|---|---|---|---|---|
| Coolant Flow Rate (L/min) | 3 | 6 | 5–8 | Met |
| Vibration Severity (mm/s) | 10 | 5.2 | ≤7.1 | Met |
| Peak Power (kW) | 80 | 122 | ≥120 | Met |
| 0–100 km/h Acceleration Time (s) | 12.5 | 9.7 | ≤10.0 | Met |
| Continuous Operation Temperature Rise (°C) | 53 (138 → 85) | 40 (125 → 85) | ≤50 | Met |
| Vibration Acceleration (m/s²) | 15.3 | 5.8 | ≤7.0 | Met |
| Insulation Resistance (MΩ) | 8.25 | 12 | ≥10 | Met |
| Range Capability (km) | 265 | 310 | ≥300 | Met |
| NVH Performance at 120 km/h (dB(A)) | Motor noise: 72, In-cabin noise: 66 | Motor noise: 62, In-cabin noise: 48 | Motor noise ≤68, In-cabin noise ≤53 | Met |
In conclusion, electric vehicle motor faults can arise from diverse causes, including cooling system failures, electrical insulation degradation, and mechanical wear, necessitating systematic approaches in EV repair. Through this case study, I demonstrated how a methodical diagnosis and targeted repairs can restore motor performance and overall vehicle functionality. As technology evolves, permanent magnet synchronous motors will continue to be pivotal in the electric vehicle industry, but fault types may become more complex. Therefore, professionals in electrical car repair must engage in continuous learning to adapt to new challenges and advancements. By embracing detailed diagnostic techniques and preventive maintenance, we can enhance the reliability and safety of electric vehicles, contributing to sustainable transportation solutions. This holistic perspective is essential for advancing the field of EV repair and ensuring long-term vehicle health.