Diagnosis of a Motor–Controller Pair: Methods and Key Points*
When a BLDC motor does not function as expected — knocking, loss of torque, random cut-offs or abnormal heating — it is essential to adopt a methodical diagnostic approach.
The motor–controller pair forms a closely linked system: an anomaly in one often affects the other.
Here are the steps to follow to effectively identify the causes of malfunction.
1. Reading Controller Fault Codes
The first step is to query the controller to read the recorded fault codes.
This information provides a direct indication of the nature of the problem:
Overvoltage: DC bus voltage too high (excessive regeneration, full battery, harsh braking).
Undervoltage: pack voltage too low (discharged battery, cables too long or insufficient section).
Overcurrent: current spike beyond the limit (harsh start, partial short circuit, excessive torque).
Overheating: controller or motor temperature above safety thresholds (insufficient cooling).
Sensor error: inconsistency or absence of Hall/encoder signals (poor wiring, faulty sensor, noise).
Each code must be analysed in its context: some faults are consequential to others (e.g. overcurrent due to phase inversion).
2. Check System Telemetry
Modern controllers offer comprehensive telemetry via serial link, USB or CAN bus.
Analysing these values allows confirming or ruling out certain hypotheses:
Motor current: consistent with mechanical load?
Pack voltage: stable under load? excessive drop during accelerations?
Temperatures: motor, power electronics, heatsink.
Speed and command: correspondence between command and actual response.
Comparing these data to expected or nominal values allows quick identification of an electrical or thermal anomaly.
3. Physical Measurements: Oscilloscope or Current Clamp
When telemetry is not enough, an oscilloscope check or DC/AC clamp can reveal hidden phenomena:
Excessive current ripples → defective filtering or PWM control.
Noise or oscillations on DC voltage → cables too long, ageing capacitors.
Significant voltage drops during current demands → line resistance too high or degraded connectors.
These measurements help distinguish a purely electrical problem from a software or sensor problem.
4. Common Problems and Their Symptoms
| Problem | Observable Symptoms | Probable Causes | Correction Paths |
| Phase or Sensor Inversion | Knocking, vibration, motor not starting | U/V/W phases or Hall wires reversed | Check pinout and commutation sequence |
| Motor or Controller Overheating | Unexpected stops, loss of torque, thermal fault | Poor dissipation, mounted on isolated chassis, no thermal paste | Review mounting, ventilation, thermal contact |
| CAN Interference | Loss of communication, erroneous frames, active watchdog | Poor termination, unshielded cable, ground loop | Check 120 Ω resistors, shielding, bus routing |
| Intermittent Contacts / Oxidised Connectors | Random faults, cut-offs, unstable current | Poorly tightened terminals, oxidation, vibrations | Tighten, clean, secure connectors |
| Under-voltage Under Load | Impossible starts or cut-offs during acceleration | Weak battery, cable section too small | Check power supply, calibrate voltage limiter |
5. Recommended Diagnostic Method
Consult recorded faults in the controller.
Observe telemetry in operation (current, voltage, temperature, speed).
Compare data to nominal values motor/controller.
Measure critical signals with suitable instruments if necessary.
Visually and mechanically inspect (connectors, cables, fixings, heatsink).
This structured approach avoids unnecessary replacements and often allows for quickly isolating the source of the problem.
Conclusion
Diagnosing a BLDC motor–controller set requires a comprehensive understanding of the system:
electrical, thermal, mechanical, and communication.
By combining fault reading, telemetry analysis, and physical measurements, most failures can be effectively identified.
Preventive maintenance (regular checking of tightness, cleaning, thermal monitoring) remains the best strategy to avoid costly breakdowns and ensure reliable and durable operation.
*: The technical information presented in this article is provided for guidance only. It does not replace the official manuals of the manufacturers. Before any installation, handling or use, please consult the product documentation and follow the safety instructions. Torque.works cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.