How to Test for Hall Sensor Phase Loss in a Brushless Motor Using an Oscilloscope
I. Pre-Test Preparation
- Ensure the driver is powered on and the motor is running stably under power.
- Securely connect the oscilloscope probe's ground clip to the negative terminal of the motor Hall sensors. Use the probe tip to measure the U, V, and W output terminals of the motor in sequence.
II. Criteria for Determining Normal Waveforms
When the motor is running normally, the three phases of a functioning brushless motor should output waveforms that meet the following characteristics:
- Consistent Amplitude and Symmetrical Waveform: The peak heights of the three-phase waveforms are essentially the same, and the overall shape is symmetrical.
- Approximately 120° Phase Difference: The phase difference between any two phases is stable at around 120°.
- Standard Waveform Type: The output waveform is a standard sine wave or trapezoidal wave.
- Complete Consistency: The height, shape, and frequency of the three-phase waveforms are highly consistent, with no significant differences.
III. Criteria for Determining Abnormalities
If any of the following conditions occur, it can be determined that the motor has a phase loss, internal winding disconnection, or poor wiring contact:
- No waveform on one phase, appearing as a flat line: This indicates no effective signal output from that phase.
- Amplitude of one phase is significantly lower than the other two: The output signal strength of that phase is markedly weaker than the others.
- Severe distortion or abnormality in one phase's waveform: The waveform shows distortion, glitches, or an irregular shape.
Example of a Phase Loss Waveform:
Example of a Normal Waveform:
Updated on: 28/02/2026
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