Is there anything to keep in mind when using a sensorless BLDC motor?

When using a sensorless BLDC motor, caution is necessary for application with low speed rotation or significant load fluctuations.

Fig. 1 Sensorless motor position detection
Fig. 1 Sensorless motor position detection

Instead of using a sensor (Hall element or Hall IC) to detect the rotor position, sensorless drive monitors changes in motor induced voltage and predicts the motor energisation timing while driving.

Fig. 2 Image of induced voltage waveforms at low and high speeds
Fig. 2 Image of induced voltage waveforms at low and high speeds

However, the induced voltage is proportional to the rotational speed. As shown in Fig. 2, if the rotation speed of the motor is low, the induced voltage is also low. If the induced voltage is not sufficient, stable sensorless drive would be difficult and tends to result in unstable operation.

Also, sensorless drives also drive the motor by detecting changes in induced voltage while predicting the energisation timing. However, if the rotation speed of the motor changes significantly due to load fluctuations, the discrepancy between the required energisation timing and the predicted timing becomes large. As a result, the normally energised motor may be out of step(fail to rotate).

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