Instead of detecting the rotor position using sensors (Hall elements or a Hall IC), a sensorless motor drive system monitors changes in the motor back-EMF voltage to predict the timing of current conduction. However, applications in which the motor RPM fluctuates considerably because of load variations make it difficult to predict this timing accurately, causing the motor to fall out of step. (The motor stops rotating.)
If the back-EMF voltage is insufficient, it is difficult to realize stable sensorless drive. When the back-EMF voltage is low at low RPM, motor operation tends to become unstable.