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High performance sensorless permanent magnet brushless DC motor operation requires proper and precise commutation sequence. In general, the average torque falls rapidly as the rotor speed increases above the base speed, hence the motor will frequently be found to have insufficient torque at higher speed. One way to counter this problem is by using third harmonic back-emf detection technique during high speed operation because the third harmonic is usually kept a constant relationship with rotor position for any motor speeds and load conditions. Therefore, we could potentially avoid the torque deterioration which commonly occurs in traditional sensorless technique due to the delay of the phase angle between current and back-emf. Furthermore this method is practically free of noise which may be introduced from inverter switches. In this work, we propose a new software scheme of phase-locked loop (PLL) of third harmonic back-emf detection in order to accomplish a precise switching strategy for improving torque produced during high speed operation.Keyword-brushless direct current (BLDC) motor, sensorless control, third harmonic back electromotive force (EMF), phase-locked loop(PLL).

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