Phase Locked Loop based Commutation Procedure for Sensorless Operation of a Brushless DC Motor

In this paper, we present the theory and implementation of a novel sensorless control strategy for BLDC motors. The proposed control technique overcomes drawbacks of filter-based methods and improves upon existing filterless methods. In order to overcome a frequency dependent phase delay induced in the back-EMF signal when using low-pass filters, a PWM based synchronous sampling technique is used to extract the zero-crossing points. An XOR gate is used in place of a digital multiplexer to combine the three zero-crossing signals so as to capture all logical transitions, while eliminating the need for digital select lines. This facilitates a deterministic transition from open-loop to closed-loop state. A square-wave phase-locked loop is used to provide accurate and timely commutations, while also suppressing RMS-jitter by nearly 75 %, thereby improving efficiency and lowering noise. Analytical and experimental results are also presented to show how the method works while potentially offering an efficiency upto 97.5 %.