Sensorless Control of BLDC Motors Using an Improved Low-cost Back EMF Detection Method

In this paper, an improved low-cost back EMF detection method is presented. With proper PWM strategy, the true back EMF zero-crossing point can be directly extracted by detecting voltage difference between the phase terminal and the midpoint of dc link without sensing the motor neutral voltage. No filtering circuit is required, and 100% duty ratio PWM control of BLDC motors is provided with the presented method. For only one of the three phase terminals is sensed to obtain six commutation signals which are used to perform the inverter commutation, the cost of the sensing circuit is reduced significantly; and furthermore, the imbalance in six commutation signals, which is caused by the asymmetrical behavior of three phase windings in BLDC motors, is decreased remarkably. The proposed method makes it possible to achieve good motor performance over a wide speed range and to simplify the starting procedure. The detailed circuit model is analyzed and some experimental results obtained from a sensorless prototype are shown to verify the analysis and confirm the validity of the proposed method.