Recent Challenges and Advances in the Sensorless Commutation of Brushless DC Motors

Abstract

Background: Brushless DC motors are highly efficient motors due to their high torque to weight ratio, compact design, high speed operating capability and higher power density. Conventional Hall sensor based rotor position sensing is affected by the heating, vibration, interference and noise. Objective: The innovative, cost-effective and easily implementable sensorless techniques are essential in order to achieve high efficiency, reduced current and reduced torque pulsations. Furthermore, a delay free, high load fast startup is also an important issue. Methods: In this paper, an extensive review of various techniques based on the detection of freewheeling diode current, phase back EMF zero crossing point detection, back EMF integration method and third harmonic back EMF was done. The effect of various PWM strategies on back EMF detection were studied. Later on, the sensorless schemes based on flux linkage estimation and flux linkage increment were introduced. The load torque observers, unknown input observers, sliding mode observers, L∞ − induced observers, H∞ − deconvolution filter for back EMF estimation were also reviewed. As the brushless DC motors have no back EMF at starting and for back EMF based commutation, minimum speed is required for sufficient back EMF. Therefore various strategies of an open and close-loop reduced current startup have been studied to achieve effective commutation without reverse torque. Initial Position Detection (IPD) schemes, which are mostly based on saliency and current response to inductance variation, are effective where reverse torque is strictly prohibited. A detailed review of these Initial Position Detection techniques (IPD) has also been presented. Results: The detailed mathematical and graphical analysis has been presented here in order to understand the working of the state-of-the-art sensorless techniques. Conclusion: The back EMF detection using direct and indirect methods of terminal voltage filtering has the problem of delay and attenuation. PWM noise, freewheeling diode spikes and disturbance in detected back EMFs are also major drawbacks. The problems such as, parameter detuning, underestimation and overestimation, offset problem, system noise and observer gain variation, etc. limit the applicability of observer based technique. Therefore, a more robust and precise position estimation scheme is essential.