A Brushless DC Motor Drive Without a DC Link Capacitor

Abstract

In recent years, economic and environmental considerations have led the industry towards energy efficient technologies. As a result, in the context of industrial motor drives, Brushless DC (BLDC) and Permanent Magnet Synchronous (PMS) motors have become popular as energy efficient and reliable alternatives for induction motors. Both BLDC and permanent magnet synchronous motors are electronically commutated based on the position of the rotor by using voltage source inverters that consist of a rectifier, a DC link capacitor and an inverter. In comparison to the other electronic components in the circuit, the DC link capacitor has a limited lifetime, which is severely dependent on the ambient operating temperature. However, with advancements in technology, direct power converters such as matrix converters that do not employ DC link capacitors are becoming popular in industry. At present, matrix converters and similar style direct converters are economically feasible in high power applications and are expected to be economically feasible for low power ratings in the future. A technique to eliminate the DC link capacitor from conventional BLDC motor drives is proposed in this thesis. Without the DC link capacitor, the BLDC motor directly operates from the rectified mains supply. A single switch control technique that allows speed and torque control of the BLDC motor is adopted. The proposed technique is simulated and experimentally validated. Also, a comprehensive performance comparison is carried out between the proposed technique and the conventional techniques. Although the proposed technique produces periodic torque ripples, the effectiveness of the proposed technique is validated for low cost BLDC motor drives. A new comprehensive buck converter based mathematical model for the BLDC motor drive is presented to analyse the torque ripple. Using the model, uncontrollable torque regions that occur due to the variable input voltage of the DC link capacitor free BLDC motor drive are identified. The reduction in torque due to the absence of the DC link capacitor is obtained by iteratively solving the mathematical model. The proposed buck converter based model is verified by comparing the analytical results, simulated results, and the experimental results. To compensate for the torque ripple, a compensation technique based on an actively controlled small DC link capacitor is proposed. A further simplified buck converter based model for the DC link capacitor free BLDC motor drive is proposed for practical purposes. The simplified model is compared with the comprehensive buck converter based model to show the accuracy of the model. Although the proposed compensation technique increases the hardware complexity of the motor drive, the overall cost is expected to be lower. A price comparison between the conventional BLDC motor drive and a DC link capacitor free BLDC motor drive with the proposed compensation technique is presented using volumetric pricings obtained through retailers. The effectiveness of the proposed compensation technique is verified by simulations and experimental results. As a solution for complex controls associated with matrix converters, a simple switching algorithm that facilitates the driving of a BLDC motor by a 1 3 matrix converter is presented. Safe commutation techniques are described in detail and the proposed technique is verified by using simulation and experimental results. In principle, the techniques proposed in this thesis are expected to be useful in manufacturing low cost BLDC motor drives with comparable performance.