Research On High Performance Drivetechnology Of Brushless Dc Motors

Technology of brushless DC motor(BLDCM) drives was studied in this dissertation in order to improve the performance of BLDCM and enlarge its scope of application. The aim is to develop new methods that can be used directly on products. High-speed control, torque ripple suppression, optimal commutation angle control and sensorless control are the four aspects being studied in detail.As to high-speed control, the influence of limited work frequency on operation of BLDCM had been studied. The conclusions show that a phenomenon of commutation delaying will always exist because of the limited switch frequency in three-phase pulse width modulation(PWM) control method or the limited software scanning frequency in the variable bus-voltage six-step control method. As the speed of the motor rises, the number of unit work cycles in one single step becomes smaller, and the motor operation performance becomes worse. The negative influences on phase current fluctuation respect to switch frequency or software scanning frequency were made into a quantitative table to direct the design of high-speed BLDCM drives. The feasibility and accuracy of the conclusions were confirmed both by reaction flywheel and high-speed energy storage flywheel.As to torque ripple suppression, a new ‘Three-segement modulation method’ was first presented in this dissertation to eliminate commutation torque ripple base on a thorough analysis of the fundamental cause of commutation torque ripple. During commutation, each PWM period is divided into three functional segments by three-phase cooperative modulation. Each of the three phases is assigned with different duty cycle by calculation, so that the average current slope of on-going phase can be kept equal with the average current slope of off-going phase in each PWM period. The commutation duration can also be adjusted as needed by adopting different duty cycle combinations, by which this method can have more flexibility. This simple and effective method is very suitable for product updating. In order to realize the presented method, a general full speed range implementation technique with minimum commutation time was given, and another five-interval implementation technique was given for low speed applications. The experimental results from a common BLDCM prototype confirm the feasibility and effectiveness of presented three-segment modulation method.As to optimal commutation angle control, the mathematical expression of advance commutation angle with PWM duty cycle, bus-voltage and load current was established, and a new variable bus-voltage six-step control method which solves the current lag problem and improves the current ripple problem was presented. The new method is implemented by adding a small buffer zone in the connecting place of two steps. This buffer zone allows the next step phase switching in before the present step is over, and lets the present step phase continue working for a short time after entering next step. The centerline of the phase current can coincide with the centerline of the back electromotive force(EMF) by determining the buffer zone’s start position which can also be seen as an advance commutation angle. The decrease of the current ripple can be achieved by choosing a proper duty cycle in the buffer zone. It only requires the value of load current to determine the advance commutation angle in this method, and applies to any speed operation once the angle is determined. The experimental results from a D200high-power fan and a common BLDCM show the necessity of advance angle control and feasibility of presented variable bus-voltage six-step control method respectively.As to sensorless control, the three-phase H-bridge structure was adopted to BLDCM, in order to solve traditional Y-connection windings BLDCM sensorless control drive’s problems of inaccuracy of zero-cross point detecting during start stage and difficulty of capturing real back-EMF during high-speed range because of distraction from its neutral point. The new structure makes two conducting phases work in parallel; meanwhile the non-conducting phase is cut off from the circuit for back EMF zero-cross detecting. Parallel connection makes each phase excited with full potential of bus voltage and non-conducting phase’s totally switching out from circuit makes the rotor position signal accurate. The theoretical derivation and analysis of presented method were given. The experimental results from a small inductance prototype and big inductance prototypes show that the presented sensorless control method can be used in higher speed applications and can greatly improve the performance at low-speed range operation. The prototypes were developed to be the next generation products.