Research On Permanent Magnet Brushless DC Motor For Small Electric Vehicle Drives

It is getting generally accepted to control air pollution from traditional inner combustion vehicles in cities by developing and expanding the use of electric vehicles (EV). Its promising future brings a new opportunity to promote electric machinery industry. High efficiency, big torque output and excellent torque control performance are highly preferred in an EV drive system and BLDCM (permanent magnet brushless dc motor) is a competitive candidate in EV systems due to its adherent advantages such as high efficiency, high power density and convenience in torque control. It is worthwhile to improve EV performance by studying the driving control methodology of a BLDCM with the consideration of EV system characteristics.Because of the increasing requirement of driving performance and continuous improvement in control methods, the design of electric machine and driving control strategies go closely by and by. Moreover, the synthetical consideration in system level of the two parts is highly demanded due to the mechatronic nature of BLDCM. Therefore, the design model and driving technique are concentrated in this article with the consideration of such applications as electric bikes, electric scooters and electric motorcycles.Firstly, the circuit-field coupled time-stepping finite element model of BLDCM was studied from the viewpoint of design, which extends from traditional static electromagnet method and sinusoidal steady state method to the non-sinusoidal characteristic analysis in time domain. Compared with the traditional methods, the circuit-field coupled time-stepping finite element model takes the terminal voltage as system inputs instead of currents and in further, the rotor is able to rotate. This makes the simulation match most practical driving system operation and offers a powerful platform to analyze dynamic motor operation characteristic. It is very difficult to solve the systemequations when an inverter is used to drive a star connected motor without neutral point. An excellent solution was proposed here, which inherited the solution for star-connected motor model with neutral point easily.The research on BLDCM control characteristics consists of two sections: motoring operation control and energy generating braking control. The research on motoring states was started with the analysis of torque-ripple-reduction control methods. It shown for the first time that there is a limitation in all the torque ripple reduction strategies because of the topology of power converter and back electromotive force. Considering the characteristics of EV power module design, its topology was studied then. The concept of coupling and de-coupling between capacitor filter and invert was proposed, based on which, a novel phase current measuring method was given to realize closed loop torque control.Constant power operation in motoring mode is always an issue in BLDCM applications, especially in EVs. According to the study of EB and EM dynamic operation, a constant power control method was realized and evaluated. A further study on the complication of constant power running states illustrated the value of system simulation method in evaluating BLDCM constant power running ability. For braking control, the particularity of EV system was studied and a unique inverter control method suited for both motoring and braking was proposed.Based on studies aforesaid, an intelligent electric bike system was developed. Its unique characteristics is no hands control operation, which enables the motion drive system to develop torque in proportion to manual torque on pedals. The author's special contribution lies in the development of the manual torque transducer using eddy current sensing theory, which is the most critical module of the system.