Electromagnetic Performance Analysis Of CPPM Brushless DC Starter Generator

Belt-driven Starter Generator(BSG) start-stop system can reduce vehicle fuel consumption and emissions by cutting down engine idle running time. As a hybrid excitation machine, consequent pole permanent magnet(CPPM) brushless DC starter generator with trapezoidal electromotive force(EMF) is a potential candidate for start-stop system applications with the advantages of high torque capability, simply control strategy, high power density and controllable air gap flux. And it meets requirements of the system that offering constant voltage in wide speed range as a generator and offering maximum torque output for quick engine cranking as a starter. However, the electromagnetic performance and control scheme of this machine should be completed and need further discussion. In this paper, the electromagnetic performance of CPPM brushless DC starter generator used has been discussed as below.Firstly, the relation of field and permanent magnetic field and the influence of armature magnetic field for field and permanent magnetic field have been analyzed by FEM. Moreover, the calculation formulas of flux per pole and electromagnetic torque have been deduced by magnetic circuit analysis method, experimental results of a 2kW prototype verified the accuracy of calculation formulas.Secondly, the distribution characteristics of air gap flux density in one pole side and compound air gap flux density have been analyzed, in view of the asymmetric air gap flux density in one pole side, a low cost rotor position detection method using six Hall sensors has been proposed, and the six Hall sensors have been divided into six groups and two signals in each group have been performed logical operation, the processed signals are the same as the Hall signals in brushless DC motor. Experiment verified the correction of the analysis about air gap flux density in one pole side and the applicability of the proposed rotor position detection method.Finally, the relation between bus voltage and field current has been deduced based on the mathematical model of phase voltage, moreover, the average phase current and DC-link current are unequal in the uncontrolled rectifier due to the conversion process, so the conversion process of phase currents have been analyzed, and its influence on bus voltage has also been discussed. Accuracy of the bus voltage calculation formula was verified by the experimental results.