The Design of Brushless DC Motor for Maximum Torque/Volt-Amps Ratio

In this paper, a PM brushless DC motor analysis is presented, which takes into account the actual voltage and current waveforms. To achieve a satisfactory motor design, the analysis has been carried out considering a prefixed torque control strategy and constant inverter volt-ampere rating, verifying the performance of the whole drive system. The purpose of the study is to investigate the influence of a set of design parameters, that are the back-to-supply voltage ratio E/Vdc, the PM cover angle 2am and the commutation advance angle a, in the torque to Volt-Amps ratio of an inverter-fed brushless DC motor. An E/Vdc ratio close to unity will be shown as the more advantageous; the simulations also indicate that a reduced PM angle could improve the overall drive performances. In addition, a suitable angle a of 5-15° is shown to be advantageous to increase the available electromechanical torque of the motor. The analysis takes also into account the torque ripple, that is one of the side-effects of the commutation advance, and that is of interest for many domestic and automotive applications. The study has been carried out on an accurate computer model of the whole drive, that comprises an hysteresis current control and uses the actual (non-idealised) voltage and current waveforms.