Efficiency comparison between Brushless dc motor and Brushless AC motor considering driving method and machine design

This paper describes an approach to establish high efficient drive such a Brushless AC (BLAC) drive by a Brushless DC (BLDC) drive. BLDC drive which is excited rectangular voltage for 120 electrical degrees per cycle is commonly used for the low cost applications of Permanent Magnet Synchronous Motors (PMSMs). This drive method contains low-order harmonics in the current waveform. These harmonics cause a mechanical vibration and the electrical losses. Therefore, BLDC drive is not applicable to the sinusoidal back-EMF machines. However, considering about a high frequency loss by the PWM carrier and also about the switching loss, BLDC drive has a potential to have higher efficiency than the BLAC drive. This paper compares the overall efficiency which includes PWM carrier frequency loss by using the coupling analysis between Finite Element Analysis (FEA) and a circuit simulator. The tested machines are two of Surface type PMSMs (SPMSM). One has a sinusoidal back-EMF waveform (BLAC machine) and the other has a trapezoidal back-EMF waveform (BLDC machine). The both driving methods of BLDC drive and BLAC drive are adapted to each machine, their efficiencies are compared by both simulation and experiments. The results show that the BLDC machine with BLDC drive has higher efficiency than BLAC machine with BLAC drive.