Efficiency comparison between brushless DC motor and brushless AC motor considering driving method and mAChine design

This paper describes an approach to achieve a high efficiency drive such a brushless AC (BLAC) drive by a Brushless DC (BLDC) drive. BLDC motors with an excited rectangular voltage for 120 electrical degrees per cycle are commonly used for low-cost applications of permanent magnet synchronous motors (PMSMs). This drive method yields low-order harmonic components in the current waveform. These harmonic components cause mechanical vibration and electrical losses. Therefore, a BLDC drive is not applicable to a sinusoidal back-EMF machine. However, with regard to the high-frequency loss caused by a pulse width modulation (PWM) carrier and a switching loss, the BLDC drive has the potential to achieve higher efficiency than the BLAC drive. This paper compares the overall efficiency, which includes the PWM carrier frequency loss, by using a coupling analysis between a finite element analysis (FEA) and a circuit simulator. Two surface-mounted PMSMs (SPMSMs) are tested. One has a sinusoidal back-EMF waveform (BLAC machine), and the other has a trapezoidal back-EMF waveform (BLDC machine). Both the driving methods of the BLDC drive and the BLAC drive are adapted to each machine and their efficiencies are compared by both simulations and experiments. The results show that the BLDC machine with a BLDC drive has higher efficiency than the BLAC machine with a BLAC drive.