Limitations of the conventional phase advance method for constant power operation of the brushless DC motor

The brushless DC motor (BDCM) has high-power density and efficiency relative to other motor types. These properties make the BDCM well suited for applications in electric vehicles provided a method can be developed for driving the motor over the 4 to 6:1 constant power speed range (CPSR) required by such applications. The present state of the art for constant power operation of the BDCM is conventional phase advance (CPA). In this paper, we identify key limitations of CPA. It is shown that the CPA has effective control over the developed power but that the current magnitude is relatively insensitive to power output and is inversely proportional to motor inductance. If the motor inductance is low, then the RMS current at rated power and high speed may be several times larger than the current rating. The inductance required to maintain RMS current within rating is derived analytically and is found to be largely relative to that of BDCM designs using high-strength rare earth magnets. Thus, the CPA requires a BDCM with a large equivalent inductance.