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The direct back-electromotive-force (EMF) detection method previously described in a sensorless brushless dc (BLDC) motor-drive system (Proc. IEEE APEC, 2002, pp. 33-38) synchronously samples the motor back EMF during the pulsewidth-modulation (PWM) off time without the need to sense or reconstruct the motor neutral. Since this direct back-EMF-sensing scheme requires a minimum PWM off time to sample the back-EMF signal, the duty cycle is limited to something less than 100%. In this paper, an improved direct back-EMF detection scheme that samples the motor back EMF synchronously during either the PWM on time or the PWM off time is proposed to overcome the problem. In this paper, some techniques for automotive applications, such as motor-rotation detection, and current sensing are proposed as well. Experimental results are presented
Jinju Shao (Fri,) studied this question.