Improvement of ASIC-based sensorless control for ultrahigh-speed brushless DC motor drive

Application specific integrated circuit (ASIC) ML4425 is often used for sensorless control of PM brushless DC (BLDC) motor drives. It integrates the terminal voltage of the non-energised phase, which is regarded as the back EMF, so as to determine the proper commutation for the BLDC motor. However, even without PWM, the terminal voltage contains pulses due to the free-wheel diode conduction. The pulse, which appears very wide in an ultrahigh-speed (120 krpm) drive, is also integrated by the ASIC, consequently, the motor commutation is significantly retarded, and the drive performance is deteriorated. In this paper, it is proposed that the ASIC should integrate the third harmonic back EMF instead of the terminal voltage, such that the commutation retarding is eliminated and the motor performance is improved. Basic principle and implementation of the new ASIC-based sensorless controller will be presented, and experimental results will be given to verify the controller.