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Purpose -The purpose is to present a sensorless control method by which high-resolution rotor position information is estimated and used for phase-advancing operation of a high-speed permanent magnet (PM) brushless DC (BLDC) motor. Design/methodology/approach -The proposed sensorless control approach uses hardware to observe the flux vector which is excited by rotor magnets. It can provide the rotor position which is the same as the phase angle of the observed flux vector. Findings -High-resolution rotor position signal of the BLDC motor for dynamic phase-advancing control cannot be directly obtained from the conventional Hall-effect sensors, or via the traditional back-EMF-based sensorless control strategies in which the back-EMF may be even undetectable at high-speed. The proposed rotor-flux-observer (RFO)-based sensorless control method overcomes these problems, and meanwhile provides high-resolution rotor position information for the phase-advancing purpose. Originality/value -The RFO-based sensorless control is traditionally applied to PM brushless ac (BLAC) operations, where the motor voltage vector can be calculated from the inverter switching status. However, this is not readily applicable to a BLDC motor since the voltage of the floating phase cannot be calculated. Moreover, during high-speed operation, the microprocessor may not be sufficiently fast to calculate the high-resolution rotor position. Therefore, in this paper, it is proposed to use hardware to observe the rotor-flux-vector. The microprocessor only samples the vector's a-and b-components and calculates the phase angle, hence, its burden is low. The proposed method is validated with a 1.8 kW 85,000 rpm BLDC motor system.

Pulse width modulation (PWM) of voltage source inverter (VSI) is common for speed regulation of permanent magnet (PM) brushless direct-current (BLDC) motor drives. However, when the motor runs at high speed, the conventional PWM technique may become unfeasible due to the low carrier ratio of inverter, which makes the motor current distorted and the motor core loss increased significantly. In this paper, two control strategies, namely single pulse variable width (SPVW) and dual pulses variable width (DPVW), are introduced for the high-speed PM BLDC motor drives, and are comprehensively investigated. Finite element method (FEM) is employed to evaluate and compare the drive system performance when using the conventional PWM, and the proposed SPVW and DPVW, respectively. Influence of the control techniques, especially on the loss distribution, is revealed.