Average torque control of brushless DC motor based on one-cycle control method

Abstract

To obtain the constant torque of brushless DC (BLDC) motor with ideal trapezoid back EMF, rectangular phase-current is required. However, due to the tolerance in manufacturing and inevitable trade-off in motor designs, the back EMF of BLDC motor is not ideal in general, thus the torque ripple occurs if phase-current maintains rectangular waveform. To suppress the torque ripple of the BLDC motor with non-ideal back EMF, an average torque control based on one-cycle control (ATC-OCC) method is proposed in this paper, which makes the output torque equal to the reference in each cycle. To observe the average torque, an indirect energy detection method is proposed, which detects the DC bus voltage and current to calculate average torque. Comparing with the present methods, this method has the following advantages: Firstly, back EMF information is not required, thus there is no offline measurement or online estimation, adaptive to BLDC motors with different back EMF. Secondly, accurate rotor position is not required, thus there is no additional measuring device or the complicated estimation algorithm, reducing the cost of the system and the requirement for the microcontroller. Thirdly, just one single current sensor is required. Simulation results have validated the feasibility of the proposed method and its effectiveness to suppress the torque ripple.