Research On Drive Methods For Torque Ripple Suppression In BLDC Machines

Brushless DC machine(BLDCM) has been widely used in applications due to its advantages such as high power density, simple-control and quick dynamic response. However, comparing to the permanent magnetic synchronous machine(PMSM), the major problem is the large torque ripple, which will damage the power system control characteristics, even destroying the machine bearings, lower the machine life and arouse the vibration, resonance and the noise. The BLDCM drive technique for suppressing the torque ripple is investigated in this thesis.Firstly, the reasons which cause the torque ripple of BLDCM are analyzed comprehensively. Where considering the common situation that the back electromotive force(back-EMF) of the BLDCM is not ideal, the torque ripples about the machine with the ideal and non-ideal back EMF are discussed respectively when they are used in the voltage adjusting and current closed-loop control systems, then the reasons for conducting and commutation torque ripple are declared.Afterwards, for the BLDCM with the ideal back-EMF, to suppress both the conducting and the commutation torque ripple simultaneously, a new power converter topology is proposed in this thesis and the control algorithm is designed accordingly. The output of the converter can be selected between the high and low voltage, thus in conducting period, the adjustable low voltage is selected to regulate the machine speed while the high voltage is selected in commutation period to reduce the torque ripple. Because the voltage is adjusted to regulate the speed, the torque ripple caused by the inverter chopper is eliminated, while the high voltage is used to compensate the torque decline in commutation period, the conducting and commutation torque ripple can be suppressed simultaneously.Then, for the BLDCM with the non-ideal back EMF, to suppress the torque ripple caused by the back EMF distortion, a new average torque control is proposed in this thesis. This method applies the ideal of the one-cycle control, the output average torque can be controlled to follow the reference in each cycle. Because the control cycle is very short, the output torque can be controlled in real-time, reducing the torque ripple. For the feedback average torque, the input energy in each cycle is observed to estimate the average torque, which can be realized merely by measuring the DC bus voltage and current. Comparing to the conventional methods, this method has no sophisticated algorithm to observe the back EMF, thus it has the strong universality and can be adaptable for machines with different back-EMF. Moreover, the precise rotor position is not need to estimate the instantaneous torque, the additional position sensor or the position estimating algorithm is not used, lowering the system cost and the requirement for the microprocessor.Lastly, for the BLDCM with non-ideal back-EMF, to suppress the torque ripple caused by the back-EMF distortion and the commutation, a novel average torque control using the overlap commutation strategy is proposed. In conducting period, the average torque control can ensure the output average torque be equal to the reference. However, in commutation period, the average torque control will fail when the machine operates in high speed and heavy load. Therefore, this novel control strategy using the overlap commutation strategy to compensate the torque decline in commutation period, where the overlap time is determined by detecting the zero-crossing point of the outgoing phase current.The corresponding simulations and experiments are conducted to verify the feasibility of the proposed methods and its effectiveness in suppressing the torque ripple of BLDCM.