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Due to the small inductance, modulation of three-phase inverter induces serious torque ripple. In addition, the increase in modulation frequency is limited by the processor speed. Therefore, the traditional torque control approaches are not suitable. In order to solve the aforementioned problems, a new instantaneous torque control method for small inductance brushless dc motor is proposed by improving the torque estimation and control. First, instantaneous torque is estimated through improved position information and back electromotive force (EMF) coefficient estimation. The former is achieved by the proposed hall sensors position calibration and compensation method, and the latter is obtained by neural network fitting. Second, the torque ripple reduction is realized in the conduction and commutation region. The ripple caused by three-phase inverter modulation is suppressed by the dc-link buck converter pulsewidth modulation control method. Upon this, an asymmetry compensation function is designed to solve the problem of unbalance among three phase windings. After that, back EMF disturbance, which is applied to current dynamics, is compensated through feedforward control. Subsequently, the commutation ripple is reduced by the outgoing phase control. Finally, the validity of the proposed torque control method is verified through experimental results.
Date of Publication: Dec. 2012