Torque ripple minimisation control method for a four-phase brushless DC motor with non-ideal back-electromotive force

In conventional control methods of brushless DC (BLDC) motor drives, back-electromotive force (EMF) is assumed to be in ideal form and the controller injects rectangular phase current commands to produce the desired constant torque. However, real back-EMF waveform might not be exactly trapezoidal because of non-ideality of magnetic material, design considerations and manufacturing limitations. This makes the generated electromagnetic torque contain ripples in its waveform which is not desirable in motor operation performance especially, in sensitive industries. Moreover, commutation states affect the quality of generated torque by producing torque pulsations because of changes of conducting phases. In this study a control strategy for a four-phase BLDC motor with non-ideal back-EMF to reduce electromagnetic torque ripples is presented. Basis of the proposed method is to inject phase currents considering back-EMF instantaneous magnitude. For this purpose, an on-line back-EMF estimation technique is used to inject appropriate phase currents to compensate non-ideality of back-EMF waveform. Moreover, the estimated back-EMF is also used for commutation torque ripple reduction. The experimental results indicate performance of the proposed control strategy in torque pulsations reduction compared with conventional control method.