3-Phase BLDC Motor Sensorless Control Using MC56F8013

Mathematical Description of the BLDC MotorFigure 2-7. Mutual Inductance EffectFigure 2-7 shows the real measured branch voltage. The red curves highlight the effect of the differencein the mutual inductances. This difference is not constant.Due to the construction of the BLDC motor, both mutual inductances vary. They are equal at the positionthat corresponds to the back-EMF zero crossing detection.The branch waveform detail is shown in Figure 2-8. Channel 1 shows the disturbed branch voltage. Thesuperimposed ripples clearly match the width of the PWM pulses, and thus prove the conclusions drawnfrom the theoretical analysis.The effect of the mutual inductance corresponds well in observations carried out on five different BLDCmotors. These observations were made during the development of the sensorless technique.NOTEA BLDC motor with stator windings distributed in the slots has technicallyhigher mutual inductances than other types. Therefore, this effect is moresignificant. On the other hand, the BLDC motor with windings wound onseparate poles shows minor presence of the effect of mutual inductance.CAUTIONHowever noticeable this effect, it does not degrade the back-EMF zerocrossing detection, because it is cancelled at the zero crossing point. Theaddition of simple filtering helps to reduce the ripples further.3-Phase BLDC Motor Sensorless Control using MC56F8013, Rev. 0Freescale Semiconductor 21