3-Phase BLDC Motor Sensorless Control Using MC56F8013

Control TheoryFigure 2-6 shows that the branch voltage of phase C, between the power stage output C and the naturalvoltage level, can be sensed. Thus the back-EMF voltage is obtained, and zero crossing can berecognized.The general expression can be found by:where:3u Vx= --u2 backEMFx(2-9)There are two conditions that must be met:1. Top and bottom switch (in diagonal) must be driven with the same PWM signal.2. No current must go through the non-fed phase used to sense the back-EMF.x =A, B,CFigure 2-6 shows branch and motor phase winding voltages during a 0–360°electrical interval. Shadedrectangles designate the validity of Equation 2-9. In other words, the back-EMF voltage can be sensedduring designated intervals.Figure 2-6. Phase Voltage WaveformsHowever simple this solution looks, in reality it is more difficult, because the sensed branch voltage alsocontains some ripples.2.3.3.1 Effect of Mutual InductanceIn BLDC motor’s back-EMF wave shape, the mutual inductances play an important role. The differencein the mutual inductances between the coils that carry the phase current, and the coil used for back-EMFsensing, causes the PWM pulses to be superimposed onto the detected back-EMF voltage. In fact, it isproduced by the high rate of change of phase current, transferred to the free phase through the couplingof the mutual inductances.3-Phase BLDC Motor Sensorless Control using MC56F8013, Rev. 020 Freescale Semiconductor