Three-phase Sensorless BLDC Motor Control Kit with the MPC5606B

BLDC Sensorless Control
6
Figure 4. Zero-crossing detection and commutation diagram
The e 1X signals in Figure 2 are the BEMF voltages. These BEMF voltages are labeled as U iX in Figure 4.
This technique is based on the fact that only two phases of a motor are energized and the third
non-conducting phase can be used to sense the BEMF voltage.
The following conditions are met:
where U N is the neutral point voltage of the star winding.
The voltage u C can be calculated:
SAb SBt PWMswitching
uN uDCB – uR L di
= – ---- – u
dt iB
uN uR L di
= + ---- – u
dt iA
The voltage u iC is null at zero-crossing, so the resultant form is:
u N
=
uDCB ------------
2
uiB + uiA – ---------------------
2
uiA + uiB + uiC = 0
u N
=
uDCB ------------
2
+
uC = uN + uiC u C
u C
=
=
3
--u
2 iC
uDCB ------------
2
+
uiC -------
2
uDCB ------------
2
3-phase Sensorless BLDC Motor Control Development Kit with Qorivva MPC5606B MCU, Rev. 0
Eqn. 1
Eqn. 2
Eqn. 3
Freescale Semiconductor