User Manual

Ps i_ A
Ps i_ B
Ps i_ C
Ui_A
Ui_B
Ui_C
Ui_A B
Ui_BC
Ui_CA
Atop Btop Ctop
Cbot Abot Bbot
Phase Magnetic Flux Linkage
Ph. A
Ph. B Ph. C
Phase Back-EMF
Phase-Phase Back-EMF
Ph. A
Ph. B
Ph. C
A-A
B-B
C-C
Acting power switch in the power stage
Speed reversal
Target Motor Theory
3-Phase BLDC Motor Control with Sensorless Back-EMF, ADC, Zero Crossing, Rev. 3
6 Freescale Semiconductor
Preliminary
Figure 3-3. BLDC Motor - Back-EMF and Magnetic Flux
3.2 3-Phase BLDC Power Stage
The voltage for a 3-phase BLDC motor is provided by a 3-phase power stage. The 3-phase power stage is
controlled by the device on-chip PWM module; which creates the desired switch control signals. A controller
with a BLDC motor and power stage is shown in
Figure 5-1.
3.3 Why Sensorless Control?
As explained in the previous section, the rotor position must be known in order to energize the phase pair and
control phase voltage of a BLDC motor. If any sensors are used to detect rotor position, then sensed
information must be transferred to a
control unit (see Figure 3-4).