Motorola M68HC08 Reference Manual page 45

3.3.1.4 Starting (Back-EMF Acquisition)
DRM028 — Rev 0
MOTOROLA
Freescale Semiconductor, Inc.
Per_HlfCmt = Period from zero crossing to commutation (half
commutation)
The required commutation timing is provided by setting commutation
constants Coef_HlfCmt, COEF_TOFF.
The back-EMF sensing technique enables a sensorless detection of the
rotor position; however, the drive must be first started without this
feedback. This is due to the fact that the amplitude of the induced voltage
is proportional to the motor speed. Hence, the back-EMF cannot be
sensed at a very low speed and a special start-up algorithm must be
performed.
In order to start the BLDC motor, the adequate torque must be
generated. The motor torque is proportional to the multiplication of the
stator magnetic flux, the rotor magnetic flux, and the sine of the angle
between these magnetic fluxes.
It implies (for BLDC motors) the following:
1. The level of phase current must be high enough.
2. The angle between the stator and rotor magnetic fields must be
90 ±30 .
The first condition is satisfied during the alignment state by maintaining
dc-bus current at a level sufficient to start the motor. In the starting
(back-EMF acquisition) state, the same value of PWM duty cycle is used
as the one which has stabilized the dc-bus current during the align state.
The second condition is more difficult to fulfill without any position
feedback information. After the alignment state, the stator and the rotor
magnetic fields are aligned (0 angle). Therefore, two fast commutations
(faster than the rotor can follow) must be applied to create an angular
difference in the magnetic fields (see
The commutation time is defined by the start commutation period
(Per_CmtStart). This allows starting the motor such that minimal
speed (defined by state when back-EMF can be sensed) and is achieved
BLDC Motor Control
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BLDC Motor Control
Used Control Technique
Figure 3-18).
Designer Reference Manual
45