Motorola M68HC08 manual Starting Back-EMF Acquisition

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

BLDC Motor Control

Used Control Technique

Per_HlfCmt = Period from zero crossing to commutation (half commutation)

The required commutation timing is provided by setting commutation constants Coef_HlfCmt, COEF_TOFF.

3.3.1.4 Starting (Back-EMF Acquisition)

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 Figure 3-18).

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

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	BLDC Motor Control	45
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