AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 6

• Manufacturer Part Number: AN1914
• Description: 3-Phase BLDC Motor Control with Sensorless Back EMF Zero Crossing Detection Using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Target Motor Theory
3.2 3-Phase BLDC Power Stage
6
motor terminals of the BLDC motor. As can be seen, the shape of the Back-EMF is approximately
trapezoidal and the amplitude is a function of the actual speed. During the speed reversal the amplitude
is changed its sign and the phase sequence change too.
The filled areas in the tops of the phase Back-EMF voltage waveforms indicate the intervals where the
particular phase power stage commutations occur. As can be seen, the power switches are cyclically
commutated through the six steps. The crossing points of the phase Back-EMF voltages represent the
natural commutation points. In normal operation the commutation is performed here. Some control
techniques advance the commutation by a defined angle in order to control the drive above the PWM
voltage control.
The voltage for 3-phase BLDC motor is provided by a 3-phase power stage controlled by a DSP. The
PWM module is usually implemented on a DSP to create desired control signals.
A DSP with BLDC motor and power stage is shown in
$WRS %WRS &WRS
&ERW $ERW %ERW
Figure 3-3. BLDC Motor - Back EMF and Magnetic Flux
Ph. A Ph. B
DSP56F80x MC PWM Module in Motor Control Applications
A-B
Ph. A
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B-C
Ph. B
Ph. C
Acting power switch in the power stage
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C-A
Phase Magnetic Flux Linkage
Ph. C
Phase-Phase Back EMF
Phase Back EMF
“Natural” commutation point
Figure
Speed reversal
3-3.
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