BLDC Motor Control
Designer Reference Manual
DRM028 — Rev 0
32
BLDC Motor Control
MOTOROLA
Figure 3-7. Mutual Inductance Effect
Due to the construction of the BLDC motor, both mutual inductances
vary. They are equal at the position that corresponds to the back-EMF
zero crossing detection.
The branch waveform detail is shown in
Figure 3-8
. Channel 1 in
Figure 3-8
shows the disturbed “branch” voltage. The superimposed
ripples clearly match the width of the PWM pulses, and thus prove the
conclusions from the theoretical analysis.
The effect of the mutual inductance corresponds well in observations
carried out on the five different BLDC motors. These observations were
made during the development of the sensorless technique.
NOTE:
The BLDC motor with stator windings distributed in the slots has
technically higher mutual inductances than other types. Therefore, this
effect is more significant. On the other hand the BLDC motor with
windings wounded on separate poles, shows minor presence of the
effect of mutual inductance.
CAUTION:
However noticeable this effect, it does not degrade the back-EMF zero
crossing detection because it is cancelled at the zero crossing point.
Simple additional filtering helps to reduce ripples further.
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Freescale Semiconductor, Inc.
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