Motorola M68HC08 Reference Manual page 111

6.2.4.1 Effect of Mutual Phase Capacitance
NOTE:
CAUTION:
6.2.4.2 Effect of Mutual Inductance
NOTE:
CAUTION:
DRM028 — Rev 0
MOTOROLA
Freescale Semiconductor, Inc.
The effect of the mutual phase capacitances can play an important role
in the back-EMF sensing. Usually the mutual capacitance is very small.
Its influence is only significant during the PWM switching when the
system experiences very high du/dt.The effect of mutual capacitance is
described in section 3.2.4.4 Effect of Mutual Phase
Note that the configuration of the end-turns of the phase windings has a
significant impact. Therefore, it must be properly managed to preserve
the balance of the mutual capacity. This is especially important for
prototype motors that are usually hand-wound.
Failing to maintain balance of the mutual capacitance can easily
disqualify such motors from using sensorless techniques based on the
back-EMF sensing. Usually the BLDC motors with windings wound on
separate poles show minor presence of the mutual capacitance. Thus,
the disturbance is insignificant.
The negative effect on back-EMF sensing of mutual inductance, is not to
such a degree as unbalanced mutual capacitance. However, it can be
noticed on the sensed phase. The difference of the mutual inductances
between the coils which carry the phase current and the coil used for
back-EMF sensing, causes the PWM pulses to be superimposed onto
the detected back-EMF voltage.
The effect of mutual inductance is described in section
Mutual Inductance.
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 wound on separate poles, shows minor presence of the effect
of mutual inductance.
However noticeable this effect, it does not degrade the back-EMF zero
crossing detection, because it is cancelled at the zero crossing point.
Additional simple filtering helps to reduce ripples further.
User Guide
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User Guide
Application Suitability Guide
Capacitance.
3.2.4.3 Effect of
Designer Reference Manual
111