BLDC Motor Control
Brushless DC Motor Control Theory
DRM028 — Rev 0
Designer Reference Manual
MOTOROLA
BLDC Motor Control
31
Figure 3-6. Phase Voltage Waveform
However simple this solution looks, in reality it is more difficult, because
the sensed “branch” voltage also contains some ripples.
3.2.4.3 Effect of Mutual Inductance
As shown in previous equations
(EQ 3-4.)
through
(EQ 3-6.)
, the mutual
inductances play an important role here. 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. In fact, it is
produced by the high rate of change of phase current, transferred to the
free phase through the coupling of the mutual inductance.
Figure 3-7
shows the real measured “branch” voltage. The red curves
highlight the effect of the difference in the mutual inductances. This
difference is not constant.
0
30
60
90
120
150
180
210
240
270
300
330
360
390
uVA
uSa
- Back-EMF can be sensed
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..
