User Guide
Designer Reference Manual
DRM028 — Rev 0
110
User Guide
MOTOROLA
6.2.2 Maximal Application Speed
The maximal motor speed is limited by the minimal commutation period:
(EQ 6-1.)
COMMUT_REV
— commutations per motor revolution, must be set
according to rotor poles:
(EQ 6-2.)
where: p = rotor poles
The minimal commutation period is determined by execution time of the
software. With default software settings and COEF_HLFCMT = 0.450 it
is 333
µ
s, as shown in
Table 2-1
. So for example, the 4 pole (3-phase)
motor can be controlled up to the maximal speed of 15,015 rpm.
NOTE:
Using PC master software in the application increases the minimal
commutation time. This is due to the execution of PC master software
routine. In this case, the minimal execution time is 520 ms. The minimal
commutation period could be decreased using pure assembler code
instead of C coding.
6.2.3 Voltage Closed Loop
As shown in
Application Hardware and Software Configuration
, the
speed control is based on voltage closed loop control. This should be
sufficient for most applications.
6.2.4 Motor Suitability
Back-EMF zero crossing sensing is achievable for most of BLDC motors
with a trapezoidal back-EMF. However, for some BLDC motors the
back-EMF zero crossing sensing can be problematic since it is affected
by unbalanced mutual phase capacitance and inductance. It can
disqualify some motors from using sensorless techniques based on the
back-EMF sensing.
maximal speed[rpm]
60 10
6
(
)
min. commutation period [us]*COMMUT_REV
--------------------------------------------------------------------------------------------------------------------------
=
COMMUT_REV
6p
2
-------
=
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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