Motorola M68HC08 Reference Manual page 29

3.2.4.1 Stator Winding Equations
3.2.4.2 Indirect Back EMF Sensing
DRM028 — Rev 0
MOTOROLA
Freescale Semiconductor, Inc.
The BLDC motor is usually very symmetrical. All phase resistances,
phase and mutual inductances, flux-linkages can be thought of as equal
to, or as a function of the position
The electrical BLDC motor model then consists of a set of the following
stator voltage equations
The task of this section is to explain the background of the back-EMF
sensing and to demonstrate how the zero crossing events can be
detected. Parasitic effects that negatively influence the back-EMF
detection are discussed and their nature analyzed.
Let us assume a usual situation, where the BLDC motor is driven in
six-step commutation mode using PWM technique, where both top and
bottom switches in the diagonal are controlled using the same signal (so
called "hard switching PWM" technique). The motor phases A and B are
powered, and phase C is free, having no current. So the phase C can be
used to sense the back-EMF voltage. This is described by the following
conditions:
i
Sa
u
backEMF a
The branch voltage u
1
u = u – -- -
VC Sc
3
BLDC Motor Control
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Brushless DC Motor Control Theory
with a 120° displacement.
(EQ 3-1.).
u i
Sa Sa Sa
d
= R +
u i
S
Sb Sb t d Sb
u i
Sc Sc Sc
S S PWM
Ab Bt
1 1
u = -- - u u = -- - u
VA d VB d
2 2
= – i = i d i = d – i = i d
Sb Sa Sb
i = 0 d i = 0
Sc Sc
+ u + u
backEMF b backEMF c
can be calculated using the above conditions,
Vc
c
∑
u + L – L
backEMF x ac bc
x = a
BLDC Motor Control
(EQ 3-1.)
(EQ 3-2.)
= 0
i d
--- - u – (EQ 3-3.)
VC
t d
Designer Reference Manual
29