Motorola M68HC08 Reference Manual page 30

BLDC Motor Control
Designer Reference Manual
30
Freescale Semiconductor, Inc.
After evaluation the expression of the branch voltage u
u =
VC
The same expressions can also be found for phase A and B:
u =
VA
u =
VB
The first member in the equation
to indirectly sense the back-EMF between the free (not powered) phase
terminal and the zero point, defined at half of the dc-bus voltage (see
Figure 3-5.). Simple comparison of these two levels can provide the
required zero crossing detection.
As shown in Figure 3-5, the branch voltage of phase B can be sensed
between the power stage output B and the zero voltage level. Thus,
back-EMF voltage is obtained and the zero crossing can be recognized.
When L = L , this general expressions can also be found:
cb ab
u
Vx
There are two necessary conditions which must be met:
• Top and bottom switches (in diagonal) have to be driven with the
same PWM signal
• No current goes through the non-fed phase that is used to sense
the back-EMF
Figure 3-6 shows branch and motor phase winding voltages during a
0–360° electrical interval. Shaded rectangles designate the validity of
the equation (EQ 3-7.). In other words, the back-EMF voltage can be
sensed during designated intervals.
BLDC Motor Control
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3 1
-- - u – -- - L – L
backEMF c ac bc
2 2
3 1
-- - u – -- - L – L
backEMF a ba ca
2 2
3 1
-- - u – -- - L – L
backEMF b cb ab
2 2
(EQ 3-6.)
3
= -- - u where x = A B C , ,
backEMFx
2
is as follows:
Vc
i d
--- - (EQ 3-4.)
t d
i d
--- -
(EQ 3-5.)
t d
i d
--- - (EQ 3-6.)
t d
demonstrates the possibility
(EQ 3-7.)
DRM028 — Rev 0
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