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Bldc motor control, Freescale semiconductor, inc – Motorola M68HC08 User Manual

Page 30

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BLDC Motor Control

Designer Reference Manual

DRM028 — Rev 0

30

BLDC Motor Control

MOTOROLA

After evaluation the expression of the branch voltage

u

Vc

is as follows:

(EQ 3-4.)

The same expressions can also be found for phase A and B:

(EQ 3-5.)

(EQ 3-6.)

The first member in the equation

(EQ 3-6.)

demonstrates the possibility

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 co
mparison 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

cb

= L

ab

, this general expressions can also be found:

(EQ 3-7.)

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.

u

VC

3
2

---u

backEMF c

1
2

--- L

ac

L

bc

(

)

i

d

t

d

----

=

u

VA

3
2

---u

backEMF a

1
2

--- L

ba

L

ca

(

)

i

d

t

d

----

=

u

VB

3
2

---u

backEMF b

1
2

--- L

cb

L

ab

(

)

i

d

t

d

----

=

u

Vx

3
2

---u

backEMFx

where x

A B C

,

,

=

=

F

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sc

a

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S

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ic

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c

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Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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