Motorola M68HC08 manual Same expressions can also be found for phase a and B

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

BLDC Motor Control

After evaluation the expression of the branch voltage uVc is as follows:

u		3		1	(L		– L		di	(EQ 3-4.)
	VC	= --u	backEMF c	– --		ac		bc	)----
		2		2					dt

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

u		=	3		1	(L		– L		di
	VA		--u	backEMF a	– --		ba		ca	)---	(EQ 3-5.)
			2		2					dt
u		=	3		1	(L		– L		di	(EQ 3-6.)
	VB		--u	backEMF b	– --		cb		ab	)----
			2		2					dt

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 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 Lcb = Lab, this general expressions can also be found:

u		3		where x= A,B,C	(EQ 3-7.)
	Vx	= --u	backEMFx
		2

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 3-6shows 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.

Designer Reference Manual		DRM028 — Rev 0
30	BLDC Motor Control	MOTOROLA
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