AN857 查看數據表（PDF） - Microchip Technology

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AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

Figure 8 is a graphical representation of the BEMF for-

mulas computed over one electrical revolution. To

avoid clutter, only the terminal A waveform, as would

be observed on a oscilloscope is displayed and is

denoted as BEMF(drive on). The terminal A waveform

is flattened at the top and bottom because at those

points the terminal is connected to the drive voltage or

ground. The sinusoidal waveforms are the individual

coil BEMFs relative to the coil common connection

point. The 60 degree sinusoidal humps are the BEMFs

of the driven coil pairs relative to ground. The entire

graph has been normalized to the RMS value of the coil

pair BEMFs.

FIGURE 8:

BEMF AT 100% DRIVE

BLDC Motor Waveforms

(PWM at 100% Duty Cycle)

1.5

AN857

1

B

C

0.5

A

ABS(B-C)

ABS(C-A)

0

ABS(A-B)

BEMF(drive on)

-0.5

-1

-30

30

90

150 210 270 330

Electrical Degrees

Notice that the BEMF(drive on) waveform is fairly linear

and passes through a voltage that is exactly half of the

applied voltage at precisely 60 degrees which coin-

cides with the zero crossing of the coil A BEMF wave-

form. This implies that we can determine the rotor

electrical position by detecting when the open terminal

voltage equals half the applied voltage.

What happens when the PWM duty cycle is less than

100%? Figure 9 is a graphical representation of the

BEMF formulas computed over one electrical revolu-

tion when the effective applied voltage is 50% of that

shown in Figure 8. The entire graph has been normal-

ized to the peak applied voltage.

 2002-2011 Microchip Technology Inc.

DS00857B-page 11

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