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Phase voltage waveform – Motorola M68HC08 User Manual

Page 31

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BLDC Motor Control

Brushless DC Motor Control Theory

DRM028 — Rev 0

Designer Reference Manual

MOTOROLA

BLDC Motor Control

31

Figure 3-6. Phase Voltage Waveform

However simple this solution looks, in reality it is more difficult, because
the sensed “branch” voltage also contains some ripples.

3.2.4.3 Effect of Mutual Inductance

As shown in previous equations

(EQ 3-4.)

through

(EQ 3-6.)

, the mutual

inductances play an important role here. The difference of the mutual
inductances between the coils which carry the phase current, and the
coil used for back-EMF sensing, causes the PWM pulses to be
superimposed onto the detected back-EMF voltage. In fact, it is
produced by the high rate of change of phase current, transferred to the
free phase through the coupling of the mutual inductance.

Figure 3-7

shows the real measured “branch” voltage. The red curves

highlight the effect of the difference in the mutual inductances. This
difference is not constant.

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390

uVA
uSa

- Back-EMF can be sensed

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Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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