Motorola M68HC08 manual Phase Voltage Waveform, Effect of Mutual Inductance

Freescale Semiconductor, Inc.

BLDC Motor Control

Brushless DC Motor Control Theory

Freescale Semiconductor, Inc.

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- Back-EMF can be sensed

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 3-7shows the real measured “branch” voltage. The red curves highlight the effect of the difference in the mutual inductances. This difference is not constant.

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	BLDC Motor Control	31
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