Motorola M68HC08 manual Mutual Inductance Effect

Freescale Semiconductor, Inc.

BLDC Motor Control

Freescale Semiconductor, Inc.

0 V

Figure 3-7. Mutual Inductance Effect

Due to the construction of the BLDC motor, both mutual inductances vary. They are equal at the position that corresponds to the back-EMF zero crossing detection.

The branch waveform detail is shown in Figure 3-8. Channel 1 in Figure 3-8shows the disturbed “branch” voltage. The superimposed ripples clearly match the width of the PWM pulses, and thus prove the conclusions from the theoretical analysis.

The effect of the mutual inductance corresponds well in observations carried out on the five different BLDC motors. These observations were made during the development of the sensorless technique.

NOTE: The BLDC motor with stator windings distributed in the slots has technically higher mutual inductances than other types. Therefore, this effect is more significant. On the other hand the BLDC motor with windings wounded on separate poles, shows minor presence of the effect of mutual inductance.

CAUTION: However noticeable this effect, it does not degrade the back-EMF zero crossing detection because it is cancelled at the zero crossing point. Simple additional filtering helps to reduce ripples further.

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