Motorola M68HC08 manual Effect of Mutual Phase Capacitance, Mutual Inductance

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

User Guide

Application Suitability Guide

6.2.4.1 Effect of Mutual Phase Capacitance

The effect of the mutual phase capacitances can play an important role in the back-EMF sensing. Usually the mutual capacitance is very small. Its influence is only significant during the PWM switching when the system experiences very high du/dt.The effect of mutual capacitance is described in section 3.2.4.4 Effect of Mutual Phase Capacitance.

NOTE: Note that the configuration of the end-turns of the phase windings has a significant impact. Therefore, it must be properly managed to preserve the balance of the mutual capacity. This is especially important for prototype motors that are usually hand-wound.

CAUTION: Failing to maintain balance of the mutual capacitance can easily disqualify such motors from using sensorless techniques based on the back-EMF sensing. Usually the BLDC motors with windings wound on separate poles show minor presence of the mutual capacitance. Thus, the disturbance is insignificant.

6.2.4.2 Effect of Mutual Inductance

The negative effect on back-EMF sensing of mutual inductance, is not to such a degree as unbalanced mutual capacitance. However, it can be noticed on the sensed phase. 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.

The effect of mutual inductance is described in section 3.2.4.3 Effect of

Mutual Inductance.

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 wound 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. Additional simple filtering helps to reduce ripples further.

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	User Guide	111
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