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I am trying to detect the zero crossing point of the BEMF voltage in order to control the motor sensorless.

The following picture shows the ideal shape of BEMF and in many application notes or internet posts looks like this. enter image description here

But what I am seeing on the oscilloscope looks like this (the capture is not mine, it is from this post BLDC BEMF not right where @Erik Friesen says that are "waveforms scope shots from a working esc". enter image description here The question is why in the second picture the second bemf does not exist ?

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  • \$\begingroup\$ Notice that the voltage is clamped slightly negative after each of the powered steps? It's being held at that point (defined by the forward conduction voltage of the body diodes in the bridge FETs) by the emf being sufficiently negative that the pull up to the mid point caused by the other phase being powered doesn't come up above zero. Although the motor is running, demonstrating that the current is generating positive torque, the current is being switched well out of phase with where the zero crossings should dictate. \$\endgroup\$
    – Phil G
    May 2, 2019 at 14:00

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