Please take a look at this waveform of a 3 phase, BLDC motor:
Check out that rising and THEN falling slope…Pretty nice.
Now, take a look at what I'm getting on my motors sensorless resistor divider(s):
Not bad, but the BEMF is backwards…It rises when it falls, and it falls when it should rise.
I've tried manually commutating this thing from hundreds of mS all the way down to uS, where it freezes…Nothing seems to get this thing to turn around.
I've taken the advice from here, but nothing seemed to straighten this out.
I've inverted the PWM/GNDing phases for each of the 6 commutations steps, and that didn't change anything. I've tried physically swapping the phases on the output terminals…nothing there (not that I would have expected it to….I would have just expected the motor to run backwards).
What causes this?
EDIT
Here are some schematics of what I have going on here:
And here's a logic trace of the noninverted outputs from the micro to the gate driver. Keep in mind that each of these has a complimentary output as well, to let the gate driver pull the phase high or low, but that signal isn't shown here… But, trust me, it's there.
Best Answer
I think that you are wrong to position energized phases after bemf falling. You should energize windings first of descending phase of bem. Provably the detection circuit dont work well , filter capacitance too big and therefore delay