BLDC: Asymetrical Zero Cross detection

Hello,
for detecting the Zero-cross of a sensorless BLDC I use a starpoint reproduction and give this Neutral-Point-signal and the evaluated
Phase to a comparator. At higher speed there is no problem with zero- cross detection. But at lower speed, the zero crossing detection is asymmetrical: The falling edge comes late, the rising edge comes very soon. So the following phases of the runngin motor are long - short - long - short ....
Who knows something about this effect?
Best regards
Wolf
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I'd like to try and help, but even though I'm more or less conversant with motor theory and electronics, the terminology here is unfamiliar to me. What's BLDC? Is this a generator or a drive application, is there speed control electronics involved. A bit more info on the situation would assist.
Is your phase set stiff and reasonably sinusoidal?
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Bruce Varley wrote:

Brushless DC? A schematic would be good.
Cheers
Phil Hobbs
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On Mon, 26 Oct 2009 18:52:41 +0800, Bruce Varley wrote:

Brushless DC. Undoubtedly drive -- if it were a generator one would just be rectifying, which is a fairly easy problem. It sounds like the OP is designing the drive electronics -- for sensorless BLDC (i.e. brushless without commutation sensing) you use back EMF to figure out the motor phase.
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It sounds like the OP is

Yes - you are right.
Wolf
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Wolfgang wrote:

A DC bias would cause that effect.
Jerry
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