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When we have hall sensors inside a BLDC to use as encoders for the processor, I saw some diagrams on web showing 3 output signals (open collector outputs, need external pull-ups to the VCC of processor). It is ok to use only 2 sensors and be able to control the motor properly? I have a situation where a have only 2 inputs... 2 signals of encoder is enough?

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    \$\begingroup\$ Hall sensors are not encoders. Please clarify which it is you are actually asking about. You need all three hall sensors. Please correct the title and wording in your post. \$\endgroup\$ – DKNguyen Jul 2 '20 at 23:07
  • \$\begingroup\$ Yes, actually Im a bit unclear, I am very new to BLDC studies, which words do you suggest to change? \$\endgroup\$ – abomin3v3l Jul 2 '20 at 23:32
  • \$\begingroup\$ Just call them what they are: hall sensors...for commutation. \$\endgroup\$ – DKNguyen Jul 3 '20 at 2:05
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You need all three hall sensors.

You can infer the third phase by measuring the other two phases in a 3-phase sinusoid but that is because there are continuously varying so you can distinguish exact points in the cycle based on value, but the square wave of hall encoder outputs have flats (since it is a rectangular wave) so you cannot distinguish where the third phase is in the cycle by using the other two phases.

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  • \$\begingroup\$ Hi. What you mean with "outputs have flats"? \$\endgroup\$ – abomin3v3l Jul 2 '20 at 23:13
  • \$\begingroup\$ @abomin3v3l I guess some sort of saturation - source \$\endgroup\$ – quetzalcoatl Jul 2 '20 at 23:25
  • \$\begingroup\$ @abomin3v3l If I gave you an instantaneous value of a square wave and told you which quarter cycle it was located in, you would not be able to tell what angle that one sample came from because all the angles in that quarter cycle have the same value since it's just a flat line from a square wave. But if I did the same with a sinusoid you would be able to tell because each angle in that quarter cycle has a different magnitude. \$\endgroup\$ – DKNguyen Jul 3 '20 at 0:33

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