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    1. quadmcfly:
      Sep 25, 2017 at 04:38 PM

      I would make the argument that these are indeed a type of induction motor, though they are not AC induction motors. They still use the principles of induction to function, though the mechanism is slightly different from the AC induction motors used in industrial applications. The loaded start ability of the AC induction motor being the biggest difference.

      On the second point, you're absolutely correct, I inverted my terminology. That's what you get when you're writing too late at night. :D Fixed it. The external drive method that you are describing is actually the most reliable way of measuring the kV of a motor. The byproduct of the BEMF signal being lower at lower revolutions means it is very difficult to start a brushless DC motor spinning due to the difficulty in getting an accurate read on the BEMF signal. Kind of a chicken before the egg issue.

      On the third point, the electrical degrees are still not literal degrees of rotation, it depends on the number of poles as to the actual degrees of rotation that is produced by 360 degrees of electrical rotation. In our case with a typical 14 pole brushless motor for instance, as you mentioned 1 electrical revolution is 360 electrical degrees, but that is not literal degrees it is only the pole to pole, so it works out to about 51.43 literal degrees of rotation per 360 electrical degrees. On a 12 pole motor such as an 11XX class, that would would work out to 60 degrees of physical rotation. In that way it is a virtual number, more referring to the way it would be plotted on a graph as a sine wave than actual rotational degrees. On a related note if we switch from a 3 phase to a different number of phases, the degrees per phase of the BEMF signal would change accordingly, it may be less than 60 degrees per phase. That means in some motors the timing advance may be measured differently. You don't typically see that arrangement in brushless motors, unless you're looking at steppers, and they don't use the BEMF wave in their typical application.






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