ORIGINAL: Matt Kirsch
A popular myth that I fell victim to myself. Turns out that even though the ESC does switch the polarity back and forth on the various phases of the motor to make it turn, it's still putting out DC voltage. It's similar to AC, but the wave is a "square" type. Unlike AC, where the voltage gradually changes from maximum + to minimum -, producing a "sine" wave, this switched DC changes from full + to full - instantly.
The picture below kind of shows what I mean. This is what you'd see on the screen of an oscilliscope if you compared true AC to what the brushless ESC is doing, sort of, kinda, maybe
Matt, your description is incorrect, and illogical. DC means Direct Current, AC means Alternating Current. The terms have nothing to do with the shape of the waveform whether squarish or rounded.
As you stated, the ESC switches the polarity of the phases. This means that the current through any given winding flows 'Alternately' one direction then the other. Also the voltage waveform is more trapezoidal, not square. And the current waveform is more sinusoidal due to the inductance of the motor, not square. The brushless motor is a true 3-phase AC motor and is described as such by the motor manufacturers themselves.
Now the ESC, on the other hand, that IS a DC device. But the current in the ESC's motor leads is AC
See my post here for actual scope photos of the current and voltage waveforms and further discussion:
Why three phase AC?