In article <e...@giganews.com>, BWallace 
says...
>  John Speth <j...@yahoo.com> wrote:
> >As a newbie, I don't understand by it's called a brushless DC motor.  The
> 
> >"brushless" part is obvious.  But the "DC" designation seems wrong.  It 
> >clearly needs an AC drive applied with just the right timing to work 
> >properly.
> >
> 
> It's DC because the current is only traveling in one direction.  

The current travels in both directions as the phase rotates (remember 
each phase on the Y is driven at different points as +, - and floating). 
Each phase sinks or sources current depending on what portion of the 
drive cycle it is in. The only difference between a BLDC and a PM AC 
motor is the name.  Some make a distinction on the drive waveform but 
then those who drive BLDC with sine waves couls claim that they were 
converting them into PM AC motors.

BLDC came to be called DC mainly as a result of them being considered DC 
motors turned inside out and having the commutation done electronically* 
rather than via brushes as near as I can tell.  Also they conventionally 
are fed from a DC bus where AC motors are conventionally fed from the AC 
line.  Of course with modern drives that has changed but we haven't 
started calling AC motors DC simply because we feed them from an 
invertor.

Switched reluctance motors are a different beast.  They are generally 
driven in a single direction through the coil (which direction doesn't 
matter).  I still don't really consider them a DC motor myself but they 
really don't fit the term AC either.

Robert

* and thus sometimes they are refered to as ECM 
(electronically/externally commutated motors).
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