AVR-Chat

On Mon, 2005-09-05 at 14:02 -0400, Tom Becker wrote:
> > ... In a PWMed brushless DC motor, for example, there are three 
> "phases"...
> Yes, I understand that quite well.  I question the terminology.
> Is a brushless DC motor just the frame, stator and armature?  I
> suggest that it is not, that it isn't a brushless DC motor unless it
> includes the drive; until then, it is a three-phase motor with open
> coils, is it not?

   No, not correct.  A burchless DC motor is NOT the same as a three
phase AC induction motor.  There are some very significant differences
in pole design, and magnetic circuit layout.

> If that is correct, PWM'ing a DC brushless motor involves just two 
> leads, the DC power to the drive; the coils are downstream from the 
> drive and semi-insensitive to the PWM frequency - they certainly are
> not locked to the DC power PWM frequency, as stated.  If the DC power
> to the drive is PWM'ed, variable power is delivered to the drive,
> modulating torque, thus speed, no?

   No. If it has only two leads, it is NOT a BLDC motor.  Period.
Controllers are seldom integrated into the motors (though they CAN be).
Even then, you're going to have at LEAST 4 leads (two for power, two
(minimum) for speed control input.

> I am balking at the notion of fixed pulse-width, three-phase drive
> being called PWM.

  As well you should.  If it is "fixed Pulse Width," then it is a FPW
control, not a PWM control.   Actually, it isn;'t even that, since
sopeed control isn't even a part of the "equation."  We are talking
about variable speed motors here, so why would anyone be thinking in
terms of "fixed pulse-width?"

Tom