AVR-Chat

Don't confuse commutation with PWM used to adjust the amount of power being
delivered to a coil of wire.  In traditional PMDC motors mechanical switches
(brushes) do the commuting so all that is needed is one PWM circuit to
control power.  BLDC are more complex and because commuting and PWM are
co-mingled finer degrees of control is possible further increasing
efficiency, hence all the exotic sounding "Space Vector Control" type drive
algorithms.  The simplest level of control simply switches coils on/off once
per cycle varying the duration of the on period for control.  Not terribly
efficient, but simple.

 -----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Tom Becker
Sent: Monday, September 05, 2005 11:03 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] R/C throttle control ?

 > ... 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?

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?

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


Tom






 
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