On 2/1/2011 10:05 PM, Robatoy wrote:
Now the 'brushless' part. It probably is a marketing attempt at
projecting a zero maintenance image, and that's fine. The DC part bugs
me, because to me, DC is a continuous flow of energy, creating an
image of linear power application, i.e. smoothness. From a technical
standpoint, and I admit to picking nits, it is inaccurate although
most people probably couldn't give a rat's ass about that.
The range of RPM they claim, would indicate to me that the pulsing of
the DC would be at a high enough frequency that the application of
power would be pretty smooth.

Most electric motors have some way to drive magnets to create a rotating
magnetic field. (There are also electrostatic motors which we will ignore
for the purposes of this discussion.)

A 'brushed' DC motor uses brushes and a commutator to vary which windings
are energized to create the rotating magnetic field. The windings are
usually on the rotor and the rotor's magnetic field rotates backwards
relative to the rotor. The power flow to a brushed DC motor is only 'somewhat'
smooth or continuous. The actual current flow into the motor goes up and
down as the commutator applies voltages to the different windings. (You
cannot instantaneously change the current in a winding from zero to maximum
and then back to zero.) Compare the noise from a 'universal' motor such
as the ones on inexpensive table top saw versus the quietness of an
induction motor such as the ones found in most better table saws. The
universal motors are both electrically and acoustically noisy.

A 'brushless' DC motor uses electronics to drive the windings. Just like
the brushed DC motor, the winding drive currents are changed as the motor
turns. The winding drive signals may be simple on/off voltages (like you
would get from a brushed DC motor's commutator) or some something more
complex. I worked on pen plotters which used brushless DC motors for
positioning. Each motor had two drive windings. The windings were driven
with variable frequency sine and cosine signals (i.e. sine waves with a 90
degree phase difference) which produced constant torque and power out of
the motor. So they were very smooth and they could be run at a wide range
of speeds down to a small faction of an RPM. They were very quiet, compact,
and extremely efficient.

Note: The driven windings in a brushless DC motor are usually on the stator
(the non rotating part of the motor) instead of the rotor. This is done
simply to eliminate the need for any electrical connection to the rotor.


Dan