On 10/08/2015 01:18 AM, Marshland Engineering wrote:
> Apparently DC Brushless cog but not AC ?
> Anyway I'm about to order a 6N.m 1.8KW AC motor and drive for $340.  It's
> worth a try.
>
> Brushes ?? My South Western Industries mill has run 20+ hours a week for + 10
> years and I have never changed a brush. Maybe I should check sometime.

My understanding of motor cogging is that it is the torque or speed 
variation due to the layout of the magnetic poles.
https://en.wikipedia.org/wiki/Cogging_torque

This has been an issue with DC brushed motors when driven by a simple 
voltage source. Generally, half the rotor coils are energized South and 
half North. As the rotor turns the brushes change the polarity of a coil 
in order to shift the magnetic phase of the rotor away from the stator 
in order to create torque. This is a one coil quantum change, so the 
torque is also a step change, therefore not smooth. One way to smooth 
the torque is to twist the rotor coil layout the adjust the torque 
profile relative to the rotor/stator position.

This still is in the context of driving a motor with a simple power 
source. When speed, position or other feedback is added to control the 
source, cogging does not apply.

Cogging could be an issue for AC or DC brushless and AC induction motors 
if they are driven by a simple power source that matches the type of 
motor -- AC induction needing AC mains power with properly phased legs, 
AC brushless needing three phase AC sinusoidal power, and DC brushless 
needing three phase trapezoidal power.


-- 
Kirk Wallace
http://www.wallacecompany.com/machine_shop/
http://www.wallacecompany.com/E45/

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