On Tue, 2009-06-16 at 15:00 +0100, Leslie Newell wrote: > Hi Erik, > > Hmm, I thought I'd read years ago that skewed brushes gave the motor a > > preferred direction of rotation. > > They do.
I tend to disagree. The features that affect motor directional performance would be asymmetrical to the armature axis. The armature twist or skew is still symmetrical. My belief is that the twist effectively widens the average armature segment width relative to the stator magnet width, while keeping the same physical segment width. This overlaps the segments and eases the segment transition. Increasing the number of armature segments would have a similar effect because more segments would be engaged with the stator, so an individual segment transition would have less effect. Brush timing and and angle relative to a radial line from the rotor affects rotation direction. The timing can be advanced to get peak torque at a particular RPM or to use back EMF reduce brush arcing, but when you reverse motor rotation the timing becomes retarded unless you have some sort of dynamic timing device. Brush angle can be used to affect brush pressure as a function of RPM or friction. It may be used to control brush bounce or float, but a brush angle that increases load with speed in one direction decreases in the other. It also widens the effective brush width and the number of armature segments that can be energized for a given brush and commutator segment width. > > I haven't heard of skewed armature > > before. Your experience with their smooth operation is worth more than > > anything I can read on a website. > > > > With straight armatures the motor torque jumps slightly as each gap in > the armature passes the magnets. With a skewed armature you don't get > the sudden jumps in torque. If you turn a cheap DC motor by hand you > will really feel the jumps. However the magnet design can also > drastically effect the torque ripple and smoothness. Treadmill motors > for instance have straight armatures but still run very smoothly. > > Les Not using brushes and using software to control nearly all aspects of armature energizing, seems to be an advantage over just the maintenance issue with brushed systems. Though, this may be overkill, since my treadmill motors with high resolution encoders seem to mask any motor vagaries, time will tell. Cost-wise at least, the motor cost is not much of an issue. That's my story, and I may or may not stick to it. -- Kirk Wallace http://www.wallacecompany.com/machine_shop/ http://www.wallacecompany.com/E45/index.html California, USA ------------------------------------------------------------------------------ Crystal Reports - New Free Runtime and 30 Day Trial Check out the new simplified licensing option that enables unlimited royalty-free distribution of the report engine for externally facing server and web deployment. http://p.sf.net/sfu/businessobjects _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
