Thursday, June 7, 2012

UABMM2 Magnetic Shunts

Now that the UABMM2 is assembled, and I have been playing with it a while, I am starting the understand the magnetic sticky points.  There are spots in the rotation where the rotor will "stick" because of the cross currents between the individual magnets.  I drew this pretty picture you help you visualize the problem here.
The Gap allows a cross current field to form between the magnets.  This creates attraction points for the opposing magnets.  I need to maximize the face currents and minimize the cross currents.  If this were electrical I would short the two together to eliminate eddy currents.  But, seeing how this is magnetic, we need a magnetic conductor, with high permeability, like a piece of iron.  What I am hoping for is the magnetic shunts will help to minimize the cross currents so they will not interfere with the face currents.
The magnets are close together, but there is still a small gap there.  It is in the gap space that the cross currents develop, and make sticky spots.  To minimize the cross fields I am going to insert a piece of iron sheet which is 0.031 inches thick.  I cut out rectangles which are 1/4 inch by 1/2 inch and insert them between the magnets.
Using a high permeability material will hopefully draw the cross currents down, into the iron sheet.  So the overall effect will be to present more face current and less cross currents.
To add all these magnetic shunts we'll have to increase the inside diameter of the stator slightly.  We're adding 0.031 times 40 magnets, so that is increasing that circumference by 1.25 inches.  So I am adjusting each Cant (flat that holds the magnet) by grinding it down about 50 mils.
Also, while its taken apart I made another, tighter flat on the shaft for the shaft key that holds the split taper bushing in place.  I made that joint a lot tighter, and can now use the split taper bushing as the bottom shaft keeper, and eliminate one of the shaft collars.  I'll find another use for it.
Turning 1/2 inch nuts into 1/2 inch shaft collars was neat impromptu experience.  When you can't find what you need, you can usually find a way to make it.

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