Short circuit CIM

The brush holders are supported by a phenolic disc that is screwed to the aluminum end cover. The center of the disc has a hole that overlays the bearing boss. Clearance between the boss and the hole is tight, maybe one millimeter. Carbon dust (from the brush) is deposited liberally on the disc and the boss, especially on the insider diameter of the disc. When the brush moves outward, as it will when the commutator is inserted between the brushes, it tilts slightly toward the phenolic disc and contacts the carbon dust. Maybe this is the cause of the short to case – not a very low impedance connection, but enough to (1) cause electrical noise on your robot’s control system, and (2) get worse at higher electrical frequency.

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yes, yours are clean. We have measured 7 k before cleaning

When Continuing the search for root cause, Richard disassembled another CIM motor, so He could examine that one and Mine side-by-side.

The image below shows a potentially significant difference – mine is on the left, and the other CIM is on the right. The copper braid brush pigtail connected to the red motor lead is slightly longer on your motor than it is on the other. In the position shown, that braid can contact the inside of the motor case. He reassembled just after taking this picture and measured a short between red motor lead and case.

Short circuit between brush braid and case seems more likely than carbon dust (per my earlier email) to explain what you saw at MSC. Dust will accumulate in all CIMs eventually, but probably only a very few CIMs are built with long brush braids?

Nathan, It sure looks like the one braid is improperly terminated. It should not touch the motor case when assembled. Either it was terminated on the wrong side of the lug or the braid was the wrong length to begin with. This is really the first I have heard of a hard short on a CIM that did not occur due to mishandling. I have seen a number of them where the armature was cooked and melted all the insulation.

Al, I still have Nathan’s CIM in my car – keep meaning to return it but have not made it down his way yet.

I am on my way to St. Louis now, and will report for duty early tomorrow afternoon. After we get all the robots inspected, I will bring this motor in for you to examine.

Cool!