CIM Motor Failure

[Al Skierkiewicz]

After long hours of practice and competition, a few of our CIM motors finally showed signs of failure. The motors were showing a reduced power output, were draining the battery faster and could not survive the one second finger test for temperature.
If you look closely at the picture of the armature, you will see a wire that is no longer attached to the commutator segments. When the motor was opened, I noticed what appeared to be fractures in the darkened varnish on several wires. A slight movement released one from the commutator. I am guessing it was already open circuit when I opened the motor. The photo shows other fracture in several wires near the bent over tabs of the commutator. Near the bottom side of the windings is some epoxy applied to the windings by the manufacturer. This is normally sky blue in color, the darkened color showing the excessive heat developed in the armature.
The second photo shows the brush assembly. Please note the discoloration of the braided brush wire. The color is off in this photo, as the braid closest to the brush is almost gold in color (in reality) while the end attached to the wire is very dark copper (not the gold that it appears in this photo). Please also note the discoloration of the red wire. Some of this is brush residue but the majority is heat damage to the wire.
This is typical of the CIM motors when they are abused or in this case, heavily used. The motor is several years old, had been used on previous robots and has seen extensive practice driving over the last few weeks. Perhaps for an hour or more straight with stops just to replace the battery.