Quote:
Originally Posted by squirrel
I found the short. The first winding on the motor was touching the amature core, at this corner. I found it by connecting my trusty 77 DMM to one end of the wire and to the shaft, and it beeped until I got this part of the winding loose. It was very difficult to remove the wire here. As soon as I pulled it loose, the beeping stopped.
My take on it: I did not find any "debris" in this motor, but it also had a complete short of winding to armature core. If you have a completely shorted motor (less than 5 ohms resistance from either terminal to case), then I'd suggest you do not use that motor. If the resistance is considerably higher, then you might possibly have a "debris" problem, and if you "fix" it by blowing it with high current from case to winding, the motor might be fine.
btw the white stuff is glue that is used to hold the plastic fan in place.
I hope this little forensic exercise was helpful!
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First off, Jim, thank you for taking the time to look into this issue in such detail and giving us this information.
Here is my concern based on what you found:
If this is in fact the root cause, and common to all the 775 motors showing this fault, then the "fix" is only truly is a poor solution.
Here is why I say that. For the "fix" to work, the current path has to be through the terminal(s), through one of the brushes into the commutator, through the short and into the armature then finally through the bushings/bearings to the case. The current that can possibly blow this short open, also has the potential to damage the brush/commutator contact point as well as the bushing/bearing assembly. Yes, the brush/commutator are designed to carry high current, but the bushings/bearings are not. (I use the term "bushing/bearings" because I am not sure of exactly how this motor is actually assembled.)
Now, if enough current is passed to open this short, then I can almost assure you that it not passing through the bushing/bearings without having a detrimental effect. How much damage is being done is indeterminable at this point. How long will these motors run without failing is also an unknown.
I for one would rather replace the motor with a "known good" one, than risk it failing at the most inopportune time.