Electric motor question


I have a PC Speed Bloc that was and has worked fine since 1989. Yesterday while using it the motor sounded like it was slowing down to about half speed and now runs at that speed always.
Does it sound like a brush problem? I pulled the brushes out and there was a dull spot on one and the springs seem weak.
Suggestions?
Thanks
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Leon said:

This is how I deal with this situation... YMMV, IMHO, etc.
The brushes are generally designed to be a certain length, and this combined with spring pressure determines the optimal contact pressure on the commutator. But this requires knowing the 'new' length as opposed to the 'now' length. If you can see the brush contact patch from outside the device, for instance through a vent hole, look for evidence of excessive sparking/trails. Heat also causes the springs to lose tension.
You could opt to simply replace them, it's been a while. Clean the commutator, but don't use emery cloth as is sometimes done. Embedded particles will cause accelerated erosion of the copper blades. Their surfaces are turned on a lathe, and the slots between blades are cut, square bottomed. Keep this in mind when attempting to clean up the armature. Check the bearings for smooth operation. Check for other worn parts in need of replacement.
A short term solution, if the brushes are not cracked and burned, is:
First - order 2 new brushes sets and springs (commonly included)     Also order any bearings and misc. parts which seem worn.
Second - stretch the springs to 150% of their original length.     Reinstall and make sure the brushes maintain their original     contact pattern. Use until new brushes arrive and replace.     This is a short term repair at best...
Good Luck,
Greg G.
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The PC site indicates the brushes are 9/16" new. I'll yank them out and remeasure.

Bearings seem fine but I'll stretch the springs as they do seem weak. I'll clean the commutator with electrical cleaner and soft brush.
Thank you Greg. At least this will let me know if it is indeed the brushes. I did now ever buy a new Speed Bloc Yesterday also. I am in the middle of a job and could not wait. I'll just have a back up or spare parts if this works.
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Leon wrote:

Probably.
> I pulled the brushes out and there was

Replace brushes.
While you are at it, check bearings.
Lew
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Leon (in G_%ff.22733$ snipped-for-privacy@newssvr25.news.prodigy.net) said:
| I have a PC Speed Bloc that was and has worked fine since 1989. | Yesterday while using it the motor sounded like it was slowing down | to about half speed and now runs at that speed always. | | Does it sound like a brush problem? I pulled the brushes out and | there was a dull spot on one and the springs seem weak. | | Suggestions?
Leon...
Brushes could be a problem; but another possibility is the eccentric drive. My speed bloc showed similar symptoms ( After only 30 years of use! They just don't make tools like they usta :-) and my machine's problem was with the eccentric rather than the motor itself.
I took it in to my local PC service guys this past spring and it cost $15 to have the old part replaced. The old cord had become brittle and cracked, so I had that replaced at the same time (for another $15). Should be good for another 30 years...
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/solar.html
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Hi Morris. I suspected something being out of balance as the sander shook much more than usual. This is not unusual however because it has always shook during start up and while coasting down to a stop. Even the new one does this. I did remove the eccentric and the motor still runs slowly. I had a part of the pad assembly fly apart a couple of years ago and with only one small part missing the balance was thrown out enough that it shook violently. I replaced the pad assembly and every thing was fine for 2 years. Greg suggested that excess sparking at the brushes might indicate bad brushes and I do see what seems to be excess sparking when I had the eccentric off and the sander was running.
I was shocked how pliable the new cord was on the new unit. LOL
Thanks for the suggestions.
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If the bearings are free, I'm guessing you have an open winding in the armature unless the sander has a defective variable speed switch. You can check the armature with an ohm meter by connecting each lead of the meter to each side of the sanders power cord (sander not plugged in). Turn the motor by hand (be sure sander switch is on), the meter needle/ or reading if a digital meter should stay constant as you turn the shaft by hand if the armature is good. If it's bad (dips in readings) you would probably be better off looking for new sander due to the costs of new armatures. RM~
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Thank you Rob, I did buy a new sander, same kind. I'll check for an open circuit like you have indicated. If the armature is shot, I keep it for spare parts.
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