My apologies in advance if I've mis posted in this category.

I have a problem with an a.c. 240 v mains powered chainsaw motor. It's by a well known and reliable manufacturer. The brushes failed a year ago, so I dismantled and checked the brushes, cleaned the commutator, checked for any burn marks etc, but the brushes were worn right down and the obvious cause of failure, so I replaced them with a non original part. When I plugged in and tried the saw, it ran well for about 2 minutes then failed again. So I dismantled again and found the brushes were intact, but the copper braid had parted company from the carbon brush. I took the opportunity this time to ensure the bearings were in good shape, all gear teeth in good order etc so there were no undue loads on the motor. Another set of brushes failed in the same way.

So this time I bought genuine manufacturers brushes, cleaned the commutator and the gaps between facets with alcohol and toothbrush. Visually the commutator looks good, no burn marks - it's not a bright coper colour, but I think it's clean.

So anyway, the genuine set of brushes has just failed in the same way and I'm perplexed as to what's going on here.

I have removed the armature and visually inspected it, cleaned and lightly oiled the bearings, and it spins completely freely when the brushes are not present. The whole thing (including the brush springs) is original and I feel its a shame to just throw it away.

Any thoughts on what might be going on would be greatly appreciated. btw the chainsaw is about 12 years old.

thanks,
Paul

 

If the first set lasted about 12yrs, it would seem to indicate you are replacing them with brushes that have a different copper/carbon ratio.
These motors rotate at very high RPM's and require the original type brushes.
If there is excessive sparking at the brush contact, it would indicate a motor short in either field or armature.

 

If the first set lasted about 12yrs, it would seem to indicate you are replacing them with brushes that have a different copper/carbon ratio.
These motors rotate at very high RPM's and require the original type brushes.
If there is excessive sparking at the brush contact, it would indicate a motor short in either field or armature.

ah, right, probably time to get a new one then - thanks for reply.