On 23/05/2021 21:25, Roger Hayter wrote:
On 23 May 2021 at 18:11:59 BST, "Cursitor Doom" wrote:

Hi all,

So I have this bench grinder (one of the ones with two wheels of
differing coarseness on either end of a spindle, the motor being in
the middle). Anyway, it packed up one day. Wouldn't spin by itself
even with a prod; just made a humming noise. I suspected a duff
capacitor which turned out to be correct. This grinder uses a 450V
2.5uF one and finding a spare online turned out to be simple enough,
if not all that cheap at a fiver. Swapped out the duff cap for the new
one and it worked fine again. Then today it packed up again, same
failure mode as before. I whipped the bottom off and checked the new
cap expecting it to have gone phut but it was fine, much to my
surprise. So I'm stumped. I can't see any other caps in evidence,
either.
Any idea what's going on here?

This type of capacitor mostly fails with no external signs. They can be tested
with the right equipment, but if not available the simplest thing is to try
another new one. About the only other possibilities are the switch, a wire
broken or fallen off or an internal break/short in the motor winding. The
last one is probably a terminal event for a fairly cheap machine.

Thermal fuse within the windings playing up?