On Mon, 24 May 2021 09:08:12 +0100, Chris Green wrote:

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

Lots of not very expensive multimeters now have capacitance ranges and
they will quickly show you if the capacitor is dead or not. A quick
resistance check as well will show whether it's leaky.

I've checked it with a specific meter that tests for all that and ESR
to boot. The old one was definitely blown; the new one is still fine
(and it's rated at 450VAC).