On 23/05/2021 22:48, Cursitor Doom wrote:
On 23 May 2021 20:25:39 GMT, 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.

Tested it with one of these:
https://www.peakelec.co.uk/acatalog/...esr-meter.html

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.

There's still a discernable humming coming from it when switched on,
so we can rule out the switch and a break in the motor winding as
possible causes at any rate.

I've read your posts but can't see anywhere where you've checked
continuity of the 2 windings that should have a common connection.

You will still get a hum if only one functions, or where there is a
short circuit turn.

Can you measure these? Also what is the voltage across the cap when
switched on?

Does the motor get hot very quickly?