On 24 May 2021 at 11:10:33 BST, "Chris Green" wrote:

Roger Hayter wrote:
On 24 May 2021 at 09:09:54 BST, "Chris Green" wrote:

Roger Hayter wrote:
On 23 May 2021 at 22:48:45 BST, "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

Possibly still needs testing for insulation breakdown. Either a Megger
or a
4-500V power supply plus 47k-100k resistor in series and a multimeter
on 500
or more volt range, plus *eye protection* from possible drama.

If it's only leaky then it's still a capacitor and the motor should
start OK I would have thought.

If it is very leaky it may not achieve sufficient phase shift.

Why? It would have to be very, very leaky to have much effect surely.

Back of an envelope calculation suggests it would have to dissipate about 100W
at a phase angle of 45deg, so you are probably right!

--

Roger Hayter