Brian Gaff (Sofa) wrote

Yes the modern cap simply loses its capacitance over time and then there
is no phase changed output on that bit of the motor and it sulks. Older
ones tended to either fry, emit magic smoke or scare everyone with a bang,
bark worse than bite in most cases.

I think, I'd possibly try to find one with a slightly higher working
voltage this time as they have to survive on a much dirtier mains than
they used to.

That last is very arguable with voltage spiked.

Roger Hayter wrote
Cursitor Doom wrote

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.

--
Roger Hayter