On 2009-09-29, Wild_Bill wrote:
One reason for electrolytic capacitors to get warm or hot, is internal
leakage.
Internal leakage develops when the insulating layer(s) between the elements
starts to break down.

I don't know if this is the reason why some of your capacitors were warm,
and excessive leakage is not easily detected with a common VOM or DMM,
instead, leakage is measured while the capacitor is subjected to the working
voltage (or rated voltage).

I'm not certain that the NP non-polarized caps you have are the same as AC
motor caps. The two types are basically the same construction, but if the
caps aren't specifically labeled xxxV AC, then they may not be suitable for
use with AC induction motors.
Although if the caps weren't intended for AC, they would likely have
self-destructed during your test.
I just don't recall seeing AC motor caps that are marked NP.. maybe it' a
european habit.

I'm pretty sure they are AC caps for motors. I should have provided their
refs. What I could read on the 13µF caps is:

BOSCH MP 0 670 312 455
13µF +- 10%a
360V ~ DB / HSPM
400V ~ DB / HSPM
430V ~ AB 20% ED
SD 24h / HSFH
-25° b/a +70°C

Followed by a small diagram that looks like this:

___||___
| || |
| 1 2 |
_|_ || _|_
_____||_____
------------
3

They have 3 pins, none of which are connected to the case, which should be
grounded (guessed from the screw at the top). I'm not sure what the third
one is for.

The 14µF capacitor looks the same, only slightly larger. The 10µF only has
two pins and is as large as the 14µF but only half its length.

You don't mention the horsepower rating of the motor, or state that the
motor is an induction type (no brushes), but the 50uF value of the cap would
indicate that it's a Run cap (not a Start cap, as I think you already
understand).

Yes, I can't disassemble the motor but it's likely an induction type. It
cannot start without a capacitor but won't buzz either. As for the run
capacitor, it's directly wired to the motor without anything else between.

The motor is rated at 2750W.

I looked at the ILCEA website, and didn't see any special characteristics
shown for the K45 series caps, but I didn't download any of their catalogs
or literature.
The 5% value tolerance is a little uncommon, since many motor caps are rated
as loosely as 30% value tolerance.

As the other replies have suggested, the best approach for restoring
operation would be to get a new, fresh stock replacement cap with the
original rating and value.

I'll do that, which brings a new question. I found a shop that sells
capacitors adequately rated, but they list those as 'start' capacitors.
Are they somehow different? I thought that it was only how they were wired
to the motor.

--
Foo