Capacitors

[Abe]

Capacitors often blow...........up, short, stop working, etc.
What is the cause?
How do we test?
Ok, I have a small piece of kit which tests capacitors
But for those who dont, is a multi tester a reliable tool to use to check the capacitor?

[Reeferjon]

You can use an anologue meter on ohms range. Watch the needle, as the cap charges the resistance will increase untill it is fully charged, discharge it and try again, as for how to measure its value without a cap meter I don't know.

[El Tel]

Reeferjohn, you're right. But you don't need to discharge it. Just spin the cap through 180º and retest. Alternatively, leave the cap where it is and swap your probes from one hand to the other.

[750 Valve]

I've found capacitors used to start small hermetic compressors usually "blow up" due to the compressor's start relay holding the start winding in for longer than it should, if you pull the relay apart you will most likely find the switching contacts all scored and marked from where the relay has been sticking in and arcing the contacts. In a pinch a file of the contacts will do but I usually replace all starting components when I find any one of them faulty. After all, if you assume the compressor starts 10 times every hour, and runs 16 hours per day then that equates to 58400 starts per year, not bad for a cheap, relatively easy to change part (compared to compressor).

[Gibson]

From my experience, the advice about using an analog meter works ok for a quick check of capacitors. Most suppliers around here sell capacitor testers and they are preferred.
It should also be mentioned that in addition to the charge/discharge test, if an analog multimeter is all you have, a run cap should be tested from both the marked and unmarked terminals to the casing for resitance to ground. If you don't read infinite resistance, replace it.
If a run capacitor's casing appears swollen, replace it.
Make sure you also test the compressor windings before you run for parts!
Start Cap's, when they blow, should be accompanied by the replacement of the complete start kit if the compressor windings check out with manufacturer's tables.
When replacing a capacitor always check with your supplier that the one you pick up is what the compressor, or motor manufacturer recommends.

[Gary]

In addition to all of the above, if the capacitor is marked, the marked terminal goes to the line side not the motor side. If the capacitor shorts out, this will determine whether the motor is damaged or not.