Questions about capacitors

[oldbandnerd]

I am looking for technical help of the electrical type. I work on commercial Dish washers and am dealing with a pump motor issue that I believe is a start capacitor problem. I don't really know much about capacitors and need some help .

First .... I need to buy a multimeter that has a range big enough to test the capacitor.
From the manufactur the specs of the run capacitor are 165V 275 mF . The multimeter I am looking at says it has range of 100 uF (micro farad) . Does this meter have the range I need ?

Second ... Will I need to discharge this capacitor first and is it safe to just short out across two terminals . Will that damage it ?

Any help would be appreciated.

[DonShirer]

I'm an electronics guy and haven't worked with pumps, but until an electrician chimes in, I can give you some quick answers which I hope will apply to your situation.
1) The spec you quoted, 275 mF (millifarad) is probably a misprint for 275 µF (microfarad). A millifarad capacitor would be humongous.
2) If the multimeter max range is 100 µF, it will not be able to verify the capacitance.
3) But that is probably not as important. If your capacitor is misbehaving, you should test its resistance. The resistance reading may be low for a few seconds but it should then move toward a high value (above 100 kohm). If it stays low, the capacitor probably is shorted out.
4) Discharging capacitors before testing is always a good idea. If you don't want a spark, connect a resistor (100 to 1000 ohm or so) across the terminals to discharge it.

[oldbandnerd]

Thanks for the help Don. I've sent the company engineer a email to ask for clarification. In his email he clearly use mF and not that funny looking Greek u symbol. I've been researching on my own on the inter webs but was still unsure. I think he meant micro farads as well.

[oldbandnerd]

He replied and he meant "micro" Farads and not mili ...So the mulitmeter I was looking at will not and I have to get one with a bigger capacity. I'm gonna just short across the terminals because I'm not worried about the sparks and wont have time to get a resistor.

Thanks for the help.

[Dan Schultz]

If a motor won't start I just replace the capacitor as a matter of routine. Those things generally just aren't that expensive.

[Rick Denney]

Electrolytic motor starter capacitors can knock you across the room, or blow a hole in their sides, or spray you with hot goo, and the mistake that causes it is not so hard to make.

They are not expensive but they cannot be repaired if they have gone bad. Their purpose is to hold a substantial static charge that gets dumped into the motor windings for a very short period to jolt the stator into motion. They are similar to filter capacitors in large power supplies, and charge capacitors in electronic strobe camera flashes. All of these are hard to test. One of those bit me once, and I launched it across the room involuntarily with enough force to put it through a wall.

The DVMs that have capacitor testers do not test electrolytic caps, which require a bias voltage to create the capacitance. They test paper disk caps on circuit boards, which have capacitances measured in picofarads. 275 microfarads (which is a LOT) will blow it up. (By the way, motor and compressor starter caps have traditionally used "m" for micro rather than the Greek mu). Shorting a cap that big for testing will require a chicken stick (for safety) and a large carbon resistor. Then, you can test it for shorts. If that is new to you, stay away.

Rick "caution!" Denney

[oldbandnerd]

Thanks Rick Denney. I trust your knowledge and have decided to just get one and install it to see if the capacitor is really the problem. I will be VERY careful pulling the wires . I've had my share of bad experiences with high voltage as well . This has been very educational !

....... as a side note these things are cheap enough that just replacing them rather bothering to test them is the better way to go.