Checking Capacitors

This is page two of information all about checking capacitors and testing capacitors for your air compressor.

This link takes you to page one of testing capacitors.

Step 2 in Checking Capacitors

Now that you're sure there's no charge on the capacitor, if you haven't already unplugged the wires to it, note where they went and unplug them.

Step 3

Switch your volt meter to the ohms range, maybe around the hundred ohm range if it isn't auto-ranging. Check the meter by touching your two leads together - the meter should go immediately to about zero ohms if everything is set right, and you have a good battery in it.

A good capacitor charges up to that voltage, so when you connect the meter to a good capacitor, you'll see the ohms start kind of low and go up really high as the capacitor charges up to the 1.5 volts or whatever your meter uses.

This can happen in a few seconds, or as fast as a half second.

A run capacitor from a Campbell Hausfeld air compressor is shown below.

air compressor capacitor

Step 4

Here's the trick - switch your probes to the opposite terminals on the capacitor. Now the capacitor is charged the opposite way from what you're measuring, so the ohm meter will momentarily read a negative number! which will move through the range to zero and back to the high number or infinity like it did before (or if your meter is not digital, it'll slam the needle to the peg as it discharges the capacitor and recharges it the other way).

I usually switch the probes back and forth a couple of times to make sure I'm getting that capacitance effect that sends the number negative and slowly rises back up. Only a capacitor does that. And it won't do it if the capacitor is shorted or an open circuit.

That will give you about a 95% certainty that the capacitor is good... again, the value of a capacitor rarely changes significantly enough to affect the running of a motor. Also, most motors have a start capacitor, which is only used to START the motor. The capacitor and the start windings are only used to get the motor up to speed. A few motors (usually the bigger ones) also have a run capacitor. While the method of checking the capacitors is the same, this whole discussion is about run capacitors because it's about a motor not having the power to keep running under higher loads...

Blowing Breakers

And one thing I would check if you're blowing breakers is whether you have too long or too light of an extension cord. You may be losing a lot of power across the cord, so your motor draws more amps to keep running, and that could be blowing your breakers.

To see if that's an issue, try to check your voltage at the compressor while it is running, especially when it is bogging down. If it is significantly below 110 or 220 volts, then you might get better power to your motor and see if it still does that.

Hope this helps someone!"

A Weak Capacitor?

If the capacitor is just weak, and not blown entirely, diagnosing that is difficult. Another anonymous contributor offered the following advice.

"It's rather difficult to test a capacitor without a multimeter that has a feature that allows you to test for capacitance directly.

To see if your capacitor is within specification set the meter to the capacitance mode and apply the test leads to the terminals - it will take a few seconds as the meter is actually charging the capacitor and then reading it in farads. Be careful when testing capacitors because they do hold a charge which sometimes can be hazardous.

A continuity tester will tell you if it's blown open or shorted so if the only meter reads Infinity or almost zero owns the capacitor is toast. If it is simply week an old meter will be very very difficult to interpret correctly. Hope this helps."