Our Ac Unit is not cooling and the compressor is making a soft humming noise. The fan is not turning on. At first we thought it was just BGE and their Peak Rewards "regulating" but now we are thinking maybe the AC is broken... any ideas? We have small kids...

Humming is a good sign. Don't run the AC until fixed.

Either the fan or the fan's capacitor has failed. With the power off try to move the outside fan manually. See if it moves freely.

Capacitor is usually physically inspected for bulges etc and electrically tested. Motor is checked for binding and open windings.

If motor is replaced, both cap and motor are replaced together.

The fan does move freely. So I guess we call someone in the am?

Thanks!

Why do you need to replace the cap and the motor at the same time?

My motor just went bad yesterday, but only bought a new motor. Cap tested out good at appliance shop..

If the new motor capacitor rating matched the old capacitor, you should be just fine. But its very important that the motor matches the cap. If not, it will shorten the life of your new motor. Capacitors are extremely cheap, so its always cheap insurance to install new matching cap when installing new motor. If you had the old cap tested, and it was rated for you new motor, you are okay.

Why do you need to replace the cap and the motor at the same time?

High value capacitance capacitors have one of the highest failure rates in electronics. The motor usually outlasts them. They degrade.

Simple capacitor meters are good at guessing whether a cap has failed. Shorts are easy. Capacitance out of tolerance is yet another way. ESR or effective series resistance is a better test. The behavior in a hostile environment where temperature and voltage surges hammer the cap, your pushing the popensity to fail.

When I rebuild audio stuff or rebuild antique radios, the caps above a certain value, are just replaced. If there is any electrolytic cap in the signal path, it's changed to a metalized polester.

5 years is a good time to replace. If the cap shorts, it cn take out the motor. Cheap insurance policy to replace the cap when the motor is replaced.

Don't mean to hijack this thread, but how do you know if a capacitor is rated correctly for the motor?

Is there some sort of marking on the motor that will tell you what type of cap. To use with it?

Don't mean to hijack this thread, but how do you know if a capacitor is rated correctly for the motor?

Is there some sort of marking on the motor that will tell you what type of cap. to use with it?

Yes,on all new motors
There is a label that tell the correct capicitor needed
It's there where it gives the wiring instructions,the voltage,the rpm`s,
Most times they even put it on the box the motor came in