Testing Capacitors

You might say: the system is running, so the capacitor must be good! That is not 100% true. Now let’s see why.

A capacitor has a voltage rating on it such as 370 vac or 440 vac. So, anytime you exchange it with a new one, make sure you get the voltage that matches or is greater than the one you are replacing. Also, remember to get the manufacturer rated capacitor for the motor. Do not replace with a different size microfarad.

Capacitors are used on motors to make them more efficient by increasing the RMS (Root Means Square) of the sine wave. Capacitors increase efficiency by taking the load off the windings and they have a range for tolerance. For example, a 5 microfarad capacitor with a +/- 5% tolerance means that 5-microfarad x .05 (which is 5%) = .25 tolerance. You can add 5 or subtract 5 and that will be your range. In this example, it would be a range of 4.75 to 5.25 microfarads. If you put your meter on microfarads and measure the capacitor from one side to the other and it reads 4.25 or 5.75, it will be out of range. This makes the motor less efficient and should be replaced. You should also check the new one as well because you can get one out of tolerance right out of the box.

Start capacitors are usually made of a composite material. They are usually black or brown but they can be other colors as well. They are rated in ranges such as 150 to 275 microfarads. That is how you can tell that they are start capacitors because they are only used for a moment to boost the starting of the compressor.

A run capacitor is in the circuit the entire time the motor is running. Typically, they are in a metallic shell and they come in various shapes. For example, it can be a round canister looking object or it can be oval looking cannister. Run capacitors can have either two or three terminals to put wires. Each terminal can take 1-4 wires. The capacitors with just two terminals are called run capacitors and the ones with 3 terminals are called dual capacitors. The only difference between single vs dual capacitors is that dual capacitors internally share one point – which they call “common”. The other two terminals are called herm and fan. Herm for the compressor and fan is for the fan.