Testing Capacitors

  September 10, 2019      Jeff Rosenblum


Testing a run capacitor should be part of your annual maintenance. When testing a run capacitor many techs pull the leads off and use the capacitance setting on their meter to test. This is an easy way to check the capacitor. However, testing the capacitance UNDER LOAD (while running) is another way to confirm that the capacitor is doing its job under real load conditions.

Here is a practical method of testing capacitors. This method is actually made up of two different tests that you should already be doing! We just need to use math to figure it out.

1. Read your Volt (EMF) and Amp (Current) readings like usual and note your readings.

2. Measure the amperage of the start wire, this will be the wire connecting the capacitor and the compressor. Record your amperage.

3. Measure the voltage between the two terminals on the capacitor. (The compressor would be HERM and C and the fan would be FAN and C). Record your voltage readings.

4. Now take the amp reading you took on the start wire (wire from the capacitor) and multiply by 2,652 then divide that total by the capacitor volts you just measured. Use the following formula.

Start Winding Amps X 2,652 ÷ Capacitor Voltage = Microfarads

5. Your reading should be within +/- 6% of the rating printed on the capacitor.

6. Repeat the same test on all of the run capacitors to verify they are all within specs.

7. Many times, capacitors are replaced which leaves us to question if the correct capacitor was installed. Remember, you can always reference the data plate of the motor.

The first few times you perform this test, it may take you a bit longer. However, as you practice it, you will get faster. There is no need to shut down the unit, discharge the capacitor and remove the terminals.

One word of advice, make sure you always check your terminals to be sure they aren’t loose or corroded.  Loose or corroded terminals can cause a voltage drop and a higher than normal amperage reading. This will affect your readings and may cause you to quote a repair that isn’t needed.

Jeff Rosenblum 
Technical Support
20 Years Industry Experience
Cell (330) 962-2491