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HVAC School - For Techs, By Techs

Jul 24, 2018

In this short podcast episode, Bryan Orr discusses the best practice methods for testing run capacitors in the field.

We understand the capacitor to be a voltage storage device. We can benefit from comparing the capacitor to a balloon that inflates and deflates with electrons as the alternating current changes (60 times per second). A capacitor causes a phase shift and allows there to be current on the start winding. So, when a run capacitor fails, you won't have current on the start winding.

The old-fashioned way of testing a run capacitor was to take an ohmmeter and charge/discharge the capacitor. Nowadays, we have capacitor testers, and many multimeters also have capacitance testers. Capacitance is merely a mathematical equation that you use when you compare the amount of voltage to the amount of current entering and leaving.

A good way to test a capacitor on a running system is to test it under load. You take the amperage of the wire feeding the start winding and multiply it by 2652. Then, you divide that product by the incoming voltage across the capacitor to get the capacitance. While this method is probably the most practical, it still has a caveat; some meters may have a hard time getting a proper amp reading on the start winding. So, tool accuracy will also determine your success when testing capacitors under load. To increase accuracy, make sure your wires are isolated from others. Under-load testing may also be unsafe in some cases, such as with a blower capacitor.

Testing with the system off is called bench testing, and it is slightly more accurate but does not represent under-load conditions. It will be more practical than under-load testing if the system is already off or if it is unsafe to test the capacitor under load.


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