Why Do Capacitors Fail? (It’s not why you think)
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One theory is that a high amp draw on the motor causes compressors to fail. Another is that systems with low charge or dirty condenser coils cause capacitors to fail prematurely. However, it looks like capacitors have a fixed amount of current that can go in and out of them, and they’re more independent than a lot of us realize.
A run capacitor is like a pressure tank for electrons forced into it; it constantly stores and releases electrons 60 times per second (60 Hz). However, at some point, it can no longer store any more electrons.
The C side typically connects to the run side of the circuit, and herm typically connects to the start winding. Electrons have to move in and out of the capacitor for there to be voltage between C and the start winding. However, there is no connection between the C and herm sides of the capacitor, so electrons can’t move between terminals. A capacitor essentially adds a 90-degree phase shift to correct the inductive phase shift of the motor, so it helps the motor run in the correct direction.
Capacitors have a capacitance rating (in microfarads). The amount of electrons that can go in and out of the capacitor (as well as the amperage) is dictated by that capacitance. However, setups that include a potential relay and start capacitor(s) will increase the electron capacity, and that type of setup may draw higher amps briefly. Without that setup, the amperage on start and run will be the same. The only way to get higher amperage on the start winding is to have higher voltage and higher capacitance.
The ambient temperature can affect life expectancy, as sustained higher ambient temperatures can make run capacitors fail prematurely. Capacitors have oil to keep them cool, so they are a bit sensitive to heat. High-voltage surges (transients) can also decrease the life of the compressor, as bridging across the plates can cause premature failure. High voltage, high amperage, and poor capacitor manufacturing are mostly to blame for run capacitor failure.
We show all of this in action with a few field tests, including reading inrush amps on the start winding (0) and then the run winding (60). According to our results, the motor behavior on startup doesn’t affect the amount of electrons a capacitor can hold; even back EMF doesn’t affect startup conditions. We also include a guide for testing a run capacitor under load, which you can find here: https://hvacrschool.com/wp-content/uploads/2017/06/How-To-Test-a-Run-Capacitor-Under-Load-1.pdf.
Overall, we can ensure that capacitors reach their life expectancy by minimizing the probability of transients (such as with surge suppressors like thermally protected metal oxide varistors), keeping the capacitor cool, properly installing the capacitor with tight connections, and using well-made capacitors.