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HVAC School - For Techs, By Techs

Oct 22, 2019

In this short podcast, Bryan talks through (once again) what REALLY causes capacitors to fail and what we can do about it.

High temperatures and overvoltage (NOT undervoltage) are what cause capacitors to fail. (There are also some poor manufacturing practices out there. Remember: they should be just foil and oil.) Capacitors create a phase shift to assist split-phase induction motors. Normally, a three-phase motor can start and run just fine because the sine waves are all angled. That is not the case for split-phase motors. Start capacitors help startup, which is difficult for the motor.

The capacitor stores and discharges each time there is a cycle change (usually 60 times per second). That rapid storing and discharging helps create a lag that gets the motor get moving. However, capacitors are limited by their design: their charge capacity (current) is dictated by size (microfarads), voltage, and frequency. If you measure amps on the start winding, you will notice that the amps are lower than on the run windings; the capacitor acts as a limiting factor.

When the run capacitor fails, you have no current on the start winding. The motor does NOT cause the capacitor to fail; there is a slim-to-none chance that a motor's back EMF can cause capacitor failure. Excess temperature or voltage is what really causes capacitors to fail, and THAT can negatively affect the motor. The hotter a capacitor runs, the shorter its life will be. Locations with high temperatures year-round and lots of thunderstorms will have more capacitor failures than places with more temperate climates.

Capacitor oil also plays a role in longevity. Oil exists for heat dissipation and should be mounted with the terminals up. Condenser cleanliness and temperature can also help or hinder the oil's efforts.


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