Below are some common reasons that run capacitors fail, but depending on how close the run capacitor is to its design life, it may be difficult to pinpoint the reason on a single factor.
Time - All capacitors have a design life. Several factors may be interchanged or combined to increase or reduce the life of a run capacitor, but once the design life is exceeded, the internals may begin to more rapidly decay and drop in performance. Simply put, a failure may be attributed to it being "just old."
Heat - Exceeding the design limit of operating temperature can have a big effect on run capacitor life expectancy. In general, motors that are operated in hot environments or with little ventilation will experience a dramatically reduced lifespan on their capacitors. The same can be caused by radiated heat from a generally hot running motor that causes the capacitor to run hot. In general, if you can keep your run capacitor cool, it will last a lot longer.
Current - When a motor is overloaded or has a failure in windings, it causes the current to climb, which can overload capacitors. This scenario is less commonly noticed, as it would usually be accompanied by a partial or complete failure of the motor.
Voltage - Voltage can have an exponential effect in shortening the design life of a capacitor. A run capacitor will have a marked voltage rating that should not be exceeded. For example, a capacitor is rated for 440 volts. At 450 volts, the life may be reduced by 20%. At 460 volts, the life may be reduced by 50%. At 470 volts, there is a 75% life reduction. The same can be applied in reverse to help increase the design life by using a capacitor with a voltage rating significantly higher than needed, though the effect will be less dramatic.
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