What Causes a Capacitor to Go Bad in an A/C Unit?

Capacitors store and release energy as needed within a circuit. Larger capacitors generally have higher current load-handling ability and greater charge storage capacity. These properties are well suited for air conditioning machinery because some A/C compressor motors need a pulse of energy 300 to 500 percent of normal to get them going. A start capacitor is used for this application. Other electric motors in the system use smaller run capacitors to keep them spinning at optimum speed.

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The largest and most expensive A/C capacitor is located in the outside unit.

Three Motors

The air conditioning system has an electric blower motor indoors that circulates the cool air, an electric condenser fan motor outdoors to exhaust the hot air, and a compressor motor that pumps the refrigerant through the system. Each motor uses a capacitor to get it started when the thermostat switches the A/C on, and often to keep the motors running at maximum efficiency. The compressor capacitor is the largest, since the compressor requires a lot more energy than the other two motors.

Overheating

The electrolytic A/C capacitor contains a moist separator inside and it will dry out of the unit gets too hot, causing it to develop an internal short circuit. The capacitor can fail if its temperature routinely goes above 150 degrees Fahrenheit.

Electrical Overload

The air conditioning capacitor is designed to get the compressor and fan motors up to about 3/4 of running speed and then disengage. It is not designed for a continuous electrical load. If the motor becomes physically blocked from turning or if the motor burns out, the capacitor will probably overload and be destroyed. Voltage above the rated value for the capacitor might also destroy it. For example, a power surge from a lightning strike would cause a severe current spike and probably "fry" the capacitor.

Worn, Damaged Parts

The capacitor must disengage after a few seconds or it will overheat. A compressor or fan motor that drags due to damage or worn bearings might cause the capacitor to burn up. A malfunctioning relay switch can also cause the capacitor to overheat by leaving it in the circuit too long. Lightning can damage a capacitor, a compressor, the fan motor or the wires in the unit. Even a weak power surge can damage or destroy the capacitor, which can lead to compressor overload and subsequent failure. A capacitor that's leaking oil is a sure sign that it has a problem.

Life Expectancy

An A/C capacitor that is protected from damage, electrical surges, overheating or incorrect electrical loads will eventually fail due to normal wear. The life expectancy of an A/C capacitor varies with the climate and usage pattern, but a typical rating is about six years. For example, the Zettler start capacitor for typical home air conditoning systems claims a service life of 50,000 starts.