Capacitors consist of two parallel conductive metal plates, separated by either air or an insulating material. When voltage is applied to the plates, one gains a positive charge, and the other gains a negative charge. The higher the voltage, the higher the charge gained. In addition, the larger the plates and the smaller the separation between them, the greater its capacitance or charge will be.
Capacitors come in many types that suit specific purposes, ranging from small, delicate trimming capacitors to large oval or cylindrical high-voltage capacitors. AC capacitors tend toward the larger end of the spectrum. Dual capacitors that serve the compressor motor and the fan are the most widely used, although some ACs implement a single capacitor.
Modern AC capacitors often require replacement. Some indicators for this include the compressor humming but the AC not starting and the capacitor not holding a charge or leaking oil. Unbalanced cooling systems with unequal refrigerant pressure on each side as well as ACs with aged motors may benefit from a special starting capacitor that provides an extra voltage boost to help the system start properly.
Replacing an AC capacitor is dangerous; if you aren't experienced with electricity, then get a professional to handle it. Even while unplugged, the AC capacitor can hold enough electrical charge to severely harm a person. For this reason, the terminals on the top of the capacitor must be shorted prior to removal. When shopping for a replacement, consult the capacitor's documentation to ensure its horsepower and voltage match that of the AC unit. Most AC manufacturers also make replacement capacitors.
Three Important Numbers
Important numbers for capacitors include the working voltage and normal capacitance. You can find these figures on the capacitor. As with resistors, capacitors also have a tolerance rating, which usually ranges from 5 to 10 percent. When replacing an AC capacitor, write down these three numbers, and purchase an AC capacitor with the same specifications to avoid damage to the capacitor or your AC.