A capacitor is an electric component that temporarily stores an electrical charge. A "run capacitor" supports electric motors, such as in air conditioner or heat pump units, a fan motor and a compressor motor. The capacitor has two terminals. Capacitors come in a variety of sizes, depending on the capacitance (µF or MFD), such as 40 plus 5 µF, and also the voltage. A 440 volt capacitor can be used in place of a 370 volt, as it is build better, but the 370 can't be used in place of a 440 volt. It will work for a while and will fail prematurely, as exceeding the capacitor's rated voltage will cause the dielectric to break down and the capacitor to short out. Same as an electric wire- 600V rated wire is better than 300V one .The capacitance(µF or MFD) must be the same or stay within 10%+ or 5%- of its original value . Example: 50 µF cap can be substituted by 47.5 to 55 µF with the same or better voltage ratings capacitor . Round oval-shaped run capacitors are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor.