There are 2 different types of capacitors used in the HVAC field. The types are start capacitor and run capacitor.

A start capacitor is called a start capacitor because it is in operation only when the motor is starting. This design increases the torque of the motor allowing a compressor to start against a load. This design is not used with a fan motor. It is used in conjunction with some type of relay that removes it from the circuit as a motor approaches full speed. This type of capacitor is encased in a bakelite case and is air cooled. It is not filled with oil and it is usually a round design.

The most common type of capacitor is a run capacitor. It is designated as a run capacitor because it stays in the circuit while the motor is operating. The purpose of a run capacitor is to increase motor starting torque as well as increasing the efficiency of the motor during operation. This capacitor is in a plastic or metal can and is filled with oil. The purpose of the oil is to help cool the internal plates during operation. This capacitor can be either round or oval shape.