A Capacitor is basically like a battery. The thing that a capacitor shares in common with a battery, is that they both hold electrical energy. A capacitor will hold electrons inside of it for any given time. Inside of it, there are terminals which connect to 2 metal plates. The 2 metal plates are parted with a non-conducting dielectric substance. You might be wondering, what is dielectric? Dielectric can pretty much be any type of non-conductive material, or some type insulator. Dielectric is a material that an electric field can be held without any real loss of power.
Some basic non-conductive materials that can be commonly found in a capacitor is ceramic, Teflon, cellulose, and porcelain. The dielectric is what dictates the kind of capacitor you have and what its used for. It depends on the type of dielectric being used and the size for the use of high frequency or high voltage apps.
Capacitors can be made for all types of products, from something as small as a calculator, to a super capacitor that has the power for a commuter bus. Capacitors are found in all types of HVAC systems, including the circuit boards, and defrost boards. The most basic capacitors found in HVAC systems are run capacitors, start capacitors, and dual run capacitors.
Run capacitors are primarily polypropylene, and are created to continuously run. They are designed to run the whole time the motor runs. Run Capacitors are measured by microfarads (mfd) from a range of 1.5 to 100. An electric motor that is single phase needs a capacitor to charge its second phase winding. If a person installs the wrong run capacitor, the motors magnetic field will be uneven. The uneven spots will make the rotor hesitate. This can make the motor noisy, take more energy, cause the performance to decrease, and/or cause it to overheat.
Start Capacitors momentarily raise motor starting torque. They allow the motor to off and on quickly. Start Capacitors ratings are above 70mfd. They have 4 voltage types: 330v, 250v, 165v, and 125v. The start capacitor will stay in effect until the motor reaches 3/4 of its full power. The motor will run more efficiently, because of a start capacitor.