A capacitor is a very common electrical component within your home’s air conditioning and heating system. Capacitors enable certain types of motors to run. These types of capacitors are called run capacitors. Before we discuss why they fail, let’s talk about why they are necessary.
Most homes are on what’s called “single phase” electricity. Very simply put, this means all the electricity in your home is current that runs the same direction. The motors used in your HVAC system have two windings and they need one of those windings to be “out of phase” in order to turn. This is where your capacitor comes in. The electricity going to one of the windings in the motor goes through the capacitor first, is taken out of phase, and then proceeds to the motor, enabling the motor to turn. Without a capacitor this type of motor won’t turn, it will just sit and hum (which isn’t good for the motor).
A capacitor is made up of an electrical conductor and a dielectric (non-conductive) material rolled up together like a fruit roll-up still in the plastic. The amount of conductor inside the shell determines the amount of “capacitance” that the capacitor has. This is measured in microfarads. See the image below.
There are many ways that a capacitor can fail, but the two main failures are from heat, and from the deterioration of the dielectric.
When a capacitor gets overheated it begins to swell which can pull the contact points (where the wiring connects to the capacitor) out of the conductive material inside. This means the wiring is no longer connected to the material inside, and the capacitor has failed, it is no longer providing capacitance to the motor.
Capacitors can overheat for several reasons. A failing motor can increase the amperage needed to run, which increases the amperage being pulled through the capacitor, overheating the capacitor, and can cause it to fail. This also works in reverse, a failing capacitor can cause a motor to require more amperage to run, overheating the motor, and can cause the motor to fail. Capacitors can also overheat from outdoor ambient temperatures. It is very common to have capacitors fail during a short heat wave. The first time it hits 90 degrees we see a lot of cap failures. The ones that make it through this heat may then fail when it hits 100 degrees, and so on. This type of failure is usually immediate. The moment it overheats and pulls the contacts out the motor ceases to operate.
The next most common reason we see capacitors fail is due to the deterioration of the dielectric material inside. As this material degrades the electricity inside shorts out, reducing the capacitance, or microfarads. This type of failure is slow. As the dielectric slowly degrades the capacitance slowly goes down, bit by bit. This type of failure can also cause the motor to fail. As the capacitance goes down the motor no longer operates as designed and begins to operate with unnecessary strain which can cause the windings to break down, the motor to overheat, etc. This is why it is so important to have your system maintained on a regular basis. We can test and determine when a capacitor is slowly failing and by catching it early we can preserve the integrity of the motors it is connected to, saving you hundreds of dollars in the long run.
Capacitors, and why they fail, is a very complex subject. This is a very simplified explanation to help you understand why this happens and why we, as professional HVAC technicians, do what we do when we work on your system.
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