What AC Capacitors Do and Everything Else

AC capacitors

AC capacitors serve a critical role in the operation of air conditioning systems. These capacitors are responsible for storing and releasing electrical energy to help start and run the motors that power essential components in an AC unit, primarily the compressor and the fan.

Capacitors provide the necessary electrical phase shift to ensure the motors operate efficiently and effectively.

There are two types of AC capacitors:

  • Start Capacitors: These capacitors provide an extra electrical boost during the startup phase of the AC system, particularly to overcome the high resistance when the motor initially begins to rotate. They store electrical energy and discharge it rapidly, allowing the compressor and fan motors to overcome inertia and start running smoothly.
  • Run Capacitors: Run capacitors, on the other hand, are in continuous operation while the AC system is running. They provide a consistent phase shift that helps maintain the motor’s efficiency and power factor. Run capacitors contribute to the long-term performance and reliability of the AC system.

Understanding Voltage in AC Capacitors

Electricity simply defined as the transfer of electrons. For these electrons to be transferred from one point to another (like from the wires in the house to the condenser unit), there needs to be a difference in potential between the 2 points.

Voltage is also described as potential difference (expressed in volts) or an electromotive force. Therefore, voltage is the force which moves electrons from one point to another in an electric system.

The higher the voltage the higher the force (or the rate of transfer of electrons). And that is precisely why air conditioners need capacitors.

Capacitors can store a high voltage (more electricity) than the electric system in the house can provide. When the thermostat therefore calls for cooling, it means that the capacitor can supply the AC system motors with forceful bursts of electricity needed to start and run the system.

I can compare voltage with water pressure. The higher the water pressure in a pipe, the faster it is likely to fill a tank.

To understand the importance of capacitors in air conditioners, let us think of how hydro power plants work. These plants are built along a river but in most cases the pressure (velocity) of the water in the river is not sufficient to turn the turbines (which produce electricity).

To solve that problem, a dam is built across the river to create a water reservoir. The water in the reservoir is then let out through channels in the dam where the pressure will be very high. The pressure created by the reservoir is now sufficient to turn the turbines and generate power.

Capacitors have the same effect on air conditioners as reservoirs have in hydro power plants. Both of them store the energy needed to start and run the systems.

AC Capacitors Ratings

air conditioner capacitor

All air conditioners are not equal. Some are bigger than others while some are more efficient than others. In the same way, AC capacitors are different for different AC systems.

When replacing your AC capacitor, you need to first know the rating of your old capacitor. AC capacitors are rated using two specifications:

  • Voltage
  • Capacitance

As I have already mentioned, voltage is a measure of the amount of electric current flowing through the capacitor. It is measured in volts. Most AC capacitors have a voltage of between 370V and 440V.

As you can see, this is why ACs need capacitors. If your home is receiving 240V from the power supply company while the motor needs 370V to start, the air conditioner will be unable to properly cool your house without the right capacitor or with a bad one.

Another thing that you need to check when buying an AC capacitor is capacitance. Simply put, capacitance indicates how much electric charge the capacitor can store. It is measured in microfarads (MFD).

The capacitance of most AC capacitors is between 5MFD and 80 MFD. Both the voltage and capacitance are usually clearly displayed on all capacitors.

Start vs Run Capacitors

HVAC systems are built differently. Depending on the type of system you have, it could have either 2 single capacitors or a dual capacitor in your condenser unit.

AC systems with 2 single capacitors will have a start capacitor and a run capacitor. So, what is the difference between a start capacitor and a run capacitor?

As its name implies, a start capacitor’s job is to supply the motor with the initial jolt of electricity needed to start the cooling cycle. Once the process has started, the start capacitor gets out of the way and the run capacitor picks things up.

In essence, the run capacitor stores up electric charge and also releases it to keep the cooling cycle going. Run capacitors are used to maintain the charge necessary to run the air conditioner.

Start capacitors have a higher capacitance than run capacitors. A start capacitor will have a capacitance of at least 70 microfarads while a run capacitor will have a capacitance of 1.5-100 microfarads.

Usually, start capacitors have a black plastic case while run capacitors have a shiny metallic exterior.

start capacitor

Most AC units have dual run capacitors. In fact if you have 2 single capacitors, you should consider replacing them with a dual run capacitor then next one of the capacitors is due for a replacement.

Dual capacitors are 2-in-1 components that perform the functions of both start and run capacitors. If you are not sure which capacitors your AC unit uses you can consult a professional HVAC technician to check things out for you.

Why AC Capacitors Fail

One of the most replaced components in air conditioners is the capacitors. Why do AC capacitors fail more than other AC parts?

Since capacitors store electric charge, they are prone to overheating which eventually cause them to wear out. Capacitors overheat mostly when the AC system runs for a long time.

When for instance you fail to change your AC filter or there are other problems with how air is being moved in the system, the air conditioners works harder than usual (runs for longer). This extra strain on the AC can cause the capacitor to overheat and fail.

Capacitors can also fail because of a power surge. A power surge blats too much power through the capacitor making it too hot. In other words you can say that the capacitor gets fried.

Capacitors can also fail due to age. This does not however happen that frequent seeing that an AC capacitor can last for up to 20 years if the system is properly maintained.

The following are some of the ways you can prevent an AC capacitor from failing:

  • Install and HVAC surge protector
  • Change the AC filter after every 1-3 months
  • Clean the AC coils
  • Have the ductwork cleaned
  • Keep an eye on refrigerant levels
  • Annual maintenance by an HVAC technician

What Happens When the AC Capacitor Goes Out?

The thing with HVAC repair is that different problems will most often exhibit similar signs. For, instance, your air conditioner will fail to cool for so many reasons and that is why most HVAC-related problems are better left to HVAC professionals.

But how can you really tell if your AC capacitor has gone out. What are the symptoms of a failed AC capacitor?

The following are the signs of a blown AC capacitor:

  • The AC is not cooling the indoor air
  • AC struggling to turn on or does so after a while
  • Humming noises from the outdoor unit
  • Sudden increase in electricity bills
  • The AC will not start at all
  • AC shuts off unexpectedly
  • Burning smell from the outside unit
  • Condenser fan not spinning while AC is still on

How Can You Test an AC Capacitor?

Unless you are trained or have proper electrical-related knowledge, I would advise you not to test or replace your AC capacitor. Ideally, that job should be left to trained HVAC technicians.

Note: Even after turning off power to your HVAC unit and disconnecting it from the power outlet, it is still not safe to handle a capacitor. Since capacitors store electric charge, you can still get electrocuted by touching the capacitor’s terminals.

One thing you can do however do is a visual inspection (without touching it). If the capacitor is bulging or leaking, it is obviously bad and will need to be replaced.

Can AC Run without a Capacitor

Air conditioners need capacitors to run. But what happens if you remove the capacitor? Can the AC still run without it?

As you already guessed it, air conditioners cannot work without a capacitor. Can ACs however run with a bad capacitor?

Some air conditioners can run with a bad capacitor while some won’t. Having said that, I should warn you that running your AC with a bad capacitor is a bad idea. It puts unnecessary strain on other AC components making them to also fail prematurely.

To demonstrate that some AC motors can run with bad capacitors, you can see a condenser unit running but the fan not spinning. However, when you manually turn the fan using an object like a stick, it starts running.

Wrap Up

And that is pretty much it in as far as AC capacitors are concerned. While AC capacitors are cheap (costs about $20), the replacement cost can be high (around $150 or higher) because as I mentioned they need to be replaced by licensed technicians due to the danger they pose.