Air Conditioner Capacitor

Air conditioner capacitors are components that are used to help start and run the motors in an AC unit. They work by storing electrical energy and then releasing it when it is needed.

The most common usage of an AC capacitor is to provide the power to start a motor. Even small AC units may have three motors: the compressor, condenser, and blower motors. Each of these motors may have one capacitor or more.

Unfortunately, capacitors are also one of the most common points of failure in AC units. If the capacitor in an AC unit fails, it can easily cripple the operation of the unit.

Complicating things further, capacitors are notoriously dangerous. Their whole job is to store a lot of energy and discharge it very quickly. So if you have a capacitor related issue, hire a professional! Getting an AC unit working is never worth putting yourself at risk of serious injury or death.

In this article, we will cover what an AC capacitor is, how it works, and how to resolve capacitor failures.

What Are AC Capacitors?

AC capacitors are electrical components, often cylindrically shaped, that store electrical energy.

Capacitors are important components of an AC system because they allow the motors in the unit to start and run properly.

Air conditioning units require a large amount of power to start up. This is because motors have a huge power draw when they first start turning. To make matters worse, there are often multiple motors all starting around the same time. This means that the AC unit needs more power than the supply is capable of providing.

Once a motor is running, it requires much less power to keep it going. Thanks to its’ motors, the power requirement of the AC unit is high when it first starts and then quickly decreases.

Capacitors are great for providing the power that motors require during start-up because they can discharge quickly. This means that they can supply a lot of power in a short period of time.

In addition, to this ‘start’ application, sometimes ‘run’ capacitors are used to help smooth out the voltage supplied to the motor. You may hear the term ‘start’ or ‘run’ capacitor.

AC Start and Run Capacitors

AC Start Capacitor

The most common capacitor in AC units is the start capacitor. The start capacitor provides the initial power required to start the motor.

AC Run Capacitor

A run capacitor may be used to smooth the input voltage waveform that is being provided to the motor during normal operation. Motors are highly susceptible to voltage fluctuation and other power quality issues.

Run capacitors essentially act as type of filter. When a capacitor is placed in series with a load, it will block the DC component of an AC signal. This can help to improve the quality of the power to the motor and extend its’ life.

Start vs. Run Capacitors

The above definitions of ‘start’ and ‘run’ capacitors are the most commonly accepted.

However sometimes the term ‘run’ capacitor is used arbitrarily to refer to a capacitor that helps the motor start.

Start capacitors are the most common and when in doubt, you should be able to determine from context or circuit layout whether the documentation is referring to a ‘start’ or ‘run’ capacitor.

AC Capacitor Lifespan

AC capacitors are robust and are designed to last for a long time. However they do eventually wear out and are susceptible to power surges, lightning, high heat, poor power quality, and moisture.

An internal failure in the AC unit can cause failure of the capacitor. For example, if the capacitor is prevented from discharging quickly then it can overheat or sustain damage.

Direct sunlight or hot conditions that cause the AC to work hard can raise the ambient working temperature of the AC unit. This contributes to the premature failure of many components in the system, including capacitors.

Most AC capacitors are usually rated for over 5 years of service but actual lifespan will vary.

It is important to realize that while capacitors can sometimes last for many years, under other conditions they may fail prematurely. If a capacitor only lasts for a short time, you may want to investigate potential issues with the power supply and quality.

How Do AC Capacitors Work?

Air conditioner capacitors are just capacitors that are specifically chosen for their use in an AC unit. Inside the AC unit, motors are used to drive the fans and compressor.

Motors take a lot of power to start running. They can easily draw too much current, leading to blown fuses and tripped breakers. If enough power isn’t supplied, then a motor may not be able to start at all. This is what makes capacitors so useful in AC units.

A capacitor is a device that can store electrical energy and release it very quickly. This means that the energy stored in a capacitor can be used to start the AC unit.

Capacitors are often cylindrical; air conditioner capacitors often resemble a battery in size and shape. Like a battery, they also store electrical energy; they just do it in a different way. Both capacitors and batteries can be charged and discharged, but capacitors are better at some things and batteries at others.

Batteries store electrical energy in chemical form, while capacitors store it by polarizing the atoms and molecules in an insulating material called a dielectric. It takes time for batteries to discharge because the energy needs to be converted from chemical form back into electrical. But capacitors keep the energy trapped in an electric field, which can be discharged almost instantaneously. This is what makes capacitors great for starting up an AC unit.

Different AC Capacitors

Air conditioners can have up to three motors, and each motor may have a corresponding capacitor or more. These are the compressor, condenser, and blower motor capacitors.

The largest is the compressor motor capacitor. It provides the power to start the compressor motor, which pumps refrigerant through the AC.

When a motor isn’t provided with enough power to start up, it may not start. On the other hand, if the motor relies on the primary AC power source, it can overload the circuit

How to Diagnose Air Conditioner Capacitor Failures

Warning: Never attempt to test a capacitor yourself. Capacitors can store a lot of electrical energy and the chance of electrocution is extremely high.

The capacitor needs to be discharged and disconnected from the circuit. Before any electrical testing, inspect the capacitor for signs of damage or dielectric fluid leakage.

Most multimeters have a capacitor testing functionality that will work for any standard AC capacitor.

1. Isolate and discharge the capacitor completely, in a safe manner.
2. Connect a resistor across the capacitor. The capacitor will discharge through this resistor so it is best to use a large value in order to reduce the current.
3. Next, calculate the theoretical value of the time constant T = RC. This is the time that it should take for the voltage to drop to 63.2% of the full voltage.
4. Apply a known voltage (for example, a 9V battery) across the capacitor.
5. Connect the ends of the capacitor to the multimeter probes and set the knob to measure DC voltage.
6. Note the voltage across the capacitor using the multimeter.
7. Next, use a stopwatch to measure the time taken by the voltage to drop to 63.2 % of the full voltage (.632 x supply/battery voltage). This is the experimental value of the time constant.
8. Compare the theoretical and experimental values of the capacitor time constant. If there is a significant difference between them, consider the capacitor is faulty, and it is time to replace the piece.