The AC Unit Capacitor: What and Why


Capacitors are the unsung heroes of your air conditioning (AC) unit. They work tirelessly behind the scenes, allowing your AC to function smoothly under the harshest summer heat. This post will dive deep into capacitors, focusing on their critical role in an AC unit. We will also explain why a faulty capacitor can spell disaster for your comfort.

The Essence of a Capacitor

Before delving into their application in AC units, let’s understand capacitors. Capacitors are passive components that store energy in an electric field. They are like temporary batteries within your AC unit, storing and releasing energy as needed. The unit of capacitance is Farad (F), though capacitors commonly used in AC units are in the range of microfarads (µF).

Capacitors: The Lifeblood of Your AC Unit

Capacitors play two critical roles in an AC unit: they help start the motor (start capacitors) and ensure the motor keeps running (run capacitors). 

The Starting Line: Role of the Start Capacitor

The motor within an AC unit requires a high amount of current at startup. This is due to the establishment of magnetic fields within the stator and rotor of the motor, which creates an induced voltage (back EMF) opposing the supply voltage. As this back, EMF is proportional to motor speed. At startup, when the motor is just starting to turn, the back EMF is very low, which allows a high inrush current to flow. This is where the start capacitor comes in.

The start capacitor releases stored electrical energy at startup, which provides a quick burst of high current to the motor. The back EMF produced is directly proportional to the speed of the motor. When the AC unit is switched on, the speed of the motor is near zero; thus the amount of back EMF produced is very low. This means that a high inrush current flows into the motor, which can be five or more times the motor’s full-load current.

At startup, the current decreases to the level of the motor’s locked rotor current (LRC) until it starts moving. As the motor speed, and therefore, back EMF, increases, the current further decreases until normal operating speed and normal operating current are reached.

What happens when an AC capacitor goes bad? If the start capacitor is missing or faulty, the AC unit will struggle to start, if it starts at all. The home wiring alone may struggle to provide the needed current to overcome the initial resistance of the motor.

The role of the start capacitor is crucial. However, once the motor achieves roughly 75-80% of its operating speed, the start capacitor is taken out of the equation by a relay, allowing the run capacitor to take over. Physically, there may be two single capacitors or one dual capacitor in your outdoor unit, depending on how the unit is built. Some smaller units may not require a start capacitor at all, but a run capacitor is typically the minimum, the importance of which is discussed below.

The Marathon: Role of the Run Capacitor

Once the motor is up and running, the run capacitor steps in. This capacitor continually sends energy to the motor, keeping it running efficiently by providing a steady and constant current. The run capacitor helps maintain a relatively consistent voltage supply by storing and discharging energy at a rate that compensates for the fluctuating power demand of the motor’s rotor (an inductive load).

In other words, the run capacitor improves the efficiency of the system by correcting the power factor. Power factor is a measure of how effectively electrical power is being used. A high power factor signifies efficient use of electrical power, while a low value indicates poor utilization of electrical power.

In the plots below, a capacitor helps in power factor correction in an AC unit. The first plot is the voltage and current of an inductive load (1 Ohm + 0.1 Henries) without a capacitor, while the second one is with a capacitor in parallel (70 microFarads).

The current and voltage are in phase when the power factor is corrected. This results in efficient operation, reduced total current drawn from the source, and decreased load on the electrical grid. See this video for an excellent breakdown of power factor correction as well as the calculations involved:

I also recommend reading this article for further power factor explanation.

Voltage Ratings and Safety Factors

The voltage rating of a capacitor is another critical factor. This rating indicates the maximum voltage a capacitor can handle. For instance, an AC capacitor with a voltage rating of 370V shouldn’t be used in a system where the voltage exceeds this level.

It’s essential to adhere to these ratings because applying excess voltage can cause the dielectric material inside the capacitor to break down, resulting in catastrophic failure. Therefore, a safety factor or “voltage margin” is often included when choosing a capacitor. This margin ensures the capacitor can handle voltage surges without damage. In practice, a capacitor with a rating of 440V might be used in a 240V AC unit to accommodate potential voltage surges.

Material Properties and Their Impact on Capacitor Performance

Capacitors can be made from a variety of materials, including ceramic, tantalum, and aluminum, each influencing the capacitor’s performance and durability. For example, ceramic capacitors are inexpensive, stable over a wide temperature range, and good for high-frequency applications, but they can’t store as much energy as tantalum or aluminum capacitors.

The choice of dielectric material (the insulating material between the capacitor’s plates) also impacts the overall properties of the capacitor. For example, a dielectric with a high permittivity will allow the capacitor to store more charge, increasing its capacitance. For AC units, the material of the plates is typically aluminum.

Common Symptoms of a Failing Capacitor in AC Units

The crucial role capacitors play in your AC unit’s operation also means their failure can cause significant issues. If your AC unit is having trouble starting, is humming but not turning on, or is shutting off on its own, a failing capacitor could be the culprit.

How to Identify and Replace a Faulty Capacitor

Identifying a faulty capacitor can be tricky, but certain signs can indicate a problem. Physical signs like bulging, leaking, or a burnt smell suggest a faulty capacitor. However, an expert testing the capacitor using a multimeter set to measure capacitance for a definitive diagnosis can confirm if the capacitor is functioning within its rated specifications.

Can I replace an AC capacitor myself? Replacing a faulty capacitor should be approached with caution. By design, these devices can store a lot of energy, which can discharge to your body if not careful. Working with electrical appliances can be dangerous, so don’t hesitate to call a professional if you’re unsure about any part of the process.


In an AC unit, the roles of start and run capacitors are distinct but equally crucial. Together, they ensure your air conditioning unit’s smooth and efficient operation. Understanding their function and importance can help in the early detection of problems, leading to timely maintenance and a longer lifespan for your AC unit. Keep these heroes in top shape, and they will serve you reliably, ensuring your comfort throughout the summer.

Always hire a professional if you’re unsure about performing maintenance on your AC unit. Stay cool, and see you in the next post!

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