Run Capacitor vs. Start Capacitor: What’s the Difference?

Run Capacitor vs. Start Capacitor: What’s the Difference?

If you’re in the business of repairing or maintaining HVAC systems, then it’s important to know the difference between a run capacitor and a start capacitor. Both are essential components of an HVAC system, but they serve different purposes. In this blog post, we’ll explain the difference between run capacitors and start capacitors, and how each one affects the operation of an HVAC system.

The Role of Capacitors

Capacitors are essential components in electrical and electronic circuits. They store energy in the form of an electric field, which allows them to act as a buffer between the power source and load, or the electrical device requiring electricity for operation. As such, capacitors can be used to regulate current flow and reduce fluctuations in voltage caused by changes in load. [1]

The Role of Capacitors

What Is a Start Capacitor?

Start capacitors are used to help provide a boost to the start winding of an induction motor. The start capacitor is charged with electricity and creates a short burst of energy that gets the motor up and running. This burst of energy helps minimize the amount of time it takes for the motor to reach its full speed, as well as reduce wear on mechanical parts that are subject to high inertia loads.

Start capacitors will typically have a much higher capacitance rating than run capacitors (100 to 1600 µF vs. 5-100 µF).

This is because they need to be able to store more charge in order to provide enough power for the motor’s startup phase.

Start capacitors will also usually be rated for single-phase AC current rather than three-phase current. This is because single-phase motors are the most common type of motor used in household appliances and other consumer goods. However, some industrial applications may require a three-phase start capacitor.

Start capacitors usually have a much shorter lifespan than run capacitors. This is because they are designed to take on much higher levels of stress during the startup phase of an induction motor, leading to components wearing out faster. As such, start capacitors should be checked for proper operation on a regular basis and replaced as necessary.

In conclusion, start capacitors provide an initial burst of energy that helps motors reach their full speed faster, while run capacitors help maintain this level of power over an extended period of time. Start capacitors typically have a higher capacitance rating, are rated for single-phase AC current, and have a shorter lifespan than run capacitors. It is important to regularly check start capacitors for proper operation and replace them when necessary in order to keep motors running smoothly. [2]

What Is a Start Capacitor?

What Is a Run Capacitor?

A run capacitor is an electrical device that is used in HVAC systems to help the compressor and fan motor run more efficiently.

It stores energy, allowing it to provide an additional push of power when needed. A run capacitor also helps reduce stress on the motor, making it last longer. In some cases, a run capacitor can be used to correct phase shifting problems and maintain a balanced power supply between two motors.

The most common type of run capacitors are round metal cans with two terminals on top, usually labeled “C” and “F” for common and fan respectively. The size of the capacitor will vary depending on the size of the motor it is powering; larger motors may require multiple capacitors wired in series.

How A Start Capacitor Works

Start capacitors are designed to provide a short-term burst of energy that helps an electric motor reach its maximum operating speed. The start capacitor works by providing additional current when the motor is first started, which then allows it to reach its full RPMs. The capacitor is typically wired in parallel with the run winding in the motor, and once the motor reaches its full speed, the start circuit disconnects from the power supply and drops out of circuit.

It’s important to note that after disconnecting from the power source, there will still be some residual voltage within the start capacitor, which can cause injury or damage if it comes into contact with exposed wiring or components. As such, proper caution must be taken when dealing with these components. Additionally, start capacitors have limited life spans and will eventually fail over time, so it is important to check their condition periodically. [3]

How A Run Capacitor Works

A run capacitor is used to maintain a steady current in the circuit and can be a permanent or temporary addition. It works by storing electrical energy and discharging it as needed. This helps prevent the motor from overloading, which can lead to serious damage or failure of the device. The capacitance of the run capacitor is measured in MicroFarads (µF) and is usually higher than that of a start capacitor.

The run capacitor also helps with noise reduction in motors, increases their efficiency and reduces heat production from friction inside of the motor. When connected correctly, it will help your devices to operate at full capacity for longer periods without needing repairs or replacements often.

How A Run Capacitor Works

What About AC Compressors?

AC compressors are the backbone of an air conditioner and they’re responsible for cooling the air. They contain two types of capacitors: run capacitors and start capacitors. A run capacitor helps a motor run more efficiently, while a start capacitor helps the motor to start up faster (which can save energy). The difference between them is that a run capacitor is constantly engaged in running the compressor, while a start capacitor only engages when the compressor first turns on.

In short, both capacitors have their place in AC compressor systems – but if you’re looking to maximize efficiency and reduce energy usage, then installing a run capacitor is your best bet. It’ll provide constant power to the compressor motor and keep your AC running smoothly without having to use a start capacitor every time you turn it on.

It’s important to note that replacing either type of capacitor should only be done by a qualified technician. Incorrect installation could potentially damage both the compressor and your air conditioner as a whole, resulting in costly repairs or even the need for a new unit altogether. Your best bet is to consult with an HVAC service specialist who can help determine if you need capacitors replaced, as well as which type will work best for your system. [4]

Can a Run Capacitor Be Used a Start Capacitor?

The short answer is no. A run capacitor cannot be used as a start capacitor, and vice versa. This is due to their different electrical characteristics and internal designs.

Run capacitors, like the name suggests, are designed to remain in continuous use during the operation of an electric motor or other equipment. They help maintain a more constant current flow which increases the efficiency of the equipment being powered. On the other hand, start capacitors are specifically designed to provide an extra boost of power at startup in order to get motors running quickly and smoothly. The momentary surge of energy provided by a start capacitor helps reduce wear-and-tear on motor components and improves performance overall.

Although they serve similar functions, run and start capacitors are not interchangeable. Attempting to use a run capacitor as a start capacitor or vice versa could lead to electrical overload, damage to the motor and even fire hazard in extreme cases. If your electric motor is not functioning correctly, consult with a professional technician for diagnosis and repair before considering any type of capacitor replacement.

In conclusion, it is important to make sure you have the correct type of capacitor for your needs. Run capacitors provide continuous support during operation while start capacitors offer added power at startup. While they do serve similar functions, they are not interchangeable and should never be used interchangeably. Doing so can lead to electrical overload and potentially dangerous consequences.

Always consult with an experienced technician before replacing or attempting repairs on any electrical component.

Can a Run Capacitor Be Used a Start Capacitor?

What Causes a Capacitor to Fail?

A capacitor can fail for a number of reasons, but the most common cause is voltage surges. If the voltage across the capacitor is too high, it can cause an electrical overload, causing damage to its internal components and eventually leading to it failing. Capacitors may also fail due to excessive heat that causes structural damage or corrosion over time. Improper installation or inadequate maintenance are other possible causes of capacitor failure.

Finally, old age can be a factor; capacitors have a limited lifespan and will eventually lose their ability to store energy as they become increasingly worn out with continuous use. It’s important to regularly test capacitors and replace them when necessary in order to avoid costly repairs or replacements down the line.

Ultimately, understanding the different functions of run and start capacitors can help prevent capacitor failure. Run capacitors are designed to stay on continuously, while start capacitors are only used for a short period of time when the motor is first turned on. If a start capacitor is left on for too long, it can overheat and fail. Additionally, making sure to choose the right capacitor for the job can also help prevent failures. For example, an air conditioner may require a different type of capacitor than a refrigerator. [5]

Signs of a Bad Capacitor

When a capacitor begins to fail, there are several signs that can alert you to its failing. The most common symptom of a bad capacitor is a buzzing or humming noise coming from an appliance or device. This sound usually indicates the capacitor is not working properly, and could be due to an electrical connection issue or corrosion.

Signs of a Bad Capacitor

Another sign of a failing capacitor is visible physical damage, such as bulging, leaking fluid, discoloration and black marks on the outer case. A faulty capacitor may also cause devices to have difficulty starting up or even cease operation altogether when it fails completely. If your device has become unresponsive or will not turn on at all, this could indicate the need for replacement of the capacitor.

If your appliance or device is exhibiting any of the above symptoms, it is important to have a qualified technician look at it as soon as possible. A faulty capacitor can cause serious damage to your electronics and could even lead to a fire hazard if not caught in time. Having a professional check out your electronic devices regularly can help detect and address problems such as these before they become worse. [6]


Can I use a run capacitor as a start capacitor?

No, a run capacitor cannot be used as a start capacitor. Run capacitors are designed to maintain the current in an alternating current motor while start capacitors are designed to provide a quick burst of energy to get the motor running. A run capacitor would not have enough power to start the motor and could potentially overload or damage it.

What is the difference between single-phase and three-phase motors?

Single phase motors use one alternating current and typically require a start capacitor for starting. Three phase motors use three separate alternating currents that create rotating magnetic fields and do not require any type of starting device such as a capacitor. Three phase motors are more efficient than single phase motors but they can also be more expensive.

What are the signs that a start capacitor needs to be replaced?

If your motor is having trouble starting, it could mean that your start capacitor has failed and needs to be replaced. Other signs include buzzing or humming noises coming from the motor or a burning smell. If you notice any of these symptoms, it’s important to have a professional check out your motor as soon as possible.

What are some common capacitance values for run capacitors?

Run capacitors typically range in value from 3-50 microfarads (µF). For single phase motors, they can range in voltage ratings from 125V – 370 VAC. For three-phase motors, they can range in voltage ratings from 250 – 440VAC.

Is it possible to test a start capacitor?

Yes, it is possible to test a start capacitor with a multimeter. You can check for capacitance, resistance and any shorts in the circuit. However, if you are not familiar with reading multimeter readings or do not feel comfortable testing the capacitor yourself, it’s best to have a professional technician inspect your motor. It is important to understand the differences between run capacitors and start capacitors so that you can properly maintain your motor and keep it running smoothly. A professional electrician should be consulted when replacing either type of capacitor.

Can a motor start without a run capacitor?

No, a motor cannot start without a run capacitor. A run capacitor is an electrical device used to continuously maintain the proper phase difference between windings of an electric motor and provide torque for starting. It is designed to keep the system operating at peak efficiency by improving power factor and energy savings. Without a run capacitor, motors will not operate efficiently or effectively due to lack of single-phase power supply. Start capacitors are also required along with run capacitors in most applications as they help the motor get up to speed sooner by providing an additional boost of energy during startup.

What happens to a motor if the run capacitor fails?

If a motor’s run capacitor fails, it can cause the motor to be unable to start and/or operate properly. This is because the missing capacitor prevents the proper phase difference between windings which results in a lack of torque for starting of the motor. In addition, without a run capacitor, motors will not operate efficiently or effectively due to lack of single-phase power supply. If a run capacitor fails then it must be promptly replaced in order to restore proper operation of the electric motor. Overall, both start capacitors and run capacitors are required components for any electric motors and help keep them running smoothly and efficiently. Without their presence, motors may struggle to start up or even fail altogether so it is important to regularly check and replace them if necessary.

Do all AC units have a start capacitor?

No, not all AC units have a start capacitor. Generally, larger AC units will include both a start capacitor and run capacitor in order to ensure that the system is operating at its peak efficiency. Smaller AC units may only require one type of capacitor or none at all. It is important to check the specifications of your particular unit to determine which capacitors are necessary for proper operation.

Useful Video: What’s the Difference Between a Start and Run Capacitor?


In conclusion, the differences between run and start capacitors are numerous. Run capacitors are designed to keep a motor running smoothly while start capacitors provide a boost of power when starting up a motor. Both types of capacitor can be found in AC motors and air conditioners, with run capacitors being used more often than start capacitors. Ultimately, the choice between which type of capacitor to use will depend on the specific motor application. With that said, selecting the correct capacitor for any given job is essential for ensuring efficient operation and long-term reliability. Additionally, it is important to remember that replacing an old or worn out capacitor with one of higher quality may help extend the lifespan of your machine’s components.

Consider all these factors before making a purchasing decision as the wrong choice can lead to costly repairs. Knowing which capacitor is right for your job will help you avoid unnecessary costs and ensure optimal performance of your motor or air conditioner.