Compressor capacitor guide: How to size a capacitor for a compressor

Capacitors are devices that work by storing electrical voltage, which is then discharged to meet the demands of the air compressor system. They are typically found on the exterior of the engine. There are many different types of capacitors; which one should you pick for your air compressor?

You must find the amplifier and voltage requirements of your air compressor to determine the capacitor size. You will receive the necessary capacitance in microfarads (µF) from this.

Use the formula (2650 x Amps) / (Volts) = Required Capacitor Size (in F) if your compressor’s manual does not specify its size.

Since capacitors aren’t used as frequently as air compressors, they typically last longer. Choosing the correct size is crucial because they can unexpectedly fail if you try to start a device with the incorrect pairing.

In this article, we’ll talk about what is an air compressor condenser, where it’s found, and how to size a capacitor for a compressor for your unit.

how to size a capacitor for a compressor

What is an air compressor capacitor?

A capacitor is an element that stores an electrical charge and then releases it. It is used to protect household electrical equipment from power surges in the network. Acting as a kind of filter, the air compressor capacitor extends the life of the engine, smoothing the load and protecting it from strong voltage drops.

Currently, they are used in almost all radio engineering and various electronic circuits.

In an alternating current circuit, they can act as a capacitance. For example, when connecting a capacitor and a light bulb to a battery (direct current), the light bulb will not glow.

Types of capacitors

Air compressor capacitors are divided into 2 types: run and start capacitors. From the name, it is clear that the air compressor start capacitor only works at the time of start-up, and the run capacitor during operation.

The run capacitor and start capacitor differ both in capacity and the conditions of use, the method of installation, and fixing. And besides – the very purpose.

Run capacitor

A run capacitor is necessary to shift the phase in the circuit qualitatively. Thus, it contributes to the fact that a magnetic field is generated between the motor windings, which sets the motor in motion. You don’t have to use mechanics to do this. An example of this would be any electric motor in tools or devices.

Start capacitor

A start capacitor’s primary function is to store energy for use by the engine both during startup and operation. The start capacitor speeds the compressor motor so that it starts spinning at the right speed with the right mode. Such capacitors are actively used in circuits of heavy hoisting mechanisms.

By capacitance, you can also easily distinguish a run capacitor from a start capacitor because this value is usually at least two times larger than the second one. This is because the capacitance directly depends on the power of the electric compressor motor and is inversely proportional to the voltage in the mains.

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How to distinguish run and start capacitors?

Differences in the connection method

Run capacitors are usually connected to the auxiliary winding of the compressor motor, namely to its break. In this case, the second winding is directly connected to the network, and the third remains free. So, it turns out, a scheme called a star or a triangle.

And start capacitor is connected after the run capacitor is in parallel with it.

To connect, you need a button (if the control is manual) or a switch (if the drive will control).

According to the operating conditions

It is not in vain that the run capacitor got its name – it has to be constantly involved in the circuit and keep high voltage loads because the run capacitor works in the compressor motor winding itself.

Because of this, a voltage of 500 or even 600 volts can form at the ends of the run capacitor at certain moments, and this is two to three times higher than the input value. In a word, run capacitors are more enduring than launchers.

The start capacitor, on the other hand, does not take on a load exceeding the incoming 220 volts, the start capacitor is activated only from time to time and for a short time. Therefore, the maximum allowable voltage does not exceed 1.15 volts. Also, a start capacitor can usually stay operational much longer than a run capacitor.

The run capacitor is a real workhorse, thanks to which the phase shift occurs, and the three-phase motors themselves can operate from a single-phase power supply. And the start capacitor is more of an auxiliary character and has a short period of use. Also, the start capacitor can store more charge than the run capacitor.

How to size a new capacitor for an air compressor

Air compressor owners struggle with the idea of identifying the appropriate capacitors required to replace the failed one when a capacitor fails.

The replacement of a capacitor involves the following three steps:

  1. Find information about the capacitor, specifically its size, in your compressor’s manual, if you still have one. If there is no manual available, get in touch with the device’s manufacturer to find out what size capacitor is needed.
  2. Making some calculations in the second step could help you determine the type of capacitor you’re looking for.The full load current is therefore multiplied by 2650, and the result is then divided by the supply voltage. You will know the microfarads of the capacitor your air compressor requires after the calculation.Electrical capacitance is measured in MFD (MicroFarad), which is typically listed on a capacitor or packaging. Although this is a general estimation and not an exact number, it can occasionally be rounded up or down to 10% to get the required number. (This is a general estimate, not an exact figure.)
  3. Third step: As an alternative, you can calculate the appropriate capacitor size for your air compressor by multiplying the motor’s operating power by the power factor specified in the manual (in kilowatts).

The fact that capacitors have terminals through which you can connect the wires to the compressor motor is another crucial point to be aware of. Therefore, when looking for a replacement compressor capacitor, don’t worry too much about whether it will fit the case.

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Which capacitor is required for an air compressor?

Be aware that there are two ratings for air compressor motor capacitors when looking to replace an air compressor capacitor:

  • Voltage Range
  • Capacity

The capacity and the supply voltage can be found in the owner’s manual.


On the side of the condenser, there ought to be a label. Voltage is one of the capacitor ratings listed on this label. The usual voltage range is 120 to 240 volts.

The voltage will be adequate for your air compressor motor as long as you purchase a capacitor with the same voltage range as you currently have.


The capacitance in microfarads will be another indicator listed on the label. An MFD rating, where MFD stands for microfarads, can be used to show this.

These are the various methods for calculating a capacitor’s microfarad value.

What kind of start capacitor is required for your air compressor motor? The capacitor you choose must have the same microfarad rating as the existing capacitor that is currently installed, or it must display the microfarad range that the current capacitor falls within. Also, some capacitors have more than one voltage ratings.

Note: If the replacement capacitor is too high, electrical problems could result; if it is too low, the air compressor won’t start.

How to identify faulty air compressor capacitors?

Loss of efficiency of air compressor capacitors may occur due to:

  1. faulty starting circuit;
  2. chain break inside it;
  3. increase in leakage current;
  4. capacitance reduction.

A failed capacitor can be detected with an ohmmeter, a special capacitance meter, or a test circuit.


Replacement of the air compressor for testing capacitors

If you are one of those people who manually troubleshoot and examine the capacitor’s health, never do it unless you are familiar with how to do it. A professional must participate in the manual process because careless handling can result in significant harm.

Analog multimeter measurement

  • Verify that the capacitor, you think, is the issue, is dead.
  • Establish your AVO meter
  • Make sure to choose the higher range when choosing an analog ohm meter.
  • Connect the ohmmeter to the existing capacitor terminals.
  • Low resistance will indicate if the capacitor has been shorted.
  • If the capacitor is open, there won’t be any movement on the ohmmeter screen.
  • Therefore, a new capacitor will initially show low resistance before gradually increasing, which is a sign of a good capacitor.

Using a digital multimeter to measure

The use of a digital multimeter is advised because it is more sophisticated, user-friendly, and accurate than any alternative method.

Make sure the air compressor capacitor discharges before taking any action.

Set the ohm range on the digital meter.

Connect the meter wires to the capacitor terminals.

Watch what is being displayed carefully and take note of the fact that the counter is displaying (some) numbers per second.

The numbers are displayed, and then everything immediately changes to OL (Open Line). You will get the same outcomes if you repeat the procedure, which shows that the capacitor is in good shape.

If the counter remains unchanged, your capacity is dead, and you must get a new capacitor.

You can test a capacitor using a voltmeter and in capacitance mode, among other approaches.

Capacitor replacement tips

Find a replacement air compressor capacitor if the original is dead. As we mentioned earlier, you should pay special attention to the voltage rating and MFD ratings. Therefore, it would be wise to check if the air compressor motor will start normally even after you have found the correct capacitor you were looking for.

Use mounting tape to hold the capacitor in place when installing a new capacitor or the same capacitor. To prevent errors, be careful to correctly connect the new replacement wires.

After a successful replacement, turn on the circuit breaker to turn on the air conditioner. The air conditioner can then be turned on by turning on the thermostat and lowering the temperature.

Check the compressor capacitor wires to see if they are connected properly, if it still doesn’t work.

To stop a further error, remove the capacitor, go back and remount it, and reverse any connections. If you are sure that you have the right location, secure the condenser’s wire connections and test the conditioner before turning it on to enjoy the cooling.

Warning: Avoid measuring or touching an electric motor capacitor with power connected and wires plugged in. You run the risk of getting hurt, dying, or starting a fire or explosion. Be aware that even after the power has been turned off, electric motors can still store energy and shock victims.

The same µF and voltage specifications of an oval capacitor can also be settled by a round capacitor.

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How do I know what size capacitor I need?

Multiply the full load current by 2650. Divide this number by the supply voltage. The full load current and the supply voltage can be found in the user manual. The resulting number is the microfarad of the capacitor you need.

What happens if you put a bigger capacitor on a compressor?

Too large a condenser can quickly damage the air compressor. By increasing the current in the starting winding, the compressor starting winding will be much more susceptible to premature failure.

What size capacitor do I need for my air conditioner?

Review the user guide. For your particular air conditioning model, the manufacturer specifies the precise size and type of condenser fan motor capacitor that should be used.

You can also find information online or by calling your nearby HVAC company if you don’t have a manual on hand. Based on the brand and model of your AC unit, they ought to be able to advise you on the correct capacitor size of capacitor you require.

What does a motor capacitor do in a circuit?

An electrical battery known as a motor capacitor is used to increase current in one or more copper windings of a single-phase motor to add additional momentum and boost motor torque.

What is a rectifier circuit?

An electronic device called a rectifier uses one or more P-N junction diodes to change alternating current into direct current. Diodes function as one-way valves that only let current flow in one direction. This action is referred to as rectification.

How do you determine a compressor’s full load amps?

According to Underwriters Laboratories, to calculate the Rated Load Amperage (RLA), divide the MCC by 1.56. To meet certain other protection requirements, compressor manufacturers frequently divide the MCC by 1.44 to determine the RLA of their compressors.

Final thoughts

Different kinds of electrical networks employ capacitors to temporarily store energy. When a high-voltage power supply is required for a brief period, these tiny (yet effective) electrical components are required.

A new capacitor for air compressors can also be used to temporarily store AC power or to amplify current. Depending on the amount of capacity your system requires, they are available in a variety of shapes and sizes.

If you need to figure out how many capacitors your device needs right away and are unable to locate a manual or read the dimensions of an old capacitor, use this formula:

Required Capacitor Size (in µF) = (2650 x Amps) / (Volts)

Please get in touch with a certified air compressor expert in your area if you have any more questions!

Also Read About Electric Motor Starting Capacitor Selection

James Cole

Considering that most people continue to pay the masters in the service for a consultation, I decided to create my blog.

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