How to Size a Capacitor for a Compressor (Tips and Buyer’s Guide)

Capacitors are devices (usually found on the exterior of the motor) that work by storing an electric voltage, which is then discharged to meet the demand of the air compressor system. Depending on the type, they can also run in a continuous duty cycle. Capacitors come in an array of variations, but how should you size one for your compressor?

To size a capacitor for a compressor, you must find the Amps and Voltage requirements of your air compressor. This will give you the required Capacitance in Micro Farad (uF). First, check the manual of your compressor to match the necessary size; otherwise, use the (2650 x Amps) / (Volts) = Capacitor size required (in uF) formula. 

Capacitors do not run as often as compressors, so they typically last longer than the air compressor itself. They can also fail unexpectedly if you try to start the unit with an improper pairing- hence the importance of selecting the proper size.

In this article, we’ll discuss what an air compressor capacitor is, where it’s located, and how to select the appropriate size for your machine!

What Is an Air Compressor Capacitor?

A capacitor consists of two metal conductors, separated by some sort of insulating material (also known as dielectric). It is responsible for storing energy and releasing it into the system when necessary; it accomplishes this by swapping electrons between two conductive plates. Air compressors use capacitors to startup, build pressure, and pump air.

More simply put: They are the magic electrical jump-starters that get your air compressor going and keep it running over time.

The main purposes of a capacitor in an air compressor system is to:

  • Circumvent unwanted frequencies in the system.
  • Inhibit the flow of DC, while allowing the flow of AC.
  • Answer the energy signal from any section. 
  • Shift phases and delay system actions.
  • Start the motor.
  • Filter ripples in a rectifier circuit.

The larger the capacitor is, the more power it is able to store and discharge over time.

Types of Air Compressor Capacitors

Your air compressor system usually contains 2 types of capacitors- a start capacitor and a run capacitor.

Capacitors also range in sizes from as little as 1 uF to as large as 50+ uF. They should never be wired directly across a line and should instead be used for startup/run purposes only.

While the names “start” and “run” capacitor are fairly self-explanatory, the details below will better explain the role each one plays.

Start Capacitor 

A start capacitor is responsible for kickstarting your air compressor.

It works by receiving a signal from the capacitor start relay, which then turns on an electromagnet. When this happens, power flows through this electromagnet and creates a magnetic field, building up until it’s strong enough to pull in the armature.

The armature is what starts the motor, as it removes pressure from the system by pushing out oil or air from between its two split parts and generating a voltage “boost.”

This process continues until the motor reaches a speed that’s high enough to sustain itself. At this point, the capacitor will continue to maintain a steady RPM level with its continuous flow of power.

Run Capacitor 

The run capacitor, which controls the current and voltage to the compressor’s run windings, is a continuous duty component.

It’s more of an energy-saving part, maintaining the motor circuit throughout the entire time your air compressor is in use.

To help you differentiate between the two- a start capacitor builds energy to jolt your machine on, while a run capacitor provides continuous energy spurts to the motor in order to keep it running. Both are crucial for your air compressor to function properly.

Where Is the Capacitor on Your Air Compressor Located?

The capacitor for your air compressor will most commonly be located near the motor- either on the exterior of your motor housing or beneath its surface.

It’s also possible for them to be at the front of an electric air compressor’s base, depending on your model. Capacitors are usually marked by a yellow or red stripe that runs down one side.

They look a little something like this:

In terms of where you can purchase a new capacitor, they can be found at local hardware stores, in various online retailers, and even on Amazon. Capacitors do not come in a one-size-fits-all, so make sure you calculate the correct size before ordering!

How Is a Capacitor’s Capability Measured?

The ability each capacitor has to store energy is known as “Capacitance.” In other words, it is the ratio of the amount of electric charge stored on a conductor to a difference in electric potential.

Capacitance is typically measured in what is called “Farad.” However, it’s easier to consider the measurements of smaller units like Kilo Micro Farad (KMFD), Micro Farad (MFD), Kilo Pico Farad (KPF), or Nano Farad (NF).

For reference purposes, one Micro Farad is equal to 1 millionth of a Farad. Remember these units when we talk about calculating the correct capacitor for your air compressor.

Determining the Correct Capacitor Size for a Compressor

As we discussed ad nauseum, capacitors are responsible for providing energy to power your compressor. So it makes sense that the capacitor size you need will be determined by how much current and voltage your machine requires.

If you run into an issue regarding a failed run or start capacitor within your air compressor system, follow the formula below to help find the best replacement!

How are Capacitor Sizes Expressed?

Capacitors sizes are determined by two factors:

  • Capacitance (measured in Micro Farad, MFD, or uF)
  • Voltage (measured in Volts or VAC)

Keep in mind that the voltage rating isn’t what your capacitor will produce but rather what it can handle. The voltage rating displayed on the capacitor includes the “not to exceed” rating, which means you can replace a 250 voltage with a 340 voltage, but you cannot replace a 340 voltage with a 250 voltage.

Here is what your capacitor size readings will look like, displayed on the outer surface:

Calculating Capacitor Size

The best practice is to first consult your air compressor’s user manual to see what capacitor size is recommended. If you can’t find that, you can also look at the script on the side of both your run and start capacitors and re-order that specific type.

If, for some reason, that’s illegible, don’t panic. You can use simple calculations to determine the size you need.

Most capacitors are sized in terms of Micro Farad (uF). In order to properly size a capacitor for an air compressor, use the following equation:

Note: 2650 = a full load of Amps

(2650 x Amps) / (Volts) = Capacitor size required (in uF)

To get the Amps and Volts:

  1. Measure the total Volts between the HERM and COMMON terminals on the run capacitor.
  2. Measure the Amps on the wire leading from HERM to START on the compressor.

Let’s say, for example, you get 5.0 Amps and 331 Volts. Your calculation would look like this:

(2650 x 5.0 Amps) / (331 Volts) = A 40 uF Capacitor required

The run capacitor should have the exact Micro Farad (uF) that the motor requires, while start capacitors should be +/- 10% of the closest Micro Farad (uF) to the motor requirement.

Air Compressor Capacitor Buyer’s Guide

Based on what we learned above, there are a few recommendations for top-performing capacitors- ready to help your air compressor do its best work.

These are the most highly rated air compressor capacitors on the current market:

The best part is- they’re all under $20 and available on Amazon! The average cost of utilizing a qualified technician to swap out your capacitor is around $150 (including parts and labor).

Genet is among the most popular line of capacitors. This company designs cutting-edge, high-quality comfort and electrical parts; their track record for high-quality, highly-dependable goods makes them the go-to capacitor supplier. 

Genet Motor Run Capacitors are the most durable and dependable electric capacitors available. They replace or supersede obsolete GE Industrial capacitors, which were previously regarded as the most reliable on the market.

Dual capacitors (like the Powerwell branded ones) come in a variety of sizes, depending on their capacitance (uF or MFD), such as 40 plus 5 uF, and voltage.

Round cylinder-shaped dual run capacitors are often used for air conditioning to help start up the compressor and the condenser fan motor. A round capacitor can also be used to substitute oval-shaped ones with the same uF and voltage specifications.

How to Replace Air Compressor Capacitors

When a capacitor blows, it’s usually fairly obvious as the air compressor smokes and stops working. In order to change out either your run or start capacitor, you must remove the old one and insert a replacement.

Warning: Handle with care; most capacitors carry a lethal charge.

Follow these steps to replace your air compressor capacitor:

  1. Put safety glasses and gloves on.
  2. Make sure the machine is unplugged and cooled off.
  3. Remove the pressure relief valve, dial, switch, and wiring of each if they are in the way.
  4. Next, remove the motor housing (if the capacitor you’re changing is beneath its surface).
  5. Unwire the old capacitor.
  6. Dispose of the old capacitor in a safety bin, as it could contain harmful chemicals.
  7. Rewire the new capacitor and push it into position (inside the motor housing).
  8. Turn the metal bolt into place using your hands.

Turn your machine on to make sure the capacitor is installed correctly. Check out this video for a more detailed how-to on replacing a motor start capacitor!

What Happens if You Put a Bigger Capacitor on an Air Compressor?

As a good rule of thumb- the size of your new capacitor (meaning the voltage and capacitance measurement) should be the same as the old one you removed.

If it’s slightly bigger- no harm will come of it, but if it’s MUCH bigger, the entire air compressor system could be thrown off. This can lead to the damage of other electrical components or complete combustion.

An air compressor can not operate properly if the compressor does not provide enough energy or provides too much power for the motor.

Every compressor is made slightly differently, so you should consult the owner’s manual before deciding on a different capacitor than the original model.

Final Thoughts

Capacitors are used in a variety of different types of electrical networks where they store energy temporarily. These little (but mighty) parts are necessary in the event that a high voltage power source is needed for a short period of time.

Air compressor capacitors can also be used to boost a current or as a temporary storage device for alternating currents. They come in many different varieties and sizes, depending on how much capacitance you need for your system. 

We recommend taking a look at Genteq Capacitors- one of the most popular on the market! The best part is- they’re all under $20 and available on Amazon.

If you need to calculate your machine’s capacitor needs ASAP and can’t find the manual or read the sizing on the old capacitor, use this formula:

(2650 x Amps) / (Volts) = Capacitor size required (in uF)

For any further questions, reach out to a qualified air compressor technician in your area!

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