Air Compressor Capacitors – What They Do? Read The Detailed Guide

Capacitors are responsible for the smooth operation of your air compressor. If your capacitor is not working properly, your air compressor will not function. When a capacitor loses its charge, it can impact several compressor functions. If you don’t replace it, you will face many problems, including run time, increase in electrical consumption, and its ability to properly self-diagnose.

But the question is, what causes a capacitor to fail? The most common cause of an air compressor capacitor failing is age. Each capacitor has a lifespan; they are not manufactured to last forever, and eventually, they will need to be replaced. A capacitor can fail after anywhere between 3 months to 3 years.

So, in this article, we will cover the functioning, testing, and replacement of an air compressor capacitor with the help of diagrams. Furthermore, you will also be able to know the symptoms to identify capacitor fails and how to troubleshoot them.

What is the function of a capacitor in an air compressor?

A capacitor is an electrical component that can store energy in an electric field. This forms a reservoir of electric energy which can be released and used whenever there is a demand. The main purpose of the capacitor is that it takes the current from a power source and stores the energy until it is ready to use. It boosts the motor and provides the necessary push so the air compressor can run with full power.

There are two types of capacitors used in the motors of air compressors.

  • Start Capacitor
  • Run Capacitor

What is Start Capacitor?

The capacitor that boosts the motor torque while starting is known as a start capacitor. Its work finishes after starting the motor, and you can find it on the start windings of the compressor motor that helps to turn on and off without any delay. It provides strong thrust to the motor and maximizes the running efficiency. If the start capacitor goes bad, the motor fails to start or makes a humming-type noise. So, we need to test the capacitor once and then replace it in this situation.

As capacitance is measured in farads, so the average value of the start capacitor is 70 µF or above. It comes in different voltages that are given below.

  • 125V
  • 165V
  • 250V
  • 330V

What is Run Capacitor?

Run capacitor works on the running winding of the motor. Its main function is to provide continuous current to the motor. If the running capacitor is not installed properly or goes bad, the required torque will not produce, the motor gets warm, and its chances of burning increase. So, always go for an expert while installing or replacing the capacitor.

The capacitance of run capacitor has a rating of 1.5 µF – 100 µF with the following voltages:

  • 250V
  • 370V
  • 440V

Finally, you’ve learned all about the difference between start & run capacitors. Let us dig more and know about the symptoms of a bad capacitor & why it fails.

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If you notice your air compressors causing problems while running and creating some humming type noise, there would be a problem. A bad capacitor can damage the motor and we need to fix it soon, but first we have to know how to check whether a capacitor is bad or not. So, here are a few symptoms below that help us recognize that a capacitor is going bad in an air compressor.

  • Air compressor is not turning on.
  • Increase in electricity bills
  • Slow while starting
  • The air compressor motor starts producing a humming noise
  • Air compressor motor keeps shutting down
  • There will be a burning smell coming out from the motor

After knowing all the possible symptoms of a bad capacitor, our next step is to perform a test to check if the capacitor is bad or good.


There are many reasons why most capacitors fail or keep blowing, including overheating, damage, fluctuation in voltage, & age. Generally, if the capacitor shows signs of swelling or leakage, you can easily identify that it is bad and needs to be replaced. But, if everything is OK, we need to check it through a Multimeter to get the final results. So, here we will conduct a test that helps to conclude whether a capacitor is working or not.

Equipment Required:

  • Digital Multimeter
  • Safety Gloves
  • Screwdriver
  • Capacitor (Start & Run)

Testing Start Capacitor

Before initiating testing, we need to follow some safety precautions to avoid getting any severe shock. For that, wear insulated gloves and use a screwdriver with the proper insulation. Also, use goggles that help save you from spark if it occurs. Here testing starts:


Discharging is the most important job while checking a capacitor, and it is compulsory. Because a high amount of current is stored in the capacitor, and if we don’t release the charge, it can damage the multimeter. So, to discharge the capacitor follow the steps below:

  • Take a properly insulated screwdriver
  • Make sure your skin is not in contact with the metal anyway
  • Put the screwdriver between the two terminals of the capacitor slowly, and try to make contact screwdriver with both terminals at once. You will see a spark coming out from the terminals.
  • Again, repeat the same procedure 2-3 times more to release the complete charge from the capacitor.

This is the easiest and safest way to discharge the capacitor. Now, our final testing begins here. It will also help to know your capacitor is in good condition.

Testing Using Digital Multimeter

We will use a digital multimeter to test whether a start capacitor is working or dead. Follow the steps:

  • Wear safety gloves.
  • Keep in mind to discharge the capacitor first.
  • Now, attach the multimeter leads with capacitor terminals. If the meter shows some reading, it means your capacitor is good.
  • Repeat the procedure 2 to 3 times by changing the direction of test probes to get the final results.
  • If the multimeter shows no numbers, your capacitor is dead and should be replaced.

Testing Using Analog Multimeter

Here we are going to use an Analog multimeter to test the capacitor. Follow the quick guide below:

  • It is a must to discharge the capacitor every time before testing.
  • Change the multimeter settings and set it to a high ohm reading.
  • Touch the testing probes with capacitor terminals; if the meter swings, then your capacitor is good. If you don’t see any swinging, the capacitor is faulty or bad.
  • Check again 2 to 3 times by changing the direction of testing leads; you will come to know whether the capacitor is dead or good.

Testing Run Capacitor

Testing a run capacitor is the same as we tested start capacitor above. First, you need to discharge the capacitor and test it using a digital or analog multimeter. You can follow the steps given above to check the capacitor on both devices.

If your capacitor passes the test and working fine, it is good. But, if the capacitor is faulty we need to replace it. Before replacing, let’s know what size of capacitor you should need for your air compressor.


First of all, you have to see the manual provided by the device manufacturer, and instructions about what size capacitor is compatible with your machine will be given there. In the second method, you can manually calculate what size of the capacitor is needed; follow this simple calculation.

For calculation, you will need two values, FLA (Full Load Amps) and supply voltage, that are available in the owner’s manual. Multiply the FLA with 2,650 and then divide it with supply voltage. The final number you will get in micro farads is the size of capacitor that your compressor motor requires.

Let’s suppose your compressor motor supply voltage is 220 V, FLA is 5 amps, 5 x 2,650 and divide the answer by 220 V; the answer will be 60 μF.

When you finish sizing the capacitor, the next step is to find the average cost. You can buy a quality start capacitor from any online store for $9-$20. Similarly, the run capacitor is also available from $8 to $30. Purchase the required capacitor, so we will move to the next step, i.e. replacing.

What happens if you use a different size capacitor?

If you install a different size capacitor in an air compressor, it might disturb the motor’s working. Installing a low microfarad capacitor wouldn’t help start the motor quickly, and if it is of high microfarad, it may overheat and blow up the motor. So, always use the capacitor that matches the rating recommended by the manufacturer.


Replacing a capacitor is an easy process; when you properly size the capacitor and match voltage & microfarad (μF) numbers, no complexity remains. The quick guide to changing the capacitor is given below:

  • Most of the time, the capacitor is located on the side or back of the motor.
  • Open the screw of housing to put the capacitor out.
  • When you see the capacitor, discharge it with a screwdriver before touching it.
  • Unplug both positive and negative terminals carefully.
  • Also, check whether the new capacitor is carrying a charge or not. If not, plug both terminals. Positive wire with the positive terminal and negative with negative.
  • Avoid changing the direction of wires; it may blow your freshly purchased capacitor.
  • Now, put the cover back and tighten the screws. That’s it!

Note: You can use this method to replace or install both start and run capacitors.


Why does my air compressor have two capacitors?

Every air compressor has two capacitors that help increase the motor’s running efficiency. One is start, and the other is the run capacitor, both of them function for starting and running the motor. Suppose you have only one run capacitor in the air compressor, enough torque will not produce, and the motor will not start easily. So, to give the compressor motor, a smooth functioning two capacitors are installed.

How do I know if the capacitor on my AC compressor is run or start?

To know the capacitor type of your air conditioner compressor, you have to see the ratings available on the capacitor in microfarads (µF). Mostly start capacitors have ratings from 70 to 120 µF or higher with different voltages of 125 Volt, 250 V, & 330 V; see the rating if the microfarad numbers are greater than 70 µF, your capacitor is start. Similarly, if the rating of the capacitor is in the range from 3 to 70 µF with voltage 370 V & 440 V, then it is a run capacitor.

Can compressor run without a capacitor?

Yes, if your air compressor’s motor is split phased or shaded pole, it doesn’t require a capacitor to run. But for smooth functioning of air compressor, a capacitor is required. It provides high torque to the motor while turning on and boost the motor speed.

Why does air compressor keep blowing start capacitor?

It may be due to overheating because start capacitors mainly function for starting the air compressor motor. If they continue their work for a long time, the start capacitor overheats and blows out. The 2nd reason is frequent fluctuation in compressor motor voltage; when the voltage spikes, it automatically increases the voltage load on the start capacitor, resulting in overheating and the capacitor blow up.

Single Capacitor vs Dual Capacitor Air Compressor

In simple words single capacitor is used to run single motor, and if a device has two motors, then a dual capacitor will be used. A dual capacitor enhances the running efficiency of the motor; on the other hand single capacitor also increases the working power of one motor.

Which terminal of the capacitor in an air compressor is positive?

Generally, positive and negative signs are available on both terminals of every capacitor. If they are missing or you’re unable to find them, use our DIY method to know which terminal is positive and which one is negative.

Checking Terminals

It’s a much easy method; take the capacitor in your hands carefully. It may be fully charged, which will shock you. So, be careful while holding a capacitor or discharge it first. A capacitor has two terminals, check if one terminal is big in length, it will be positive, and if the other terminal is small, it will be negative.

What happens if a too big capacitor is used on an air compressor?

Using a big sized capacitor instead of recommended rating is not a good practice. If you have used an oversized capacitor in your air compressor motor, it will soon burn out; the windings may damage and cause overheating. It would be best if you used the fixed capacitor ratings suggested by the manufacturers; it is also best for the health of your device.

Can I use a higher voltage rated capacitor for the air compressor?

Yes, using a higher voltage capacitor in your air compressor is OK. But the voltage should not be low; it may lead to capacitor failure. However, the capacitance (µF) should be the same, a capacitor with high or low capacitance may also blow up. So always use the capacitance rating recommended by the makers, and its high voltage will not cause any issue.

Why do capacitors in air compressors always get broken?

Mostly, the compressors which have capacitors housing outside face this issue. It may get hit by a heavy thing and damage the capacitor. To avoid it, keep the air compressor in a safe place where there are no chances of hitting anything to the machine.

Do you need to charge the capacitor before using it in an air compressor motor?

There is no need to charge the capacitor separately; when you turn on the power, the capacitor automatically gets the charge and starts your compressor motor. If you have replaced the capacitor, then first check whether the new capacitor is charged or not. If it is not charged, switch on the power button it will store the charge within 5 to 15 seconds and start running the motor.

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