Air Compressor Just Hums

Why is it that an air compressor just hums?

We are not talking about the noises the air compressor is making when it is running, gosh knows air compressors are loud for sure.

What we are talking about is the scenario  that you have just switched your air compressor on, or the air compressor tank pressure has dropped to the normal cut in pressure setting, and your air compressor just hums! Doesn’t start, just sits there humming.

And it’s not even a nice melody, at that!

To understand why an air compressor just hums, and to try and fix yours if that is what is happening, it is helpful (I hope) to understand a bit about the air compressor motor.

Dayton electric motor

Inrush current needed

Air compressor motors needs to have more current available to them when they start than is necessary when they are running.

To help provide the added current on startup, electric motors above a certain size normally have a start capacitor. Think of the start capacitor as a quick-discharge battery. When it sends power into the circuit, it does it all at once and very quickly.

The electric motor uses the current available from its start capacitor to add the additional start up current that is necessary to kick the motor into action. Once going, the motor uses less current to stay running.

120 AC motors run close to the edge

Some electric motors, particularly those that run on 120 AC current, run very close to the edge in terms of power available from the 120 AC supply versus the minimums needed to keep the motor running, let alone, get it going.

Even with the boost from the start capacitor, if there is additional load on the motor at compressor start up, it cannot get going, and it sits their humming, as it is getting power, just not enough.

A humming motor is an overheating motor

You do not want to let your air compressor motor sit there humming as it will quickly overheat and cause damage to the motor.

It is necessary to address the “why” as quickly as possible when your air compressor just hums.

How to fix when air compressor just hums

Power Supply

The power supply to the air compressor must be clean and through the shortest line possible. If you are using an extension cord or a power bar, don’t. Plug the compressor cord directly into a wall socket that has few (or preferably no) other electrical devices that could pull power from that same circuit when your air compressor is starting or running.

Plug the air compressor directly into a socket, preferably one with a 20 amp circuit breaker rather than a 15 amp circuit breaker. Try to eliminate any other electrical draws on the same circuit when you are attempting to start the compressor. If the compressor starts now, an insufficient current could have been the problem.

Intake Filter

Over time the filtering media inside the intake filter on an air compressor can become plugged with dust and dirt. If there is enough crud in the filter to slow or prevent air from passing easily through the filter into the compression chamber in the pump, then that could add load and stall the motor.

A quick check is to simply unscrew the intake filter and try to start the air compressor with the intake filter removed. If the compressor starts, that could be the cause. Test again by putting the filter back in. If the compressor will not start when the intake filter is in, and it starts with the filter out, replace the filter media.

Unloader Valve

The next thing to eliminate as a cause of the air compressor humming instead of starting is a failed unloader valve.

If the compressor unloader valve did not work the last time the air compressor reached cut out pressure and stopped, then it is possible that air is now trapped over the piston(s) and that additional load may be enough to stall the motor and make it hum instead of run.

Empty all of the air from the compressor tank. This will also allow any air that is trapped over the piston(s) to exhaust to atmosphere too. Close the tank drain and try to start the compressor. If it does start, monitor the unloader valve to be sure that it works.

Start Capacitor

The electric motor usually has a quick-discharge battery on it called a start capacitor to help in starting. If the start capacitor has failed, it is not giving the electric compressor motor the added boost necessary to get the motor going.

If the other remedies have failed, then the odds are improving that the start capacitor is at fault. The start capacitor cannot be repaired, and must be replaced.

Other issues

To try to identify a possible mechanical failure in the pump as the cause of the motor overloading, decouple the motor from the pump and see if the motor will start. This is fairly easy to do if the pump is belt driven, more difficult if it is a direct drive, of course.

If belt driven, once the belt is removed, try rotating the pump sheave by hand to determine if there is binding.

Try to start the motor when it has been freed from the pump load. If the motor starts, that reinforces that it is a load issue causing the stalling and humming.

However, since the motor still needs the start capacitor boost to normally get going with any load at all, a failed capacitor could still be the problem, since now the uncoupled motor has virtually no load it may still start with a failed start cap.



  1. Chille Franklouis says:

    Hello Willyr, am so grateful for this forum it has helped me a lot, bravo. However, I have a problem with my air compressor now.

    It is a Clarke Raider air 120. It has worked normally for quit some time now about 2yrs with normal servicing.

    Suddenly one day as I plugged it to power it started to run normally with out humming for about 2 minutes and then started to smoke with a terrible smell while still running like 5 seconds and quickly I turned power off first from the wall switch then the machine.

    However smoke still still continued to flow from the motor section and the black plastic box which holds the thermo cutoff switch. Certainly somewhere it was burning. when I checked the thermo cutoff smitch it had melted terribly. I tried to reconnect it’s terminals with a thick copper wire to see if the motor still runs, yes it did run for 5 seconds but I turned it off. I tried again to reconnect the thermo cutoff wires directly with a much much thinner copper wire like that of a fuse, when I switched power the thin wire blew instantly disconnecting power to the motor. To date I don’t use the compressor waiting for a solution.

    What caused the smoking in the motor and why didn’t the the thermon cutoff switch trip off power but instead was burning & allowing current to the motor? Why is it possible that the motor still runs when current is passed with a thick copper wire but blows a thinner fuse wire, is the motor still safe?

    • Chillie, I moved your question to a different page, as I suspect this one is more appropriate for your question. The compressor symptoms you describe are what typically happens when a start capacitor fails badly. See the page on this site about how to test them.

  2. Paul Krivanec says:

    Thanks for your checklist on 2hp pancake compressor with hum only. FYI my crimp on elect leads to cap were broke by vibration, put new crimpon connectors Walla!

  3. Bill homuth says:

    I have an 8 gallon 2hp central pneumatic oil lube compressor. (Harbor freight tools) Currently, it will run insanely slow for about 3 seconds, and then stop running and just hum. Any ideas?

    • Please let me know what things you have checked, per the list on this page?

    • No cheap extension cords, those need to be plugged right into 15 or 20 amp electrical outlet.

      (By no cheap extension cords I believe Beano means use shorter cords better than longer, and and extension cord that has heavy enough gauge wire to feed the compressor motor over the distance needed. It is almost always better to move the compressor closer to the power source – eliminating the extension cord entirely, and use a longer air line.)

  4. Hi willy,please advise on a direct drive ryobi 50ltr,it started all normal ran tank full, then all of a sudden ive heard a noise then i switched it off,now its just humm.pls help.

    • Since we did not hear the noise we can’t be sure what the cause was, but all of the things to check are noted here: // . Please do the checks and add a comment here with your results.

      • Hey Willy I have a husky 5hp 120/250volt 26 gallon belt driven compressor just looking for some advice it has been sitting for every bit of 8 years it just hums when switched on and it appears I have a smaller battery looking copacitar and a bigger one sitting on top of my pump I believe both with wires going down into is there a way to test them why is there two and different sizes just kind of wondering where I go from here the belt wheel moves just fine with no restriction

        • Well, the reason it’s humming is, I expect, because the compressor motor power is compromised. If the pump sheave can be turned by hand, that typically lets out a mechanical issue back loading the motor and preventing it from starting. So, the next check would be the caps. You’ve got two, so one’s a start capacitor and the other is likely the run capacitor. Which is which I don’t know. Check ’em both. See the page linked under compressor info about is your capacitor working to find out how to do that.

    • Check the oil, the oil most most likely low or no oil left. So the motor seized. That’s causing the humming sound.

  5. I have a compressor that says on the plaque that it is rated for a 13 amp fuse. Having not used it for a couple of months I went to use it and it starts and then after 3-5 seconds the fuse blows and that’s it.

    I have had an electrician look and they have suggested that the motor need to be serviced ??
    Is this right ?

    many thanks

    • Sure, it sounds right. Of course, it depends on what is meant by servicing. As it alludes to on this page, a humming air compressor indicates the motor needs servicing, and that servicing starts with inspecting the compressor motor capacitors, which is what we suggest is done first.

  6. Captain David Bowley says:

    I have an ANTLIA AN-2021 direct drive compressor 21 Litre.
    Was working fine until yesterday when I pulled start switch, nothing happened.
    I located the reset buttun, pushed it back in and pulled start switch again and the motor just hums. It is empty of air and moisture drained and oil OK.
    Could it be poor initial power supply or dud capacitor.
    It is not that old and not used much,
    Any suggestions anyone.
    many thanks,
    Captain Dave Bowley ( Lybster, Scotland UK)

    • Yup, the logical thing at this point is to test your capacitors. How to do that is covered on a page on this site.

  7. I have a six gallon compressor that has been giving me issues. When I try to start it, all it does is hums. Switched to multiple outlets and still wont start. Emptied the tank and still won’t start. Took the guard off the top and it appears that the motor shaft is bound up. If i get the motor shaft to turn over, the compressor will then start (once plugged in). Am i looking at a mechanical issue here ?

    • Likely. The question is why is the motor shaft bound up? Is it because the pump cannot reciprocate, or is it that something in the motor has failed? Decouple the motor from the pump, and see if the motor starts. Try to operate the pump manually. That should point out where the problem lies.

  8. Erik Hansen says:

    Hi, first time caller long time reader. My 2HP 30gal Husky has been slowly laboring more and more. It’s gotten a lot of use but I’ve changed the oil and drained the tank so it seems like it’s still in pretty good shape. With an empty tank it hums a bit but then starts like a champ then starts to build pressure. Then it starts to bog down eventually overloading the motor. The pulley on the end of the motor gets extremely hot from friction so I thought it was a loose belt but I adjusted the slider on the motor and still no difference. It’s plugged directly to a 20Amp breaker on a 120V circuit all by itself and the length to the breaker is about a foot. I’ve replaced both capacitors and the blow off valve is working fine. Is it possible that the clutch inside the motor is bad? I’m trying to get the belt off but both the motor and the air pump seem to rotate without too much trouble. Thanks.

    • Might be friction heating the pulley Erik, but then, it may be heat transfer from the motor, too. The symptoms point clearly to the capacitor as being a problem, but then, you’ve changed them, and as long as they are the same voltage and MFD rating as the original, that’s no longer the issue. Clutch? Maybe. If there is less lugging on startup with no air in the tank, make sure you check the tank check valve. Else, it could be the motor wearing out yes, but it also could be a valve in the pump, or the pump itself is adding additional motor load through piston binding in the cylinder. We suspect that you’ll want a motor load test first, and if the motor comes through fine, it’s time to tear down the pump.

      • Erik Hansen says:

        How can I do a motor load test? You know the crazy thing about this is that motors are $250 while the new compessors are like $200. Maybe not the best but still good. Not sure how far I want to bother with this but I’ve already spend $50 on the capacitors. Ugh. Thanks for your help though!

        • You would need to take the motor (or motor and compressor perhaps) to an electrical motor repair shop. They have the setup to load test a motor to see what’s what. Your point is valid. Consider instead buying another of the same make, and then using the old one for spare parts which the new compressor, inevitably, will one day need.

  9. My compresser has always worked fine, use it rite regular, now after a cold nie around 30 degrees it would only hum, but that afternoon when it warmed up in the 70’s it started up fine, but cold mornings just hums? what you think? motor r pump! it’s not outside but and open shed.

  10. I have a Puma 60 Gal w/3hp single phase. It kicks on fine, runs to about 50psi and trips the reset on the motor. the motor is very hot and takes a good 5 min to cool down before I can reset the motor. thoughts?

    • this just recently started after about 2 years of sitting is storage. Worked awesome before going into storage.

      • This compressor is specified as having a 208-230V motor and it requires at least a 16.6 amp circuit. Assuming that you have supplied adequate power Homer, that the motor is very hot by the time the tank pressure reaches 50 PSI could be a load problem. In other words it may be mechanical. But, we actually think it’s a power supply problem, and our best guess is you have a bad run capacitor. Please have a look at the page on this site about how to test them, do the test, and please let us know if that was the source of the compressor problem. We’re always learning too. Thanks.

  11. I have a Compressed Air Systems 80 gal 5 HP vertical tank compressor on my home shop. I bought it new and have been using it for around 4 years with intermittent starter problems. I had a professional electrician wire it according to the Manuf. specs., but it will burn the starter contacts to the point that they will no longer touch or in a few instances, they will fuse together and the compressor will continue running past the shut off pressure until the Tank relief valve pops off. The first time it happened the compressor was less than a year old and I contacted the manufacturer and they sent me a complete new starter, which was different from the first one that came with the unit, claiming that they had a bad run of those starters. Then about a year later the new one started acting up. I had the electrician come out and look at everything again and and he found the burnt contacts and said it appeared like it had taken a lightning strike through the line> He cleaned up the contacts and it worked for awhile longer, then the same problem arose, and he just suggested I get a new starter, since just the contacts weren’t available. I did this a few months back and have been running fine until today. You guessed it, same thing. Ideas??? I am at a loss since the electrician says it is wired correctly. Thanks,

    • We can’t give you electrical advice as we are not licensed electricians and we are certainly not going to second guess your electrician. We can tell you that often points burn because the motor is pulling too much current on start up. That would suggest that your start capacitor needs checking. Another thing we would have checked is the condition of the wires and the wire terminals in the motor circuit. A bad connection there could be an issue for burnt points. Another issue that you don’t touch on is cycle. If using the compressor all day every day is the norm, particularly if the air demand is greater than the compressor can deliver, then the pump will have to cycle frequently, a source for point wear. Is the cut in and cut out pressure setting of the pressure switch at factory specs? If the gap between cut in and cut out has reduced, the compressor will cycle more often, leading to increased point wear and burning. These are a few things to consider. Other suggestions are welcome.

  12. I have a mastercraft 8gal 2hp that hums, has been emptied, brought inside, but doesn’t want to run.

    • Jerrold, you need to make sure that the compressor is plugged into a 15 amp breaker circuit and that that nothing else is pulling power when the compressor is trying to start. If the compressor hums when it is trying to start, and the tank is empty, and the power supply is good, and the pump has not seized up for some reason, then a good spot to start is to look to the start capacitor as being weak or failing.

  13. Dale Collvins says:

    I have speedaire compressor. After sitting unused for a couple of years I pluggedit in and turned it on. It pumped up the first time, but when the pressure dropped and it needed to kick on the motor couldn’t make the compressor run and it would throw the 20 amp breaker.

    It is on a circuit by itself so I replaced the wire with a heavier gage and put a 30 amp breaker. Also, bought a new motor, pressure switch and cleaned the check valve.

    Turned it on, it ran and cut off as it should.

    Worked great for a few days and then went back to the way it was before I did anything to it.

    I then put a new check valve in the tank. It came on, ran, cut off, unload valve worked.

    Next time it was struggling to come on like it the unloader valve isn’t working. I’m at a loss as to what to do now since I have replaced everything except the pump and tank.

    • Yikes, that’s an odd one. Had you not said that you checked the unloader valve and it was working, we would think that a crudded up or stuck unloader valve might be the cause. If you empty the tank of air entirely, and then try to start the compressor, does it? if so, then check the unloader again and, sorry about that, pull the pump head and check the intake or pressure valves. If one of them is broken, the back pressure may cause the problem.

  14. I have a Sears 30 gallon compressor and I plugged an extension cord into it. It started and was running perfectly until I did this. I could hear the motor was not running as loud as before when I had it plugged directly into my outlet. After I used some air and needed more, it will not start now! Any ideas

    • Off hand, Royce, we would say that you’ve used a too-small gauge wire extension cord, and you are underpowering the motor. Hence the motor sound change, and, if you continue to do this, you may damage the motor. Try plugging the compressor cord directly into a wall socket fed by a 20 amp circuit breaker, and see if that makes a difference.

  15. I have a campbell hausfeld twin tank 4 gallon 3 horsepower compressor. Model wl506206aj I have been having trouble with it lately chugging for maybe a second then just humming when plugged into any 15 amp circuit. It always worked fine on 15 amp circuits previously. If I plug into a 20 amp circuit, it’s fine. I tried replacing the start capacitor but that didn’t help. I had the motor inspected today by a motor service company and they checked everything and said the motor is fine. Once the tanks reach the full load, you can hear the unloader switch vent off air for a second or two then stops. I also changed the air filter and that didn’t help. There is no air leaking sounds so I don’t think it’s the check valve, but the only things I can think of now is either the check valve or unloader switch. Any ideas? Don’t want to keep wasting money on parts that are not broken. Thx.

    • That it works OK on a 20 amp circuit suggests that the issue is electrical. As compressor parts age, they don’t work quite as well, hence the issue with the compressor motor not working so well on a 15 amp breaker. The logical solution is to power the compressor with a 20 amp breaker. I would think that the problem is an aging electrical motor or a failing, but not yet failed, run capacitor if the start capacitor checks out.

      See the parts list and compressor drawing here:

      • The motor shop said it was fine and they had it hooked up to meters? They also said both capacitors were fine? They did witness the same issue I have been having while it was there though (several times). They ran the motor and unplugged it (didn’t use switch) and many times it chugged for a second then just hummed. They said that the amp meter spiked way up during this. They said that it is something mechanical. Most of the time they turned it on/off it ran as it should, and they said the motor sounded good. Is there any way to definately rule out a bad check valve, unloader switch, or even the compressor part? Thx

        • I cannot refute the motor shop, George. If they say the motor and the caps are fine, no way I can argue with that. The compressor symptoms can be a reflection of binding in the pump, adding load to the motor, and preventing if from starting. That increased load could account for the spike in amps. If you disconnect the line from the top of the pump, the one that goes into the tank,then your compressor will run without any backload from the tank, the tank check valve, completely. If the motor starts repeatedly and with no problem, then that would suggest it’s not the pump, but another component. You can check the tank check valve by emptying the tank, removing the check valve, and seeing if it actually does allow air flow in only one direction by blowing through it. Opening that line will also remove the unloader valve from the equation, as any air trapped over the piston will empty when you break that line. I don’t know what this… ” or even the compressor part ” means.

  16. colin foster says:

    Hi, I have an old HPC 240v MOB 15E continuous running compressor and am wondering if I should add a pressure switch to the system? would this cause any problems? would I need to modify or disconect the offload valves?



    • Colin, typically an air compressor with a pressure switch has a reservoir tank that fills while the compressed air is not being used, and when the cut out pressure is reached, the air compressor motor stops until the pressure in the tank drops to the cut in pressure. A demand air compressor like your HSC rotary screw is not likely designed to have the motor start and stop, but rather, have the motor run continuously, and just stop producing compressed air when it was not needed. This is making me wonder about the motor maintenance considerations. If you don’t have a tank to monitor with the pressure switch, then your air compressor motor would be cycling on an off very rapidly as the minimal air in the air lines was consumed. That wouldn’t be good for the motor, I should think. It is, of course, your compresor to do with as you wish.

  17. Norman Roberts says:

    I have a 33 gal Craftsman compressor that was given to me. So far I have replaced the piston/sleeve set up, and also the valve plate. It now builds pressure, but for some reason it also keeps tripping the breaker. This is plugged into a wall, or a brand new generator. It also continues to leak air from unloader valve once it has cut out. Any ideas?

    • If this were mine, I’d be looking at the run capacitor to see if it’s up to snuff. If it is leaking air from the unloader after the compressor stops, you need to look to your tank check valve, it’s probably leaking.

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