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.
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
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.
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.
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.
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.
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.