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We seem to be getting more and more questions lately about why an air compressor slows and stops, and buzzes or hums, sometimes at the same time, on this site.

That an air compressor slows and stops does not seem to relate to any particular brand, either.

The questions about this seem to be more frequent about the smaller DIY air compressors than larger industrial air compressors.

To understand how to diagnose and troubleshoot this compressor problem, we also need to dive a little into how exactly air compressors operate in principle.

Please note that a little further along in this article we will talk more about air compressors that hum or buzz on start up.

This page about a compressor that slows ans stops is prompted by a question about a Speedaire pump that is slowing and stopping at around 40 PSI.

Speedaire 3z170 Compressor Pump
Speedaire 3z170 Compressor Pump

What’s happening when the air compressor slows and stops?

We assume the compressor starts properly and the pump is working. You can monitor the tank air gauge, and see the pressure in the tank rising.

Suddenly the sound from the air compressor starts to change. The tank gauge pressure reading is nearing 40 PSI (for example) and now the compressor pump is running slower, starting to bog down, starting to load. What’s happening?

Well, for a start, there is now back pressure building in your air compressor. The pump is driving air down into the tank, through the tank check valve, filling the compressor tank with more and more air. Since the volume of the tank is fixed, as air is driven down into it, the pressure of the air rises.

What is the result of that? The compressor motor has to work harder to drive the pump, as now the pump piston has to drive air down into the tank against an increasing load of air already in the tank.

That the air compressor motor has to work harder to drive air into the tank is expected, what is designed for, and is not the issue. The issue when an air compressor slows and stops, is that something else on the air compressor is causing the problem.

One of those issues is air compressor power supply.

Have you enough power to run your air compressor motor?

Are you trickle-feeding your air compressor motor?

By trickle-feeding your compressor I wonder are you using at least a 20 amp circuit breaker on the circuit that feeds the plug where you have your air compressor plugged in? Or, have you put a power bar or extension cord in the circuit between the air compressor and the plug? Or, are there other electrical devices pulling power from the same feed that your air compressor is using, at the same time that the air compressor is trying to run?

If you have done any of the above, then you are, what I call, trickle-feeding your air compressor motor. Simply, it isn’t getting enough power to do the job against the increasing load of the air in the compressor tank, and if the electrical feed to the compressor motor is slowed enough, your motor will slow and might even come to a halt.

That you are underfeeding the compressor motor also means that the motor is running hotter than it should, and this is leading to an increasing likelihood of compressor motor failure.

So, plug your air compressor into a clean circuit, do not use any cord or power bar in between the compressor cord and the outlet, and ensure that the power supply is clean and plentiful. Did this help resolve the issue that the air compressor slows and stops?

If not, the problem with your air compressor slowing and stopping may have to do with the fact that it has multiple pistons, and the valves are affecting the pump operation.

Compressor with multiple pistons

A multi-stage, multi-piston air compressor pumps air from piston, to piston, and then into the tank.

Each one of the succeeding pistons will have a set of valves. The intake valve will open as air is pumped into that piston, and then the discharge valve opens as the piston cycles, to drive air into the next piston or down into the tank.

If any of the valves in the sequence of pistons, or the valves in a single piston air compressor for that matter, seize up or break, the ensuing blockage of air flow could result in a pressure buildup and slowing of the pump / motor.

To determine and to rectify that requires a pump tear down and replacement of the valves in question. If you do this, I strongly recommend that you replace all gaskets in that pump as well as gaskets will be damaged in the process of disassembly, and blown gaskets are, themselves, another trouble spot on some air compressors.

If the multiple compressor piston doesn’t relate to your compressor, what else could be wrong, then?

Compressor Hums and Buzzes 

A humming or buzzing compressor is telling you that something is wrong with the power on the compressor. It could be a power supply issue, or a problem in the delivery of power to the compressor motor through the various electrical devices on the compressor.

When a compressor hums and buzzes

A 120 VAC household circuit, with a 20 amp breaker, cannot provide enough power for a compressor motor exceeding 1 HP in size to start properly. However, most DIY air compressor owners are powering their air compressors with a 120 volt circuit.

In order to make up the shortfall in the power needed to get an electric compressor motor running, the motor will have a start capacitor. Some have a start / run capacitor, with the run capacitor providing a power boost to help when the compressor motor is trying to run under load, like when the air tank is filling up, for example.

Is the Start capacitor humming?

If you air compressor is humming or buzzing, that’s an indication that the motor cannot get going. And that often means that the start capacitor is failing or has failed.

How do you check? First find the capacitor or capacitors on your compressor motor.

Look at the electric motor. You may have to remove some shrouding or shielding. Your air compressor motor will likely have one or two tubular bumps at the shaft end, likely attached to the outside of the motor housing. One of these bumps contains the start capacitor, and if there are two, the other contains the run capacitor.

Once you have found the capacitor, then see the page on this site under troubleshooting for information on how to check a start and run capacitor.