By that, I mean that that the air compressor starts normally, it begins to build pressure in the tank, but at a certain point, the compressor motor begins to slow down. The compressor motor begins to labor, the compressor motor slows, the sound changes, and the buildup of air pressure in the tank either stops completely, or slows substantially.
Sometimes, when an air compressor bogs down, that process causes a problem with the power. The motor may pull too many amps, and the circuit breaker lets go. Or, the on-compressor thermal cut out will shut down the compressor due to motor overheating.
Many things might be the cause. A list of the most common reasons why an air compressor bogs down follows:
If the compressor is belt driven, remove the belt (power cord out please) and turn the pump sheave by hand. If it turns relatively easily with no binding, that helps eliminate the pump as the cause.
If the start capacitor checks out, and your compressor has a run capacitor, check that one next. If either is not up to snuff, they must be replaced.
Now it's decision time. Do you take your compressor motor to an electric motor shop for a load check, or do you tear down the pump to see what's what? Your rotating of the pump sheave may help. If there was any hint that the pump may be malfunctioning mechanically, tear down the pump to check the piston rings, intake and pressure valves, and general condition of the pump innards.
If you feel that the compressor pump is OK, and the power supply to the motor and the motor caps are good, it may be time to pay for a motor load test.
If you opt for tearing down the compressor pump, depending on the age of the compressor and level of use (ie; did you forget to put oil in a lubed compressor?) you may want to replace the piston ring, valve plate and ALL gaskets to ensure that when you reassemble the pump, none of these items can be an issue for some time.
Make sure when you install the valve plate that it right side up, else your compressor pump will not work.
One last thing. Know the duty cycle of your air compressor and make sure you run the compressor within this time frame. Over use of the compressor can, too, be a factor in why an air compressor bogs down.