Compressor slows down at 100psi

by Joe

Hello I have made my own air compressor. It is a central pneumatic compressor rated at 145 psi and requires a 5hp motor it is twin cylinder.

The motor is a 220v marathon 6hp sitting on a 60 gal tank from my previous compressor.

After getting everything bolted down and plumed I kicked it on and started great.However when it reached 100psi i noticed that the compressor started to slow along with the motor.

I have the pressure switch set to kick off at 130psi and then back on at 80psi.

I decided to turn it back on thinking it was me and the motor would turn but very slowly so i turned it off. drained the tank back to 60psi and it started fine, got to 100psi same thing.

I have checked voltage which was fine i cracked the line from the tank to the head to check the hold back valve on the tank and no air came out as well the head release valve dumps the air to.

Thinking that maybe I had pressure switch problems i wired it direct and same thing. I would think that the motor would slip the belt if caused by the compressor which it wont and i have checked belt tension.

This is the second motor that i have tried i thought the first one which was the same thing might have been a lemon, but apparently not.

My only thought is that somehow the hold back valve on the tank is messing up but as i said when i crack the line to the head from the tank it does not leak or I wondered about my plumbing, two lines out of the heads are 3/4 to a T and reduced 5/8 from the T to the tank (hope that makes since)thinking that the lines are too small and the compressor cant get the air out of the lines fast enough. I cannot think of anything else. Thanks.

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Bill says...

Joe, I doubt if the line size from the heads to the tank is the problem.

If you have pulled the tank check valve, checked it, cleaned it, and it seems fine, then that is not likely the problem either.

It might be intake valves too, but likely not.

I'm thinking that you have a mechanical issue in the pump that, when it starts seeing back pressure, that as that load increased, the pump mechanicals are being affected, and they slow the motor.

If you hadn't tried a second motor, I would have suggested look to the motor first, but since you have changed motors and the symptoms are the same, I'm guessing a pump issue.

Bill