Compressor Project

I picked up a compressor that was posted on Craigslist for $100 dollars. The seller said it had a bad motor, but actually, the compressor pump had seized. I didn’t test the motor to see if it worked (I was only after the tank). The tank is 60 gallons and has a maximum working pressure of 150 psi. It came from a shop that made granite countertops. The whole thing was coated with a thick layer of granite dust. Here is the tank, stripped of all parts, and cleaned with a pressure washer.

I am using a 5 HP motor I bought from the Surplus Center for $200. I bought the compressor pump from Harbor Freight on sale for $99. The compressor produces 15.2 SCFM @ 90 PSI.

Compressor Pump

The top plate was not large enough for the compressor pump. I spot welded additional pieces to support the new pump.

It is not a good idea to weld on a tank. Note that I welded to the existing mounting plate and not on the tank itself.

The finished welds.

I painted the extension. The color is not exact, but I think it’s close enough.

I made a brass extension for the drain so that I wouldn’t have to reach under the tank to use it.

Using a hammer drill and a 5/8” SDS bit, I bored 4 holes for 1/2” “drop in” anchors. I bored a 5/8” hole through the center of each of 4 hockey pucks. I then set a puck over each anchor in the floor. I used a fender washer over each puck and then set the tank in place. I added additional fender washers as shims until I had the tank level. I bolted the tank to the floor with 1/2” bolts and blue loctite. The pucks are made of hard rubber. I do not think they are too hard to be used as compressor isolation mounts. The compressor is very heavy.

I used brass fittings and permatex thread sealant with Teflon. I used a ball valve for the tank outlet.

This picture shows the control manifold. At the front is the pressure gauge. Next is a safety valve which pops off if the tank pressure reaches 150 psi. At the back is the pressure switch. An unloader valve is also part of the pressure switch assembly.

I used brass fittings to make an oil drain that would clear the mounting plate. This is to avoid a mess when changing oil. I bolted the compressor pump in place.

The compressor pump has a 3/4” NPT output fitting. I used a 3/4″ to 1/2″ NPT reducer, followed by a 1/2″ NPT to 5/8″ flare adapter. I bent a 5/8″ soft copper line to the shape I wanted and then made a 45 degree single flare on each end. This line connects the compressor pump to the tank check valve assembly.

A little more about the control function…

The pressure switch has an on/off switch to allow the compressor to operate. If the tank pressure is low, the contacts in the pressure switch will be closed, allowing current to pass through the switch to operate the compressor. When pressure in the tank is high, the contacts will open, stopping the compressor motor. The air pressure in the tank acting against the pressure spring determines at what pressure the contacts will open or close. Mine closes and turns on the motor at 105 psi. At 125 psi, the contacts open and stop the motor.

The output from the pump goes down through a “T” and then through a one way check valve into the tank. The check valve allows air into the tank, but not back out again. The “T” also has an unloader port. A 1/4″ copper line connects this port to the unloader valve in the pressure switch assembly. When the pressure switch opens the contacts and stops the compressor motor, there is a little arm that swings down and presses a pin on the unloader valve. This bleeds all the air out of the line from the top of the check valve to the compressor pump head. The little valve works similar to bleeding air out of a tire. Without an unloader, each time the compressor started, the motor would have to turn the pump against tank pressure. Because the air above the check valve is bled off, the first few revolutions of the pump are made against ambient air pressure. This makes for easier starts for the compressor motor.

I drilled holes in the top of the mounting plate and tapped threads for 5/16” bolts. This picture shows the motor mounted.

The pressure switch is rated for 15 amps. My motor can draw 19.5 amps. I used a magnetic starter so that the motor current would not pass through the contacts of the pressure switch. This picture is of a 6” x 6” x 4”, nema 1 enclosure that will contain the starter. I am not using a thermal overload with the starter because one is already built into the motor.

This is a picture of the magnetic starter sitting on top of the enclosure.

The starter is installed in the enclosure.

Here is my wiring diagram. The compressor operates on 240 volts, single phase. Incoming power is line 1, line 2, and a ground wire. There is no grounded (neutral) conductor, because this compressor does not use any 120 volt devices. If you were to include some 120 volt devices in your application, you would need to run a 4 wire 240 volt service that included a neutral conductor. A ground (green) wire is not supposed to be used to carry any current. The two hot wires go to the top of the magnetic starter. A wire jumps off of each of these and goes through flexible conduit up to the pressure switch. A ground wire also goes up to the pressure switch. When the tank pressure is low, the pressure switch contacts close. The two hot wires are switched to both sides of the control coil of the magnetic starter. This applies 240 volts across the control coil (my starter has a 240 volt coil). This pulls in the starter contacts and sends 240 volts to turn the motor. The control coil does not use much current to energize, so very little current travels through the contacts of the pressure switch. The large current to operate the motor goes through the larger contacts of the magnetic starter.

This picture shows the flexible conduit that houses the wires that run from the magnetic starter housing to the pressure switch assembly.

This picture shows the control wires on the pressure switch.

This picture shows the magnetic starter with wires installed. There is a white wire because I used 10/3 cable to connect the compressor motor to the starter. Also note that the magnetic starter can be used for 3 phase, so the center contacts are not used.

I installed a disconnect switch on the wall next to the compressor.

Here is the finished project…