|These photos represent a couple of weeks of searching and trial and
The first photo is going to require a lot of explanation. There is a lot
My garage has been in constant growth and disarray since 2003. Only half
of it has a concrete floor. The other half is partly gravel, partly
dirt, and partly tongue and groove oak. In the upper left of the photo
you can see the end of some kind of weight lifting equipment. That part
of the garage is a weight room. It was the bribe I used to get my son to
nail the gussets to the gambrel trusses we built to expand the garage in
2003. 76 nails per truss, 31 trusses. His hammer arm may be a bit
stronger than his other, now. The rough OSB door behind the compressor
system was the exterior door of the original 20x24 garage. In the upper
right, you can see the rough-sawn oak that sheathed the original garage.
Center right is dirt, still awaiting excavation, gravel and concrete.
Anyone who is an electrician or electrical inspector may want to stop
reading or have someone stand by with the smelling salts. It gets a
On the dirt is a Square D "General Duty" safety switch, rated for 3 HP
single phase at 230/240V. I nabbed it from an old water well on the
place. It had a pair of time-delay 20A screw-in fuses. I pulled those
and created a solder bridge across the sockets. My 125W soldering gun
couldn't get the copper busses hot enough to melt solder so I made a
soldering iron from a piece of copper pipe. I pounded the end closed and
pointed, held it against the busses using vise grips, and heated it with
a propane torch.
230V was tapped from my welder outlet. Since that uses a 5-50 plug and
all I had was the "pigtail" from an old electric range, I adapted the
range plug to fit. This involved twisting the two "hot" prongs until
they were parallel and then changing the flat ground blade into a tube.
The wires in the pigtail are #6, so it could handle the current. Notice
the fancy mounting of the safety switch? It has a pile of disc rotors
and a big cast 22 tooth A550 sprocket laying on it. That's so it won't
move when I yank the switch with the garden hoe. That's the handle of
the garden hoe on the right.
On the left is my old rinky-dink compressor system. The compressor
cylinder on it is smaller than one of the intake air filters on the big
one. The tank is connected to the big compressor via that hydraulic hose
and a collection of assorted 3/4" galvanized pipes and fittings. There
is no automatic pressure switch. The safety valve on the little 20
gallon tank is smaller than my thumb and so would likely just make a
good warning whistle before allowing the tank to rupture from
First test, I terrified my wife. I showed her the circuit breaker in the
house that controls the welder outlet, then had her follow me to the
garage. There, I gave her instructions, "If this test fails and I'm
twitching and smoking on the floor, don't touch me, just go flip that
breaker and dial 911." That went over really well. It took a little
explaining about ground, wooden handles and worst case scenarios before
she'd hang around and not go flip that breaker right away and stand
guard over it. With her 20 feet away and not happy, I pulled the switch
with the hoe, the motor made that familiar rising pitch hum of start-up
and the compressor made a beautiful bloopida-bloopida-bloopida sound. I
pushed the switch back with the hoe and all was well.
Even with the drain plug out of the tank and no hose connected, the
compressor managed to move the gauge. I had to try it again. Same
results, same welcome hiss of air. This time I decided not to use the
hoe to shut it off and got a nice ZAP when I touched the switch.
Something wasn't right. I flipped and taped the breaker then opened the
switch to check the wiring. Everything looked right so I rechecked my
wiring in the welder outlet. It looked right, also. Nothing was out of
place in the motor's wiring box, either.
Some time later I repeated the test, using the hoe of course, with my
son standing by. This time we had the hose connected but the drain plug
still out. It built up 20 PSI and blew a mud dauber's nest out of the
drain. I think my son travelled only slightly slower than the mud plug.
(Just in case anyone reads this and is unfamiliar with a "mud dauber",
it's a wasp that makes nests using mud. The nests range from tube-like
things resembling TIG welds to non-descript globs). It must have been an
industrious wasp; I only left that plug out for two days. The only
reason I know it was such a nest is that part of it froze to the floor
instead of being blasted all over the place.
My overgrown football fanatic son has two fears: anything that resembles
a snake and electricity. As I hit the switch to turn the compressor off
it shot sparks and smoke out of the switch box and my son shot over
piles of junk to land about ten feet from the evil machine. I
completely disassembled the switch this time. My solder bridge had
dripped through and a point of solder was touching the metal box,
hidden by the big insulation plate on which the switch parts were
Electricians and safety inspectors may resume reading. The rest of this
tale should cause no more than grimaces and moderate pain.
The proper controller for an air compressor is a pressure switch, of
course. I could not find one rated over 3 HP at 230V single phase. Since
I didn't want to weld the contacts with an overload, I spent a few
nights at the computer digging through almost all of the electrical
listings in the http://www.grainger.com online catalog. (Don't believe
the prices listed in that. The motor listed for $556 and I got it for
$361 from a local branch. A man named Jeff at that branch saved me at
least two weeks of searching and waiting for parts to be shipped). What
I found is a "definite purpose contactor" rated at 40 amps inductive
full load current and 7.5 HP at 230 VAC single phase with a 230 VAC
coil. So an ordinary pressure switch could trigger this relay which
would deal with the motor.
The contactor is too big for ordinary electrical device boxes: 2.5
inches wide, 4 inches long and 3.11 inches tall. Fortunately, I'm a
packrat and hardly ever throw anything away that might conceivably be
useful someday. Buried in amongst some car, lawn mower and chainsaw
parts was a 100 A circuit breaker box my wife had purchased at some yard
sale years ago because she thought I might need it. It's a Cutler-Hammer
box and none of the Square D breakers on the place will fit it. Gutted,
it has just the right depth for the contactor.
Photo 2 shows the wiring for the next test. I had to crank up the
brightness and contrast in the image in order to see the contactor,
which makes the pressure switch too bright. The large black cable is an
8/3 w/gnd (3 conductors of 8 AWG plus a bare ground wire) coming from my
welder outlet. Red and black, the "hot" wires, are connected to the
input side of the contactor along with red and black from a 10/3 w/gnd
cable feeding the pressure switch. The T1 and T2 terminals of the
pressure switch, instead of feeding the air compressor motor, are
instead connected to the coil of the contactor. The output side of the
contactor is connected to the motor. In the photo you can see the white
neutral wires from each cable are simply curled out of the way and
capped. A white wire is being used as ground for the motor just because
it was there and handy. That will be changed to a proper ground wire
when I rewire my garage and install a new 100 A service.
This test was successful and completely uneventful. That's a good thing.
That allowed me to move on to the final steps, which were attaching the
pressure switch to the tank, installing the new 3/4" safety valve, and
mounting the box with the contactor to the frame around the compressor
and motor. The belt guards will be bolted back in place after I'm
certain that everything is working correctly, sheaves are in line, and
setscrews are going to stay tight.
The 3rd image is a composite of two photos. The left shows the new
safety valve and the new pressure switch with its twin cables, one for
power input and one for the output which controls the contactor. The
right half shows the cabling temporarily taped to the back rail of the
frame around the compressor and shows the box containing the contactor.
(File sizes, in bytes: image 1 = 142528, image 2 = 126178, image 3 = 66266)
(click photo to enlarge)
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