[TheForge] O.T-. compressor wiring question

[Steve Smith]

Bob Smolen wrote:
> I bought an older Ingersoll compressor from an estate. I did not see it in
> operation but it looked to be in pretty good shape.. I believe a model 30T.
> It has a 3 phase 7.5 h.p. motor.
> 
> To test the unit,I hooked my shop 3 phase directly to the 3 wires on the
> compressor motor and it started no problem. I now want to hook up properly
> and want advice on proper wiring to pressure switch and low oil sensor.
> There is a switch and pressure gauge mounted to the tank. There is also a
> low oil sensor for the compressor pump which is monted on the pump and the
> wired into the switch. There are two sets of wire that were disconnected
> when the unit was moved from the last shop.One set is from the motor and has
> 3 wires.These are the wires to run the motor which I have verified will
> start the motor when hooked up to shop power. There is also a set of 2 wires
> extending out of the switch (which I will decribe as enter/exit because I
> guess they can lead current into or out of the switch).I do not see where
> any other wires could previouly could have come out of or  entered the
> switch box.
> 
> The switch has 4 contacts(I believe) marked line, load,load, line.The oil
> sensor has 2 wires  entering the switch box. One oil sensor wire is simply
> connected to one of the 2 wires that exit/enter the switch. The other oil
> sensor wire is hooked to a "line" terminal.The second wire that
> enters/exits the switch is hooked to the load terminal of the switch.
> 
> My uneducated guess is that one of the 3 current source lines  should be
> connected to one of the wires extending from the switch. The other switch
> wire is then connected to one of the wires extending from the motor. In
> other words, current goes into the switch and leaves the switch and enters
> one of the lines connected to the motor. The two remaining wires coming from
> source are connected to the 2 remaining motor wires. If the oil is low or
> the pressure sensor signals the switch to cut the motor, the one leg of the
> 3 phase hooked to the switch will  open, shutting down the cuurent in that
> leg and therby shutting down the motor. Does this make sense? If not, any
> suggestions?
>  Thanks,
> Bob

Bob, opening one leg of a three phase motor will not stop the motor, it 
will just overheat the other two windings (assuming a full load like a 
compressor).

The other issue, if I understand your description right, is that the oil 
sensor and pressure switches are made to handle much lower current than 
your 7.5HP motor. If you use one of those switches to directly interrupt 
the motor current, you will fry the switch contacts. Again, I may not be 
following your description.

The piece I don't hear you describing is usually called a "magnetic 
starter". All they are is a large, high current relay (large like the 
size of two fists, lots of black bakelite plastic and thick metal 
contacts) with some built in "heaters". The heaters open up when too 
much current is flowing to the motor, like if it is stalled or if one 
phase is open.

The way I would hook this up is to have the oil sensor and pressure 
switch interrupt current to the magnetic starter coil. This is a low 
current leg, and also is just one phase.

Turning a three phase motor on/off involves connecting or disconnecting 
all three legs--when off, then there is no voltage on the motor at all. 
The magnetic starter is usually set up so that the output side of it 
sends power to the coil (through the pressure and oil switch), so that 
once it turns on, it latches on. You turn it off with an off pushbutton 
in series with the starter coil.

I probably have some (low current) start/stop pushbuttons you can have 
if you end up putting a control box together.

Clear as mud? Tricky stuff to do in words long distance. Where are you 
at? Maybe there is a motor jockey in your neck of the woods.

Steve
Brownfield, Maine