[TheForge] O.T-. compressor wiring question

[Chuck Robinson]

Steve is absolutely correct.
Your best bet is to get a magnetic starter switch big enough for your FLA 
rating and have some one near you ,who knows what he is doing,  help you 
wire it correctly.
Check on ebay for a used one. They are pretty pricey new.
Chuck
----- Original Message ----- 
From: "Steve Smith" <sos at alum.mit.edu>
To: "Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Wednesday, June 29, 2005 7:23 PM
Subject: Re: [TheForge] O.T-. compressor wiring question


>
>
> 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
>
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