Compressor question: 240v to 120v

[Doug W Swanson]

Hey all,

I just bought a spare compressor from my fire dept. Currently it is set to run at 240v but I'm hoping to run it at 120v. The motor has a diagram showing the wires that need to be switched on it but I have a question about hooking up the wires to the compressor. It has this contraption on the side and I'm not 100% sure what it is. Maybe a pressure relief valve?

Anyway, it has two black wires coming in and two black wires going to the motor. Can I just change one of the black wires to a neutral? Or can I bypass this and wire directly to the motor?

Any help would be appreciated!355791355792

[Bill Orbine]

Don't bypass the pressure switch! You do that, somebody may call the fire department if the compressor goes BOOM!:p Obviously, you need qualified help.....any of your fireman buddies an electrician?

[Chris Hachet]

If possible I would just run a 220 line to the shop.

[Scott T Smith]

Doug, you need to check the nameplate on the electric motor to see if it is designed to run on 120. If so the plate should have a wiring diagram on it that shows you what wires need to be moved around.

Most compressors 3hp or greater are 240V only due to the high current inrush on startup.

[Jerome Stanek]

You will have to switch the wires on the motor and then wire the cord that plugs into the wall with one black one white and one ground

[CPeter James]

I suspect that the compressor is too big to run on 120V. 1.5hp is about the max and for compressor with "real" 1.5 hp motor the start current would probably trip even a 20 amp breaker. What does the data plate on the motor say about HP and Amps?
CPeter

[Bill Dufour]

What is that switch connected to? does it have any plumbing connected or is just an electrical overload with no pressure sensing? No reason not to switch the neutral as long as the hot lead is switched as well. Never switch the ground lead, ever!
Bill D.

[CPeter James]

The switch is screwed into the side of the tank to read tank pressure.
CPeter

[Tim Bueler]

Looks like a pretty large motor to run on 120V. Is there a possibility you could install the compressor where you have 240V available and just extend your air line to it? There are nylon and/or polyurethane air lines that can handle plenty high enough pressures for a woodshop, they're flexible and wouldn't rot if you had bury it. Just a thought FWIW.

[Don Sundberg]

Hey all,

I just bought a spare compressor from my fire dept. Currently it is set to run at 240v but I'm hoping to run it at 120v. The motor has a diagram showing the wires that need to be switched on it but I have a question about hooking up the wires to the compressor. It has this contraption on the side and I'm not 100% sure what it is. Maybe a pressure relief valve?

Anyway, it has two black wires coming in and two black wires going to the motor. Can I just change one of the black wires to a neutral? Or can I bypass this and wire directly to the motor?

Any help would be appreciated!355791355792


Your motor nameplate has 2 wiring diagrams on it (low voltage and high voltage) Wire the motor as shown on the plate for 120V. You don't need to change anything at the pressure switch. You then need to put the correct plug on the cord. If you want to be completely correct you should trace the L1 wire to the plug to be sure the hot wire is on the correct spade of the plug. It will run fine if it is swapped with the neutral though.

Don

[Bruce Page]

The big question is what HP is the motor and how many amps does it draw at 110v? Compressors draw heavy amps at start up. Wired for 220v reduces the amp draw to almost half.

[Doug W Swanson]

Here's a picture of the data plate. It's a 1hp motor and draws 14 amps so I should be able to run it at 120v, right? I only have 6 or 7 receptacles on my garage circuits and this is pretty much the only big draw I'd have. I don't have 240v close to where I want to put the compressor so that's why I wanted to switch it over to 120v. I could always run a 240v line but I prefer not to unless I have to.

355804

I wasn't 100% sure what the box was on the side of the compressor as my other compressors are 120v and they don't have it. Since I'm not one for risking injury (although I am a fire fighter :)) I won't be bypassing the pressure relief valve. Also, the compressor does have another pressure relieve valve on the front by the pressure gauge.

[Bruce Page]

You shouldn't have any problem running 1hp on 110v. Is it the original motor to the compressor? It seems small for a tank that size.

[Dan Friedrichs]

Should run on 120V fine. Open the junction box on the side of the motor (on the far right of your picture), and there should be 6 wires (labeled "T1" through "T6"), plus two supply wires coming in. Reconnect the wires as illustrated in the picture (T1, T3, T8, and one supply wire connected together, and T2, T5, T4, and the other supply wire connected together).

[Doug W Swanson]

The motor is not original. The compressor originally came from the Twin Cities Army Ammunition Plant and was installed in the fire station maybe 15 years ago. My plan is to switch the motor to 120v and see how it runs. If it works, great. If not, I'll run a 240v outlet and switch it back.

And I had a brain toot last night when I said my 120v compressors don't have the relief valve. They have them but they're just covered in a big plastic shroud :)

[Bruce Page]

Doug, I'm just curious, has that motor ever driven the compressor pump?

[Doug W Swanson]

Doug, I'm just curious, has that motor ever driven the compressor pump?

Yes. I think the motor was installed whenever the compressor was installed at the station. I made sure I tested it out and ran it for a short period before I put a bid in on it :).

[Art Mann]

I agree that 120VAC, 15A power will most likely work well. There is one warning I will toss out. If you are running multiple machines off that off one circuit (breaker), you may occasionally trip the breaker if the compressor cycles on while you are also using some other power tool. I have that situation but I don't leave the compressor on unless I am using it a lot.

[Doug W Swanson]

Should run on 120V fine. Open the junction box on the side of the motor (on the far right of your picture), and there should be 6 wires (labeled "T1" through "T6"), plus two supply wires coming in. Reconnect the wires as illustrated in the picture (T1, T3, T8, and one supply wire connected together, and T2, T5, T4, and the other supply wire connected together).

Currently the pressure switch has two black wires and a ground going to it. Then two black wires run to the motor. I've got the motor wired as Line/T1/T3/T8 and Line/T2/T4/T5 as the diagram shows. Am I right in thinking that I need 120v running to both of those wire bundles? Or is just one OK? Do I need a neutral? If so, where would it go?

I've looked online and really can't find a good wiring diagram of what I'm trying to do so that's why I need a little help. I've done plenty of wiring on my house but it's either been 120v or 240v. I haven't had to go from one to the other....

[Bill Dufour]

For 120 you need one hot lead and one neutral. Not sure how that relates to the internal motor windings but only two wires and ground will come out of the motor wiring box and into the supply line.
Normally neutral is white but in your case you will use one of the black leads as neutral. You should mark it white at both ends with tape, whiteout or paint. I would leave the pressure switch as is and switch both neutral and hot at the same time. You do not normally switch the neutral wire but in your case it is already done and it would be a waste of time to jump around the switch for neutral.
Bill D.

[John K Jordan]

... and it would be a waste of time to jump around the switch for neutral.

And especially if someone switches it back to 240 some day!

BTW, Doug, I suspect the switch on the side is not so much a pressure relief but simply a switch to turn the motor on when the pressure gets low and off when the pressure is high enough. Some have a knob or screw for adjustment. A pressure relief is often a mechanical spring-loaded pop-off valve that releases pressure in the tank in the event the compressor fails to cut off or maybe if the tank gets too hot. At least that's the way it works on the four compressors I have.

JKJ

[Doug W Swanson]

And especially if someone switches it back to 240 some day!

BTW, Doug, I suspect the switch on the side is not so much a pressure relief but simply a switch to turn the motor on when the pressure gets low and off when the pressure is high enough. Some have a knob or screw for adjustment. A pressure relief is often a mechanical spring-loaded pop-off valve that releases pressure in the tank in the event the compressor fails to cut off or maybe if the tank gets too hot. At least that's the way it works on the four compressors I have.

JKJ

Yes, you're right, John. I've been calling the pressure switch the wrong thing. Fortunately everyone here knows what I'm talking about :)

[Doug W Swanson]

355852

I've got the motor rewired as per the diagram and here's the pressure switch. The two outside terminals are labeled 'line' with the two black wires coming from power while the two inside terminals are labeled 'motor'. So what I'm hearing is that I change one of the two black 'line' wires to a neutral and everything would be ok?

I think what I may do is just buy a new power cord and replace the flex conduit that is currently there.

[Thomas Canfield]

I don't think I saw it, but to change from 220 to 110 you need to make 2 changes. The plug on line to the pressure switch will need to be changed for 110 service. Then you will need to change the L1 and L2 connections and other terminal connections in the motor housing. It was mentioned that the 14 amp at 110V may present a problem if it is not a dedicated circuit and there are other users on the circuit (shop vac, drill, etc) and cause frequent tripping of breaker. I just bought a new air compressor and changed it to 220V to keep from overloading the standard shared 110V circuits. The 220V circuit used is shared but all loads are only 15 amp, well under the 20 amp breaker.

[CPeter James]

If this was in a fire station, it was most probably used to maintain the air pressure in the fire truck air brakes so that when the trucks have to respond to a fire, they don't have to wait for the air pressure to build up. For this purpose, they would not need a high volumn pump and the 1 hp motor would have been sufficient. Being a government installation, I would suspect that it is a high quality USA made compressor and worth putting a little money into it to get it going.
CPeter

[Doug W Swanson]

Went to Menard's this morning and bought the necessary supplies and got the compressor fired up this afternoon. Everything works great!

Thanks for all of the advice!