Hey all you electrical engineers and electricians. I need some help with an air compressor I just received. It is between ten to fifteen years old, but it is in really good shape. It has an electric motor that can be wired for 115V or 230V. It is currently wired for 230V, and that is the way I would like to keep it. The problem I have is that it has a three wire system...two hot wires and a ground wire. According to the current building codes, all 220V lines must be a 4 wire system. This means that there will be two hot wires, a neutral wire, and a ground wire. I was going to rewire the motor to be a four wire system, but I am not sure how. What exactly is the neutral wire for? What happens if I do not use it? Here is a diagram of the current setup:

P P T T T T T
1 2 8 3 2 5 4

P1--unground line(one of the hot wires).
P2--taped off(has a wire nut on it).
T8, T3, T2--taped together(using a wire nut).
T5, T4--wire nutted with the other hot wire.

now, according to the label and by looking at the wires, T5 and T8 have been switched so that the motor rotates in a counterclockwise direction.

Can anyone give me an idea on how to convert this from a 3 wire system to a four wire system? If this can't be done, I will probably install the outlet from the breaker box using a 4 wire setup, and will wire the three wires on the compressor to use a four blade plug. What will this do if I do not use the neutral wire?

Thanks for your help

 

[updated:LAST EDITED ON 27-Nov-02 AT 01:27 PM (EST)]What you have there is two hot leads (120vac each), a neutral and earth ground. The green/yellow (most common colors) should be connected to earth ground. If this is a commercial installation, the gang box should be grounded if it is metal, if not there should be a fourth wire leading to ground. Earth ground is very important, if you eliminate it, you have a floating ground. If the compressor is not properly grounded you can have voltage floating around from the chassis to ground. I have seen this voltage go up to 120vac. Not fun to find this by mistake.

If you call an electrical motor shop and give them the numbers off of the plate, they should be able to provide you with a spec sheet with all of the possible wiring configurations for the motor.

Hope this helps.

 

I re-read your post and while the wire numbers you provided are some help to me, it appears that your ground (on the motor) may be a frame ground. or the earth ground may be tied into your neutral line (wire nutted together). I do a lot of work with both A/C and D/C motors. Respond to me with the numbers on the motor and I will look them up for you to locate your ground lead.

Steve

 

Thanks for the replys.

I called the city building inspectors and asked them what the codes were on wiring circuts for power tools. They had to do some research to find the answer. When they got back with me, they told me only circuts for oven/ranges, refidgerators, and dryers required 4 wire circuts. Anything else can use a 3 wire circut. I have wired the circut and the compressor is up and running.