3 Phase to Single Phase motor

[Chuck Ramstack]

I have an opportunity to buy a Rolair Air compressor. 3 phase, 5hp, 60hz, 230 volts. It has an Eaton Magnetic Starter. Not having 3 phase in my small work shop I would like to just buy a single phase motor and convert it. Since its a compressor I know I need something pretty heavy duty. Will a 5HP do the job
and can I keep the Eaton starter?

 

[LarryFine]

1. 5HP is 5HP whether the motor is 1ph or 3ph, so it should be enough. Don't be suprised if your new motor is larger than the original, though.

2. 5HP at 1ph will require more current than 5HP at 3ph. The starter should contail labeling. If the starter has overloads, it may trip too easily.

 

[augie47]

Few suggestions/comments...
First, check the motor nameplate and make sure it is actually a 5HP motor. Compressor folks have a habit of using "special ratings" and 5HP in compressor salesmanship does not always mean a true 5 hp motor.
Secondly, as Larry pointed out, a 5HP single phase motor is going to be considerably different in size from a 3 phase unit. Be prepared for significant mechanical modifications.
Third. A different starter will be required. Normally a 5 HP 3 phase requires a Size 1 starter, a 5 hp single phase a size 2 SIGNIFICANT COST DIFFERENCE.

Even with all the above, you still may want to discuss the change with a manufacturer Representative. Compressors are often designed to the limit of the motors and single phase may not provide adequate torque, etc.

With cost, mechanical difference, etc. I have seldom seen such a conversion desirable.

 

[LarryFine]

Actually, Gus, I was considering your first and second points in my first point.

 

[augie47]

Actually, Gus, I was considering your first and second points in my first point.

sorry, Larry. I wasn't trying to steal your thunder. only substantiating it.:grin:
I did acknowledge it in my post. No offense intended.

 

[LarryFine]

No offense intended.

None taken.

I was just saying that a real 5HP motor will probably be larger than his existing motor.

 

[Chuck Ramstack]

Thanks for the info!! Rolair offers this same compressor in single phase as well which why I thought it should be an easy conversion (tongue in cheek).
I'm trying to find the right Frame size of the motor so I can use the same pulley. Don't want to bush the shaft or bore out the pulley.
I was told by an Electrical Engineer this morning that I can use the existing 3-phase magnetic starter (Eaton) and pressure switch by leaving the 3rd pole empty. Then take L2 and wire it to one side of the coil, then run a wire from
L! to the pressure switch, and another from the pressure switch back to the other side of the coil. That should give me control from the pressure switch.
I was planning on using #10 wire and a double pole 30 amp breaker.
Does this make sense to you guys???

 

[augie47]

Thanks for the info!! Rolair offers this same compressor in single phase as well which why I thought it should be an easy conversion (tongue in cheek).

I'm trying to find the right Frame size of the motor so I can use the same pulley. Don't want to bush the shaft or bore out the pulley.
I was told by an Electrical Engineer this morning that I can use the existing 3-phase magnetic starter (Eaton) and pressure switch by leaving the 3rd pole empty. Then take L2 and wire it to one side of the coil, then run a wire from
L! to the pressure switch, and another from the pressure switch back to the other side of the coil. That should give me control from the pressure switch.
I was planning on using #10 wire and a double pole 30 amp breaker.
Does this make sense to you guys???

Not necessarily. Two questionable areas
Your "engineer" may be correct, although I doubt it seriously. His "method of wiring" is correct but if you check the nameplate on the starter I think you will find the starter is not rated for 5HP SINGLE PHASE (unless they oversized it for the 3 phase motor).
Secondly Art 430 tells us to use the tables for everything except OL settings. 430.248 gives us 28 amps fro 5hp at SP 230.
28 x 1.25 = 35. The limit for a #10 in the 75? column. Just be sure you are using a wiring method that doesn't require 60? rating (NM, SE)

 

[jminer99er]

Check out practical machinist forum under RPC, Transformers, VFD.

They have good info on this topic and compressors.

You could also pm Jreaf on this forum. (not sure if the spelling on his name is correct)

 

[Chuck Ramstack]

So Augie whats the bottom line? Are you saying I need a bigger breaker? I'm assuming #10 wire is adequate? I can check with Eaton on the starter.
Chuck

 

[augie47]

Your asking that raised enough doubt that I checked the profile. I will refrain from any additional "how to" until a Mocerator says it's o.k.
sorry

 

[charlie b]

Gus is right. You are not an electrician, and forum rules prohibit us from providing you any "how-to" assistance. I am closing this thread.