Electric Motor / Air Compressor Question

[GSequoia]

So I've got my trusty old 230V Craftsman 4HP 20 gallon compressor, right?

Well currently I only have a 230 outlet in the back yard, kinda a PITA, I plan to move soon, I won't be owning the place and I won't be able to have a 230 outlet anymore. So, I'd like to get a 115 motor if I can... Here's the details of the motor.

Voltage: 230V
HP: 4
RPM: 3450
Of course it's Compressor Duty

Now, what options do I have. I'd like to get the compressor in the garage for now, and still be able to use it at a non-230 house. Any options there? I'd rather not have to buy a new compressor, it's prefectly fine except the motor (okay, it needs a coat of paint on the tank..)

Thanks
Sequoia

 

[Eagle]

Since you said it's a trusty "old" compressor, I assume it has a capacitor-start motor, correct?

I think you're out of luck. I have the largest compressor Sears used to sell for 115v service. It's a 2-cylinder compressor, with a 3-hp capacitor-start motor, and it requires a 20-amp circuit or the start surge blows the circuit. When I worked in a swimming pool filter factory, some of the motors we dealt with could be wired either 115v or 230v, but there's a "gotcha."

The motor requires a finite amount of energy to run. If you reduce the voltage by half, you double the amperage required. So a motor that requires 15 amps at 230 volts would require 30 amps at 115 volts. You are not likely going to be able to run a 4 horsepower motor on a 115 volt circuit.

Check into replacement motors, but I think you'll have to accept a smaller motor size, probably 3 horsepower. Do you have W. W. Grainger in your part of the world? Around here they are a great source of replacement electrical motors. Also check Northern Tool and Equipment (formerly Northern Hydraulics).

 

[GSequoia]

Doh! I knew that half the voltage is double the amperage at one time...but that was a lot of beers ago....

No dice in W.W. I think (never heard of them), but I actually did check into Northern Tool and didn't find anything (Those guys have a lot, an engineer at one of our buildings turned me onto them a couple years ago). I'd say the compressor is probably around 10 - 12 years old (My grandfather gave it to me when I was a young'n), as for wheter the motor is a cap start, not sure, I'll have to look at the data plate again.

So what'chu think about going lower horsepower, forgive my ignorance here, but I expect I'd lose pressure in the tank, correct? Right now the cut-off is set for around 125 psi, any idea what pressure a 2 and 3 HP motor will put me at max (I don't know what the tank is rated at, but I won't be push it over that I assure you!) I'd like to keep around 125 pounds because I have a 7 gallon portable tank that I like to fill to it's max, good for mobile applications (It won't run an impact gun long..but it'll do it just long enough to fix me!)

Sequoia
Man...I guess I'm feeling kinda long-winded tonight!

 

[Goatman]

You're sure there won't be an outlet in the garage for an electric dryer? Do you know where the panel is in the place you're moving into? If there's not an outlet for a dryer, I'd wire a 230V outlet close to the panel and make an extention cord to run to the compressor. You can easily do both, make the cord out of a length of wire (3 wires) and ends from any hardware store. Running a 230V outlet from the box is very easy, and you could take the parts with you to the next house depending on where the panel is and how you did it. If that's a possibility, I can tell you how to do it.

 

[5-90]

Goatman beat me to it - I do the reverse. I have a gas dryer now, so I made a "splitter" to break the 230VAC dryer service into two 120VAC/20A single outlets - I can feel the welder, lead pot, or other high-current draw device without popping breakers...

5-90

 

[GSequoia]

Well that's always a posibility (I've done my share of wiring, one of the nice things about being raised without money, you learn to do a lot of things yourself!), but one of the main reasons I'm planning on possibly not doing that is that there is a big possibility we'll have to settle with an apartment with a two car garage instead of a house (have I mentioned lately how much I HATE apartments?)

 

[Georgia Mike]

I just put in two new 50 amp breakers in my box. One for my compressor and one for ol' reliable-the stick welder. I'm going to be dismanteling my old compressor and saving the pressure switch and relief valve for my OBA project (if I ever find time to get around to doing it ) The motor on it is probably a 1-2 HP unit. I don't know if it'll do you any good or not,but you can have it if you'll pay for shipping on it. It may be a few weeks because I've still got to plumb my garage with all new air lines and bleeders and such.

 

[Eagle]

I don't even want to get into the logistics and legalities of rewiring a rental property. That's between you and the landlord.

My 3 hp compressor is set for a 100 psi shut-off. The regulator switch is adjustable, but I have never tried to change it. I usually keep the output regulator set for about 90 psi anyway.

I spent some quality time with the Northern Hydraulics (oops -- Northern Tool & Equipment) master catalog last night. They have two pages of compressor motors. The good news is that they have some that should work for you. The bad news is that the largest rated for 115/230v is a 3 horsepower, and that requires a 20-amp circuit. The largest listed for 115v/15-amp is only a 2 hp.

The 3-hp sells for $89.95. I think you could get away with that one. The heaviest draw is at start-up, after that it probably doesn't draw more than 15 amps to run. So even if the circuit is a 15-amp circuit, you could get by with replacing the breaker with a 20-amp breaker, or (much better) using a "slow-blow" breaker that will resist the heavy surge yet continue to protect the circuit against prolonged draws of more than 15 amps.

 

[GSequoia]

That's why I want to replace with a 115V motor, bending the rules is okay, breaking them sucks sometimes! Me thinks I'll get the 3HP/20A motor, I can change out to a bigger or slow-blow breaker if I find that I blow circuits a lot.

Thanks guys,
Sequoia

 

[Georgia Mike]

It's not the breaker that you need to be concerned with. It's the wire that's attached to it. Sure,you could slap a 20 or 30 amp breaker in the box and be done with it. It won't trip anymore. But it's NOT SAFE!! Breakers are rated at different amperages for a reason,wire size. The minimum safe gauge for a 20 amp breaker is 12 gauge. 30 amp breaker-10 gauge. Check the wire sizes out FIRST before you go off and burn someone's house down. :lecture:

 

[GSequoia]

Don't worry man...that will certinaly be a consideration, and if I were to be doing that it would only be for a short spike, I wouldn't run high ampearage devices on low ampeage, I have scars to proove to me what happens when you overload wires. (Got a good burn from a hot unfused 10g wire that got "welded" to the frame)

 

[CheapXJ]

umm... just a thought...

but why not run it on gasoline?

it'd be a little more work, but really, all you'd need to do it is a solenoid to kick the throttle and a clutch on the compressor. you could run it off of pretty much anything then (hell, it'd be completely self contained)

all you'd really need to find is a decent 4-cycle horizontal shaft motor, and you're golden!

 

[Eagle]

It's not the breaker that you need to be concerned with. It's the wire that's attached to it. Sure,you could slap a 20 or 30 amp breaker in the box and be done with it. It won't trip anymore. But it's NOT SAFE!! Breakers are rated at different amperages for a reason,wire size. The minimum safe gauge for a 20 amp breaker is 12 gauge. 30 amp breaker-10 gauge. Check the wire sizes out FIRST before you go off and burn someone's house down. :lecture:

Very true, which is why I suggested that a 15-amp slow-blow breaker is the preferred solution. However, even using a 20-amp breaker isn't going to burn the house down unless the compressor runs almost continuously for a long period. It's the start-up surge that spikes the current. Once running, I don't think mine uses more than 15 amps. A 20-amp breaker will still protect the circuit against direct shorts and severe overloads.

CheapXJ is correct -- according to the electrical codes a circuit wired with 14-gage wire should not have more than a 15-amp breaker, 12-gage wire takes a 20-amp breaker, and 10-gage wire can use a 30-amp breaker. The slow-blow 15-amp breaker is fully code-compliant, the 20-amp breaker on a 14-gage circuit is not. It will work and it will be safe as long as you are cognizant that you risk overheating the wires, and keep an eye on it, but it will not be legal.

If you luck out, maybe the garage circuit will already be 20-amp and there won't be an issue.