HELP running air compressor with two motors

[hempdiddy]

I recently acquired a complete Kellogg American 331 compressor circa 1959. The 5hp three phase motor is bad.

I can't run three phase at my home shop, so I'm planning on running two 120v single phase motors to run the pump. (It would be more work than I want right now to run a new 240v circuit. I have two 20 amp 120v circuits avail and ready to go).

Can this be done? What tips and design elements should I be concerned with?

 

[The Cobbler]

if you have 2 20 amp circuits, you likely have 240 at the garage already . the circuits should be on separate legs of your main panel.

 

[kythri]

if you have 2 20 amp circuits, you likely have 240 at the garage already . the circuits should be on separate legs of your main panel.

If you've got 120, you've got 240. I don't think that's really his issue.

The OP would have to confirm that the wire in one of his 20A circuits is sufficient to run a 240V circuit at the appropriate amperage for whatever 240V motor he might be interested in.

Past that, however, in regards to the question, I'm not going to speak for any certainty, but I don't know how you'd make two motors function on a single compressor pulley, and how that would even be beneficial if you could.

 

[monkeyspanners]

Could you run the compressor at half speed on just one motor?

 

[sk farmer]

forget 120. you more than likely have 240, you just don't know it. 120 is fine for anything less than 1 hp. if you need more than that 240 is the minimum. i once had had farm duty motor rated for 120 or 240 that ran an auger. we fought and fought with it. wouldn't pull the load, over heated, tripped the breaker. switched it to 240 and have never had an issue.

 

[scw1991]

A 1-1/2hp motor is the max you can run on a 120v/1ph/60hz circuit, and that is pushing it and most likely not feasible for an air compressor application due to high starting torque requirement

 

[The Cobbler]

.....
The OP would have to confirm that the wire in one of his 20A circuits is sufficient to run a 240V circuit at the appropriate amperage for whatever 240V motor he might be interested in.

.....

assuming that it is wired/ fused properly, the two circuits are rated for 20 amp @ 120. it's the same as 40 amp @ 240 . a 40 amp 240 circuit will operate a higher HP than a 120 20 amp circuit will.

 

[kythri]

assuming that it is wired/ fused properly, the two circuits are rated for 20 amp @ 120. it's the same as 40 amp @ 240 . a 40 amp 240 circuit will operate a higher HP than a 120 20 amp circuit will.

Don't quit your day job.

Unless someone went seriously overkill, a 20A 120V circuit is likely going to be wired with 12AWG.

That is absolutely NOT suitable for a 40A 240V circuit, which would require 8AWG, if I'm remembering sizing properly.

Two individual 20A 120V circuits are absolutely NOT the same as a single 40A 240V circuit.

 

[The Cobbler]

ok, maybe my wording is wrong. I mean that a 20 amp 120 circuit won't handle as much as a 20 amp 240 circuit. he basically probably has available 20 amp 240 circuit in his garage already, easier to use that than 2 120 v motors . for the same draw he can pull on that at 240 . a 40 amp 120 would be required

 

[hempdiddy]

I know that I have 240v available at the panel.

The two 20a 120v circuits I mentioned are wired w 12ga wire.

The only 240v circuit I could make out of those two circuits is 20a. Anyway, I'm not interested in running a 240v circuit right now however.

If two motors on the same pulley is a stupid idea, I'll just run the beast pump at half speed I guess with a 120v motor.

 

[sberry]

You need to run a circuit period,, interested or not if you want to use this. You will find out it is the easier way after the effort to come up with a scheme wont work at any speed anyway.

 

[BDT/NWMN]

A 1-1/2hp motor is the max you can run on a 120v/1ph/60hz circuit, and that is pushing it and most likely not feasible for an air compressor application due to high starting torque requirement

I agree.. Back in 1975 I bought a new CH portable air compressor. 1 1/2 hp, 120 volt. Kicked in at 80 psi and shut off at 100 psi.. It would run ok on a 20 amp circuit, but 15 amp circuits would not always handle it.

 

[justanengineer]

If two motors on the same pulley is a stupid idea, I'll just run the beast pump at half speed I guess with a 120v motor.

Make sure you know the pump's upper and lower speed limits if youre changing pulleys, and verify the result with a tach. Even the pressure lubed compressor heads dont take well to being run below their limit, nevermind a splash lubed head.

 

[Mikerodrig27]

I don't understand why two motors couldn't run on one pulley. People don't design a system like that because it is not too user friendly requiring two circuits. I think that if the two motors were the same and they weren't ran on the pulley without a load then it would be fine.

I'd say give it a shot. I cannot see any reason not to. Just do it with two belts

 

[sam.coll]

Ha whats the big problem with running a new circuit? do a job once do it properly

 

[kythri]

I don't understand why two motors couldn't run on one pulley. People don't design a system like that because it is not too user friendly requiring two circuits. I think that if the two motors were the same and they weren't ran on the pulley without a load then it would be fine.

I'd say give it a shot. I cannot see any reason not to. Just do it with two belts

What would be the benefit?

 

[sberry]

I don't understand why two motors couldn't run on one pulley. People don't design a system like that because it is not too user friendly requiring two circuits. I think that if the two motors were the same and they weren't ran on the pulley without a load then it would be fine.

I'd say give it a shot. I cannot see any reason not to. Just do it with two belts

How much work does anyone think it would take to rig 2 motors to a scheme like this. Would take a home brew type a day just to mount it and due to the problems we have with math here I am with the guy says,,, dont quit the day job.
As the engineer said the pump may not work right at reduced speed but we should get him back here to splain in further detail how 2 1 hp motors are in any way going to replace a 5.

 

[sberry]

Take a master mechanic a work week just to put it together from used junk

 

[kythri]

I'm still trying to figure out how two 1HP motors are going to do anything different than a single 1HP motor.

 

[aka Larry]

I'm still trying to figure out how two 1HP motors are going to do anything different than a single 1HP motor.

Me too.

 

[sberry]

It would double the power.

 

[sberry]

I am 55 and been a mechanic all my life. Not teribbly bright and no formal academic education and have great respect for engineers. I do real basic stuff. At 55 just beginning to get some insight that there is more to it than just making it work which a lot of people can do.
I got a lot of respect for the UL listing, all the nema and the codes. Very few people including myself are qualified to build the machines we do in some respect, I look at a lot of the older work I did and realize now I am clueless.
It doesn't help that I am a generalist but today on occasion I see stuff in simple design that didnt even occur to me.
I am a lot less tempted to "modify" and quicker to see how many ways factory adjustments can be used, some make a big difference, some a little. A simple cultivator dragging thru the dirt has a totally different finish with 1/2 a turn on one thing.

 

[kythri]

It would double the power.

How, exactly?

Are we talking adding some kind of differential to this?

 

[scw1991]

yes, if you put two 1HP motors on a compressor, you have twice the horsepower of a single 1HP motor as you are effectively doubling the torque. HP Equation = (Torque x RPM) / 5252.

However, if the OP's compressor requires a 5HP motor and OP intends on using (2) 1HP motors, he'll need to drastically slow down the speed of the compressor by using 1HP motor pulleys that are about 1/3 the size it currently is on the 5HP motor.

The issue becomes slowing down the compressor to the point where it could potentially damage it due to lack of lubrication (assuming it is a splash oil lubrication system).

Then the next major obstacle is how to mount (2) 1HP motors. You'd almost need one motor mounted on each side of the compressor (assuming compressor flywheel has a two v-belt drive system), one v-belt for each motor. Motor brackets would need to be slotted to take up belt slack.

 

[hempdiddy]

It looks like the pump runs in the 300 - 700 RPM range. http://www.aircompeq.com/ka.htm

Maybe I size a pulley for just a single 120v motor and get it to run around 350-400 RPMs and confirm it with a tach. Even at 400 RPMs, I'm probably gonna get about 12 CFM at 90 PSI.

The thing is, the pump spins freely by hand, and I have some spare 120v motors. The whole Kellogg American compressor cost me $60 from a failed auto shop. I'm in this at almost no cost and if I can get even 10+ CFM out of this for $60 and a little bit of my time, that's a home run in my book.

 

[scw1991]

It looks like the pump runs in the 300 - 700 RPM range. http://www.aircompeq.com/ka.htm

Maybe I size a pulley for just a single 120v motor and get it to run around 350-400 RPMs and confirm it with a tach. Even at 400 RPMs, I'm probably gonna get about 12 CFM at 90 PSI.

The thing is, the pump spins freely by hand, and I have some spare 120v motors. The whole Kellogg American compressor cost me $60 from a failed auto shop. I'm in this at almost no cost and if I can get even 10+ CFM out of this for $60 and a little bit of my time, that's a home run in my book.

You are not going to get anywhere close to 12 CFM @ 90 psi output from a 1HP motor. You'll get 4 CFM @ 90 psi at most. In addition you'd have to slow the pump down well below 400 RPM (closer to 100-150 RPM) to keep from overloading a 1HP motor and this would surely cause the compressor to self destruct due to lack of internal lubrication.

Rule of thumb for reciprocating compressor output is 4 CFM/1 HP

 

[hempdiddy]

That makes me so sad.


Rules of thumb would tell me to just sell the compressor, it seems.

 

[kythri]

Buy a 5hp motor and run a 240V circuit, man.

Baldor, $350
Leeson, $250

You said you paid $60 for the compressor? Add that $250-ish motor, and you're only in for $310...

Edited to add: After looking more, the Leeson might not be all that great, but the Baldor looks pretty good...

 

[scw1991]

sure, it's a pain in the *** to run new wire but they make 8/2 Romex w/ ground and the cost is about $1.75/ft.

I've built up a few compressors with the L1410T or L1430T Baldor 5HP 1725 RPM 184T frame motor. One thing you'll also need is a dedicated mag motor starter (NEMA 1.5) like the Square D 8911DPSG32V09.

For $60, I'd just hold onto it for a rainy day until you have the time/money/resources to run a dedicated 230v line.

 

[MacMcMacmac]

How, exactly?

Are we talking adding some kind of differential to this?

The same way putting two locomotives on a heavy train doubles the pulling power.