View Full Version : Air compressor motor

11-04-2006, 04:03 PM
I have the chance to get a large air compressor for free. The present owner states the motor on the machine is damaged and a new one is needed. I have looked the compressor over and it is in great shape. The motor is a 5 hp compressor motor nothing fancy. The capacitor (sp) has been torn off the motor and that looks to be the only damage. If the motor has to be replaced can a new one be had cheap. I have done some looking around and everything seems to be going for around $300.00 for the motor. I can buy a similar new compressor for a couple of hundred more. I would like to get into this compressor as cheap as possible any help would be appreciated.

Al Messer
11-04-2006, 04:15 PM
If you can get it for FREE, haul it home, remove the motor and take it by an electric motor shop and have it checked out. It may be that you will only have to replace the Capacitor. Also, the fellows at the shop may be able to point you to a cheap, used replacement motor if it comes to that.

Alistair Hosie
11-04-2006, 06:27 PM
Wouldn't you be morraly obliged to tell the present owner of your findings I E the motor is not dead just needs a new capacitor I personally couldn't sleep at night thinking I was taking advantage of a decent mans generousity.

11-04-2006, 11:56 PM
All a capacitor does in a normal single phase motor is provide phase shift to start the motor in the right direction. Staggers the pulse to give it direction, that is why you can change the motor direction by changing the capacitor-winding logic from 120 to neutral, and other end (5&8) ?? I can't remember at the sec.

YOu can test the motor and points *(that open when the motor spins up to speed) by isolating the wiring if it is loose, taking the belt off, applying power as you spin the motor.. It will start in either direction and run if nothing else is wrong..

THE compressor I had the points corroded. I polished them with a slip of paper till they were shiny once again. That was about six months ago. I bought a V4 7.5 hp compressor from www.eatoncompressors.com and love it.. THE other one was a joke next to this behemothe. UNLOADING it liked to have hurt me thou. It weighs in about 1800lbs.

We did more on testing a single phase motor on Adrians site in the cnc section.

11-05-2006, 06:08 PM
Well got the compressor home today. The motor is shot and the head gasket needs to be replaced. I hooked up a 1 hp motor to the compresser and the pump seems to work well, with the exception of the head gasket (minor leak issue). The 1 hp motor can pump up to about 50 psi and them it starts to slow down and stall. What type of oil goes into the compressor. I need to change the old stuff out. THe compressor is a Magna Force (made my Coleman). Any idea where I can get a 5 hp motor new or even rebuilt. Over all the compressor looks pretty good.

Bill Pace
11-05-2006, 06:36 PM
Hey Gear...
My 5hp, 2 stage motor recently threw craps. I floundered around googling and wasnt being inspired, so tried flea bay and immediately got some good prospects. Ended up ordering one from .....
ummm...around $250? -- I think it was, -- a competitive price from those I had looked at. and they shipped it really quick, packaged well, etc, etc.

If that compressor is a 2 stage---and with a 5hp on it, it prolly is---then youre well ahead by fixing it.

11-05-2006, 06:43 PM
The eaton site I listed above also has parts, motors.. real motors.. Real pumps.

I am impressed about everything about my compressor except the paint job. It has never hardened. Compressor is starting to show rust around the edges of the fins, it is inside a building.

I could not build one as cheap as I bought it, Seems was near 1500 or so. FOR a compressor you can sandblast with, has a continous run valve, IS quieter than any compressor I have ever been around, it is actually a ten horsepower pump turning slower on a 7.5 hp motor.

It should outlast my need without further investment. IT Did require running a 50 amp service to it. #6 wire and 50 amp breaker.

Mike Burdick
11-05-2006, 07:33 PM
David brings up some good points. Before you buy anything first determine what SCFM you need and what costs you are willing to pay for it. If the compressor can deliver an adequate SCFM with a smaller motor geared down (smaller pulley on motor or , if possible, larger pulley on pump) than go for that. The sound will drastically be reduced! This way you can use it for all the work that doesn't require the high delivery air volume - which is probably 90% of time.

Since this is basically a "found compressor" I'd try to get it to work for me as inexpensively as possible - even if it doesn't meet all your needs. Also, if you decide it needs the 5-hp motor, be patient, and I bet you’ll find a good used 5-hp motor for a good price.

11-06-2006, 05:02 AM
Never thought about changing the pump pulley. I have a large pulley at the house so that is a really good idea. One thing I see is the pulley that is on the compressor is a really heavy duty type. The pulley I have is a thinner material but larger (ID). Do you guys see a problem with the lighter weight of the replacement pulley.

11-06-2006, 07:47 AM
Forrest Addy wrote a good bit about air compressors round here or on pm , but also i think in one of the Magazines.

11-06-2006, 09:12 AM

Many compressor pulleys have a fan built into the spokes to blow cooling air over the compressor fins. If the old one has them, the replacement should have them or rig a separate fan.

11-06-2006, 12:15 PM

Have you examined the motor? If the windings aren't fried or damaged, it likely can be repaired. A motor shop can determine this or you may be able to find out yourself.

The capacitor(s) are used for STARTing (to provide phase shift on single phase motors), and if there's a second cap it would be a RUN capacitor. Some motors have the run cap as well as the starting cap.

Suppose your motor is a capacitor start motor (only one cap).
You can test the motor, less the starting cap, by powering it and manually spinning the shaft to get it going; it usually will run which ever direction you start it turning and run up to full speed. If it continues to run without getting hot, the motor is prob OK. Then you need to replace the start capacitor.

Jack C.

J Tiers
11-06-2006, 12:51 PM
Since the pump pulley often is a special, what about the motor end?

I put a 3/4 HP on an old Craftsman A/C that came with a 1.5HP. I then turned the motor pulley down proportionately to compensate. It has a serpentine belt, which is easier to use with small pulleys than a V-belt.

I needed to run it off a 2500watt inverter.

11-06-2006, 07:26 PM
I have looked at the motor and the windings appeared to have been fried. I noticed the copper windings at the rear portion of the motor were fused together by and electrical arc, and that's how I determined the motor needed to be replaced. Both of the capacitors are exposed and are hanging from the wires that attach them to the motor. The motor has a strong burnt smell also. I just took it for granted the motor needed to be replaced after seeing all the problems with it. I did not try to hook the motor up to any electrical current to confirm the motor was shot, but the way the motor looks and smells made me to believe it was a goner.....

I will try to forward some pictures If I can figure the system out......

11-06-2006, 08:55 PM
The overriding all important first step is to fiigure out why the old motor burnt so the new motor doesn't do the same.Chances are there is a reason it fried.I would check the unloader valve if it has one and the check valve which it must have and also the pressure switch before going any further.A stuck valve or welded contacts in the pressure switch are all suspects.

Also,post if you will the motor nameplate HP,voltage,rpm,amps,shaft size and frame size.That might be a 5hp motor,and it might be "rated" at 5hp,there is a big difference.

As a word of caution,I have been down this road before,that might be a good compressor and it might be a barbeq grill in disguise.I wouldn't spend too much time or money on it just in case.

11-07-2006, 04:50 PM

Well I just tried sending pictures not sure if it came out.

11-07-2006, 04:57 PM
As you can see from the pics the motor on the compressor is a 1 hp. I put it on just to test the pump. The other motor is the one that was originally on the compressor. The photo that shows the back portion of the motor was taken to show the electrical arcing that took place in the motor (not real clear). The capacitors were already off the motor and hanging free. I originally thought it was a 5 hp motor but it may be a 6.5 ????

11-07-2006, 08:24 PM

Okay,see where the HP rating is listed as "SPL",that's short for of course,special(I.E.mfgs are lying about the HP on the tank)the term"compressor duty" lets them off the hook legally.

Notice the amp rating is 15 at I am assuming 115vac,at best it's a 2hp motor,at worst a 1.5hp.

It's also marked as a 56 frame,although on a compressor that's not gospel,if it has a 5/8" shaft it is most likely a 56 frame,bigger than 5/8" all bets are off.

A suitable replacement motor can be had on sale from Surplus Center,item #10-2319-H,$89.95+shpg.It matches the specs pretty close,1.5hp,115/230 vac,reversible,56H frame,14.4/7.2amps 5/8"D x 1-3/4"L shaft.

I wouldn't sink too much into it though,at best it's a $500 pump new.

11-08-2006, 02:42 AM

Thanks for the information. After reading your post I thought about just putting on a 3 hp motor from a local discount tool store. It can be had for $99.00 plus tax. For what I will be doing with the compressor I believe that will be just fine. The only real work I would like the compressor to do is run a small sand blasting cabinet, you know one of the small Clark brand cabinets.

THe pump has a small leak and a new gasket will fix that, but all in all if the 3 hp motor works then the compressor project will be a success.

11-08-2006, 02:57 AM
Just a note of caution. On some compressors the "pulley" may also be a flywheel (balance problem...) & act as a fan for an aftercooler. Does it have spokes pitched like fan blades & a coil of tubing or finned tubing behind it ?
Preferrably, change the motor pulley to change belt ratios. Probably easier to do, anyways. Don't go to too small a pulley as that will affect belt life & drive efficiency. (Check a handbook on drives, Gates used to have a good one.)

11-08-2006, 04:43 PM
Can standard motor oil be used in the compressor pump??

11-10-2006, 04:15 AM
Just a note on the compressor, took the motor to a local compressor repair shop. The sales rep took one look at it and called it dead. He read the motor spec's to be a 1.5 or 2 hp at the very best. Mentioned to the sales rep that I was going to HF and buy a 3 HP for $99.00 to run the compressor. He had not problems with that. I did buy new upper and lower gaskets for the pump and installed them yesterday. Looks like I'll have around $130.00 in a $600.00 compressor not to bad all in all. Thanks to all who made comments and suggested how to make the fix.....gear

11-10-2006, 12:02 PM
Good deal...glad it is working.

To answer a late question you asked about using motor oil....don't do it.

Compressor oil is low in ash content and some other stuff that will carbon the valves. Yep...really...carbon. Not from combustion,but from oil hitting the backside of the hot valves. Additionally, motor oil has detergents to keep wear particles in motion so they can be removed by an oil filter in a pressurized oil system. In a non-filtered, non-pressurized system, you want those particles to settle out rather than remaining in circulation where they can cause more wear.

I let my shop cool down to maybe 40's or so when not used for a while. I went to using synthetic compressor lube as it doesn't turn into tar at cold temps. Most compressors are splash lubricated....and need the oil to be able to splash. My Ingersoll-Rand came with a 2 year warranty good only if synthetic lubricant was used.


11-11-2006, 09:47 AM
I am so tickled this has worked out for you. I was pretty tickled when I fixed my lil blue compressor too. I had already ordered the big un thou.

YOU seem to have learned a great deal from it. AIR hp is a actual function of input power. It takes energy to make air, and heat, and noise.

Right now I got gremlins in a pc here giving me a fit.

11-11-2006, 02:22 PM
David, your right I have been taught a few things here. I belong to other like sites but even when I ask a foolish question here, someone always responds with help. Even the very seasoned guys on this site still give advice and help.
Again thanks for eveyones input.

11-15-2006, 05:25 PM
All right guys here we go again. Looks like someone was into the pressure setting screws on the pressure switch. The pressure switch has a single metal lever that operates the compressor pump and starts the process off. Once the plastic cover was removed I observed the pressure switch to have two screws one metal and one which appears to be plastic or the like. The low end pressure or the point were the compressor will start to refill the tank is set at 85 psi. The tank will fill past 140 psi and that makes me a little nervous. Really I have not let the tank go past 140 psi before I will turn the compressor off.

Question: which set screw sets the low end pressure and more importantly which one will set the high end pressure and which way do you turn the set screws to adjust to the desired pressure.

Mike Burdick
11-15-2006, 05:53 PM

When the tank is at its highest pressure and shuts off, turn one of the screws gradually one full turn to the right until the motor starts; if nothing happens turn it back to where it started and slowly take one full turn to the left. If still nothing happens, return the screw to where it was and try the same with the other screw. If again nothing happens, then start over - only this time make two turns. Once you find when the motor starts you'll have the information you asked about.

Glad to hear the compressor is in good working order and the deal turned out well for you!

11-15-2006, 06:17 PM
If it's a cheapo, you may have to experiment (carefully). If it is a standard American made pressure switch (Square D etc.) then you may find docs on the company web site.

If that is a single stage compressor, then 140 may be a bit high....and I suppose could account for your original problem....a toasted motor. As the pressure climbs, so should the (mechanical) resistance the motor sees. I can't help but notice that most of the single stage compressors shut off at 125-130 pounds. Both of my reciprocating single stage compressors shut off at 130. If you are using a motor below rated HP as I think you indicated, I would err a bit on the light side.

I personally find that I end up setting the regulator at 90 PSI for the air tools and that is usually where it stays for everything else, so you likely don't need gobs of pressure for most stuff.

Just my $.02

11-15-2006, 09:04 PM
The trouble you will have is a fixed differential.This means the cut on/cut/off limits will be at a fixed differential and not adjustable.Most times those type pressure switches will have a 30-35psi differential.On your pump it should cut on at 85 and cut off at 120.The fact that it is cutting on at 85 and off at 140 may mean the pressure switch is shot in which case $40 for a new switch should fix it.Don't even consider a well pump pressure switch,too low a pressure rating.

11-16-2006, 12:57 AM
I have a compressor motor that has very similar tag on the motor 15 amps 220 volt hp = spl same manufacture i bought the compressor new it was advertised as 5 hp ,,, i Knew better i was guessing about 2 hp or a little more its been a good compressor i think mine runs between 80 and 115 lbs pressure, i have noticed as it gets closer shutoff it labors abit nothing troubling,,

11-16-2006, 05:48 AM
After reading all the posts on the set screws I did as suggested and was able to set the low end pressure and the high end pressure to were I wanted it to be (roughly 85 and 120 psi). The switch still works good so I saw no reason to pitch it or put anymore money into the compressor.

Thanks to all who took the time to give some advice. Gearhead

11-16-2006, 05:40 PM
I bought a nice little portable twin tank that was abused, for $35.

Many contractors run them onlong extension cords and burn up the switches.

I replaced my switch for a few bucks and now have 3 compressors that work.

If you have to use an extension cord, keep the wire size big and the length short. It's better on the compressor to use a longer hose, as opposed to a longer cord.

My extension cord is 10 gauge and 50 feet.