Advise needed on replacing a compressor motor

[Lonstar]

A friend has a Mac air compressor, model ACP680V, don't know how old it is. I can't find anything about it online. It's an upright model, specs are 80 gallon, 175 psi max, 19.6 ACFM @ 100 psi, originally had a 6 HP motor. Don't know if it's a single or dual stage (how can I tell?) The motor went bad, was replaced with a Century 5 HP motor. After a period of time the 5 HP motor caught fire and burnt. Did some research on compressor motors and now understand why the 5 HP motor burnt. I have a chance to get the compressor for a good price so looking a motor options. The original motor is long gone so don't know what it was.

One of my uses for the compressor is to run a blast cabinet, so need big air. I was looking at a Baldor 7.5 HP but they're pricey, want to be sure it'll be ok. The burnt Century motor has a 56Y frame, 7/8 diameter shaft. I assume that's what the original motor was. The 7.5 HP is only made with a larger 184T or 213T frame, 1 1/8" or 1 3/8" shaft.

Are there any compressor gurus out there that can tell me if this is ok or make any other recommendations?

 

[md21722]

A quick Google search shows a Century 6HP 56Y frame:

https://www.amazon.com/dp/B007ATNHSG/?tag=atomicindus08-20

Leeson, Baldor, or WEG motors can usually be found on eBay from motor wholesalers or even sometimes Zoro for reasonable prices. But you are correct, generally the frame size and as a result, the shaft diameter, is going to change. You would need to look at the baseplate and see what is machined for to determine level of effort on using a different frame size. Then you'd need to replace the motor shieve for the correct diameter. You may find it to be lower cost to find a separate shieve and bushing than an exact diameter replacement shieve that is machined to the exact diameter of the motor.

In some cases its feasible to buy a adjustable motor mount base and bolt is to the compressor to plate.

You may also try to find out who the OEM of the MacTools compressor is and see what direct replacement costs would be. I'd expect them to be higher than a generic wholesalers. A parts list for ACP680V shows the motor is M182703 and the parts list is dated 2005.

If you don't need "6HP" worth of air, you may consider buying a cheaper 5 HP motor and adjusting the motor shieve to a smaller size so the compressor pump turns slow enough that it won't burn up a 5 HP motor.

These are just some ideas you can pursue. Other than finding a direct replacement motor, you are probably going to have to do some fabracobling.

Motors are not usually cheap, especially single phase over 5 HP.

 

[Lonstar]

You found more info that I did! I searched the P/N for the motor you provided, one of the results was the parts list you referred to. Strange thing is the results also point to a 5 HP motor for that model, not 6 HP. Attached is a pic of the label on the compressor that shows it to be 6 HP.

I'd like to upsize the motor, not downsize it. I assume a bigger motor won't raise the CFM output, but I was interested in doing it for 2 reasons. One, minimal chance of smoking it like the 5 HP Century, and two, will a bigger motor allow the compressor to maintain its peak CFM better?

I currently have an Iron Horse compressor rated at "6.5 peak HP". The label on the Marathon motor doesn't list the HP, just says "SPL" for HP on the label. Since it's 15 amps I'm guessing it's around 3 HP. Output is rated at 11 CFM at 90 PSI. A die grinder and a air sander outrun it, I wouldn't even attempt sandblasting with it.

If there's no gain in putting a 7.5 HP motor on the Mac over the OEM 6 HP, I'll just go with a 6 HP, 56Y frame motor. I've found none of the motor manufacturers make a motor larger than 6 HP in a 56Y.

 

[Bert_]

As far as real motor ratings go there are no 6hp motors. That's marketing HP. I would guess a real 5hp motor would be equal to original motor.

Did the second motor have overload protection? Did anyone ever check current draw to make sure the motor wasn't overloaded?

 

[md21722]

I think a true 5 HP would be running into its SF rating with a 19 CFM rating? The simplest thing to do is replace with the Century B387 as it will be a direct replacement without any modifications. A true 7.5 HP would take the load just fine, but generally require a mag starter and some adjustments to adapt to the baseplate...

 

[Milton Shaw]

I ran into a problem on my 5 hp compressor. First motor lasted about 12 years, second motor lasted another 8 years or so. The third motor started tripping overload within about 4 months. Would not trip if the belt was off the compressor. I think the compressor had bearings tightening up after running 3 or 4 minutes. Since the whole thing was over 25 years old by that time I just replaced the whole thing. I bought a Quincy from Northern. IT was a customer return that they had no idea what was wrong with it as they had no circuits for testing one. They gave me $500 off and a full warranty on unit. Motor was bad and Quincy sent someone 150 miles one way to replace no charge to me within one week. I am very happy with what I have now.

 

[Citation]

The compressor is on Mac's 2009 price list as a 5hp, 2 stage. Given the ratings I suspect it's a true 5hp with a magnetic starter. If you pay a picture of the pump I bet people here can tell you what it is. From there you can get the pump specs and the needed pulley ratio for what ever motor you get.

 

[brownbagg]

on compressor motors its rpm, get one that match the rpm of the first motor

 

[Lonstar]

As far as real motor ratings go there are no 6hp motors. That's marketing HP. I would guess a real 5hp motor would be equal to original motor.

Did the second motor have overload protection? Did anyone ever check current draw to make sure the motor wasn't overloaded?

My guess is no to both questions, especially since it caught fire.

Attached are pics of the fried motor I sent to my friend (current owner of the compressor) when I took it apart.

 

[Lonstar]

I think a true 5 HP would be running into its SF rating with a 19 CFM rating? The simplest thing to do is replace with the Century B387 as it will be a direct replacement without any modifications. A true 7.5 HP would take the load just fine, but generally require a mag starter and some adjustments to adapt to the baseplate...

Here's a pic of the label on the burnt motor. What's the difference between a B387 and the dead B384?

 

[Lonstar]

The compressor is on Mac's 2009 price list as a 5hp, 2 stage. Given the ratings I suspect it's a true 5hp with a magnetic starter. If you pay a picture of the pump I bet people here can tell you what it is. From there you can get the pump specs and the needed pulley ratio for what ever motor you get.

I wish I would've taken more pics of the compressor when I took the motor off and brought it home. Only pics I took were of the label I already posted, and the label on the pump which I'm posting to this reply. My friend owns a farm about an hour away, it's in one of the garages there. I'll take more pics next time I'm there (a few weeks or so from now)

You mention a mag start. I remember when I was looking at it I thought it might have had a mag start on it originally. No idea if it did, just noticed a bare mounting bracket that made me wonder if that was a provision for one. The current wiring didn't look like a mag start was bypassed. Also could have been an option it never had it to begin with.

 

[Lonstar]

Full disclosure - (I'll use made up names to make it easy for me to type it out)

Fred bought the compressor new. No idea how long ago and no idea how long the original and second motor were in service. When he replaced the original motor he wrote a date on the replacement with a paint stick - 4/30/13.
Fred is the owner of a 3 man car repair shop (him and two others that work for him). Don't hang around him enough to call him a friend, but enough to know him as a person. He's a nice guy and is honest. Definitely not a shady type guy, and he's very particular about his tools and equipment.

George is a close friend of mine. He and Fred are friends, they hang out together from time to time, and George will only let Fred work on his cars. When the second motor went, Fred bought a new compressor. Some time later he sold the Mac to George.

George paid him $200 for it. He's had it for at least 2 years, maybe longer, and hasn't done anything with it. He'll sell it to me for what he paid. That leaves me thinking I can have a much larger compressor for the price of a motor plus $200. I priced new compressors with like specs and think it's a very good deal, just have to determine what motor to put on it.

 

[Citation]

OK, based on your picture and this link here:
http://www.usedprice.com/items/power-tool/mac/compressor/acp680v-526883.html

I think that is the same as the Bel Air 318VN
https://www.aircompressorsdirect.com/manuals/318VN_Spec.pdf

The pump is the same Look at the part where it says "Extra Details" and it says " May have 1121001.... marked..."
https://mastertoolrepair.com/new-pump-t39-1312101036-p-633.html?osCsid=nhr9vsiah0g91i2hjh8trrn0n7
And now that I look at your earlier picture it also says T39 pump.

If you look at this Bel Air manual you can see the 318VN and then find the pulley part number that goes with the 5hp motor.
http://www.belairecompressors.com/BelAire-Compressor-Manual.pdf

This link suggests the pulley size is a 5.95"
https://mastertoolrepair.com/stationary-twostage-air-compressor-parts-318vl-318v-318vn-p-785776.html

So if your pulley is bigger than that it's likely the motor was getting over worked.

Of course, I'm telling you all of this with limited information but hopefully it helps.

 

[Lonstar]

Citation, you've done a lot of research....very much appreciated. Thanks!

I followed the links and understand your logic, and I mostly agree. I have 2 reservations though.

Your last link and the suggestion that the pulley should be 5.95". I have the pulley that was on the burnt 5 HP. It's 4.25". I think the mastertoolrepair site has a flaw. The first time I followed your link I saw 5.95", so we both saw that. Then I searched the part number of that pully and found it's 4.25" on other websites. I went back to your links several times to find that 5.95" number again and it's no longer showing up (at least I couldn't find how I got to it the first time).
No matter really, from your info I continued to research and came up with the same result - the pulley size for a 5 HP motor is 4.25". The pulley I have is a MasterDrive 2AK44H. 4.25", 3.66" pitch.

Second issue is the conclusion that it originally had a 5 HP motor to begin with. I believe it had a 6 HP motor as stated on the Mac label. When he replaced it with a 5 HP, he probably just swapped the pulley from the old motor to the new one. Then, as md21722 suggests, the 5 HP was running at its SF rating and couldn't handle the heat.

md21722 - when I asked what the difference was between the 384 and 387 I thought they were both 5 HP. Wasn't till I was comparing them today that I realized the 387 is 6 HP.


I don't know much about electric motors and SF ratings. Is a 6 HP motor really that much more powerful than a 5 HP? I see the 6 HP pulls more amps, I guess that says something. Last thing I want to do is spend $500 for a 6 HP Century motor and have it fail prematurely. Yet the install hassle and higher expense to go with the larger frame 7.5 HP Baldor isn't something I find appealing either.

 

[Citation]

I'm glad the links helped. My first thought would be figure out the size of the pulleys (pump and motor) used on the 5hp model. If they match what you have then you should (per Bel Air) be able to drive that pump with a 5hp motor. Can you see if any of those Bel Air models had what appears to be a 6hp motor? The failure of the 5hp motor could be dumb bad luck (but it might be work checking the pump too).

If you are still worried you could reduce the load on the motor with a change in pulley size. There are two ways to check the load. One is a power meter. The other would be checking RPM.

As I found recently with my furnace fan adventures, you can't always measure voltage and current into an induction motor and expect a good reading thanks to this thing called power factor. The small 1/3hp blower motor in my furnace draws nearly the same 6.5A under load and when the fan belt is removed! That's an artifact of power factor. The power into the motor isn't just Amps*Volts. Too bad as my plan was to look for current under FA to indicate the motor wasn't fully loaded. However, in my research I found that peak current measurements are more realistic on larger motors. In the end I rigged up my "Kill A Watt" type meter to the motor to read power consumption.

But you could measure motor RPM. If the motor is overloaded I would assume the speed would drop to under the rated speed. So long as it maintains rated speed I'm assuming it's not overloaded. A digital tach like this would be a good way to check
https://www.amazon.com/dp/B073X4ZY74/?tag=atomicindus08-20

If the motor is too slow look at getting a smaller pulley or, if compatible, an adjustable pulley.

Since I'm guessing this system doesn't have a magnetic starter I don't think you can really upsize the motor much.

BTW, have you looked at the contacts in the pressure switch? It's possible they got dirty/damaged due to arching. That's going to drop the line voltage a bit and can result in slow motor speeds and higher current. Thus the motor might have gone bad due to an issue with the switch. BTW, I'm a bit out of my league with that last bit. It does seem reasonable to me but I haven't fully thought it out so maybe I'm wrong. Still, it doesn't hurt to check.

 

[Toothaker]

There's really no downside to buying more motor than you need, except for the purchase cost. A 6 HP motor isn't going to draw more current than a 5 HP motor, given the same 5 HP load. Of course, if you really have a 6 HP load, you need the bigger motor.

I've not seen this discussed yet: what is the motor RPM supposed to be? Is it a 3450?

 

[The Cobbler]

... A 6 HP motor isn't going to draw more current than a 5 HP motor, given the same 5 HP load. ...

I disagree

 

[Lonstar]

One thing is certain - I need to go get the compressor, probably do it this coming Saturday. The Bel Air site showed the model 318 compressor is made with 5 HP to 7.5 HP motors depending on model. Makes me wonder if the plate already has provisions for a 184T frame. I have not checked the pump, was told it's fine, but need to confirm that for myself. I can check the contacts as you suggest. I like the digital tach and simply must have one (why don't I have one already?). I'll be priming one my way as soon as I'm done typing this.
The only reason I'm pursuing this compressor is for the CFM gain. Originally I wanted 22 CFM or more, but the (potential) low cost to repair the 19 CFM Mac appealed to me. If I have to downgrade output to make it work I'm better off looking at other compressor options.
I work for a company that has many machines, we use motors from <1 HP to over 150 HP. Pretty sure I can get my hands on a 184T framed motor to check mounting options. I asked one of the maintenance managers about a motor a few months back, he said they throw away motors less than 10 HP no matter what's wrong with it. Buying a new one is more economical vs the cost to have one of our guys spend the time to rebuild it.

Toothaker - RPM for the motors being discussed is 3450, not the 1750 variant. According to the specs on Century's website, the 5 HP was rated at 22 amps, the 6 HP at 24. I realize specs aren't always exact, I was just commenting that the 6 is rated higher.

 

[Citation]

With the correct pulley size you can use a 1750 motor. It might reduce friction loses on the motor pulley. Do look at the starter. I'm not sure what the rules are but I've it seems cheap 5hp compressors switch the motor with the pressure switch directly while the better ones use a magnetic starter. There has to be some reason to add the cost and my guess is reliability.

 

[Lonstar]

I'll probably add a mag starter no matter what motor I put on it. Friend that has compressor and I have been texting.....might go up there after work tomorrow and get it. We'll see.

 

[Toothaker]

Yes, a 6 HP motor draws more current - if the load is there. Since your compressor originally needed a 6 HP motor, get a 6 HP motor. Don't monkey with it by installing a smaller motor and changing a pulley. The CFM capacity and/or the duty cycle of the compressor will be lower if you do that.

My prior comment was about installing a motor that was too large for the compressor. In this case, 6 HP is not too large. As Goldilocks said, it is just right.

 

[Lonstar]

Found a Leeson 7.5 HP motor coupled direct drive to an Atlas Copco LE7 compressor on craigslist. Compressor is bad, motor is fine, price was good, I bought it.

The motor has a 215T frame, have to make a mounting plate that will allow it to be positioned about 2" rearward (when facing the belts) for the pulleys line up. Other than that it's a matter of getting the correct pulley for it.

Specs on the Leeson - 36 amps @ full load, 1.15 SF (40.5 amps), 1740 rpm.

The T39 compressor rpm range is 700-1400 rpm or 960-1350 rpm depending on the website. Specs are (states motor rpm is 3450, pulley is 15") - 7.5 HP, 27.6 amps average, 32 amps full load. Compressor rpm with this HP and amperage isn't listed, I'm assuming it's to run it at the upper end of the range.

I'm trying to decide which pulley to put on the motor. Obviously I want max CPM but also don't want to stress the compressor. A 10.25" pulley would spin the compressor at 1189 rpm, an 11.25" 1305 rpm and a 12.25" would be 1421 rpm.

I'm leaning toward an 11.25". The motor should be able to handle it without coming close to it's full load, but don't know if that's too hard on the compressor. Should I keep the rpm down with a 10.25" or is it ok to run it close to it's max rpm with the 11.25"?

 

[md21722]

I generally like to run them as slow as possible to deliver the desired CFM. As you turn a compressor pump faster it gets louder and hotter. A faster running (and hotter running) compressor is likely to introduce additional moisture into the compressed air. Maybe additional noise and the potential for additional moisture are not issues in your setup. I do not know. If the direct drive Atlas came with a tank, run it in series to the outlet of your existing tank. It will act as an expansion tank and drop enormous amounts of water out of the air.