Compressor Motor Fried...

[krooser]

Came home a couple weeks ago to find my air compressor motor inoperative. Got a chance to look at it on Saturday.

The contacts on the pressure switch were welded together so I "unwelded " them and cleaned 'em up with a little emery cloth. Hit the breaker and "pop"... more welding... so I did a little snooping and found smoke coming from the motor... not good.

Did some searching on-line for new motor prices.... Amazon had the best price of $331.00. This is a 208/240V 3450 rpm 5 HP deal. Others were $349.oo and up. My local farm store was $499.00.

Today I took it to a local motor repair shop... he took it apart only to find burnt windings... lots of burnt windings. He's going to get a little more involved in it and let me know the cost to repair it. He sez these modern motors are built on fast assembly lines and they don't use enough laquer on the windings nor do they use any paper insulating materials so wind up with problems like I have in my motor.

If he can't repair it economically he can sell me a new one OR a better quality used motor from the days when they used better insulating techniques that avoided problems like we have here today.

He also recommended I use a 1725 rpm motor if I replace it. He sez it's a big load to ramp up a motor to 3450 and, in the old days, all the compressor motors were slower speed affairs. I can change the pulley to get back some of the lost air output if I change the motor speed.

BTW... the motor is an AO Smith Century. I'm lucky to have 5 motor repair shops within a 1/2 hour drive of my shop.

Film at eleven...

[Chris Adams]

Well, say you have a compressor that needs to turn at 1200 RPM and you have been running a three inch pulley on a 3450 to get it there. OK, no problem. Now you put a 1725. Turns half as fast, so you need to get a 1.5 inch pulley. Now it turns at 1200 right? Thus the startup load is exactly the same. The motor turns half as fast but must make twice as much power.

Don't know if some motors are just better because they turn slower, but if the compressor needs 1200 RPM to make 5 HP (or whatever, just examples) then it takes the exact same draw.
Now you could under drive your compressor, say put a 2.25 pulley on it (again, just an example) and it will turn that much slower. Easier on the motor, sure, but not gonna make the CFM.

3450 is no amazing amount for the motor to turn. Bad motor can happen at any RPM, if the motor was poorly made to run at 3450, then you could get a poorly made motor that runs at 1725.

When I thought I might need a 5 HP motor I checked local shops and found one guy selling refurbished with 90 warranty for about 125. Found them on e-bay new for about 225. Then found them on Craig's list from vendors across the state for 90-110 some new some used, and most would ship.
So you might just look around and you might want to think about pro-actively cooling that new motor.

[krooser]

The short circuit was caused by vibration wearing away the laquer coating on the field windings. The thin coating didn't hold up.

I agree that 3450 is not a problem to achieve. But everytime the motor starts it starts under load when on a compressor. It's like having a load on a grinder/lathe/drill press, etc. everytime it starts... or having your car in gear when you hit the key. It takes a split second for a motor to spool up to speed... the compressor motor is always under load. There are "compressor duty" specs for these motors... I have a hunch that's why.

Many high quality compressor manufacturers run their pumps at lower speeds to help the unit live (and make it quieter). My friend's Curtis uses a 1725 motor... maybe I can swap the 5HP for a 7.5 and keep it at 3450. That might help with handling the load at start-up.

I'll take a ride over to the local air compressor shop tomorrow... Midwest Air Compressors. they do every possible commercial and industrial application. I'll get his feedback. I would have bought my unit from them but I got a good credit deal from NAPA when I bought mine. And "saved" a couple hundred bucks. That may turn out to be a case of penny-wise... pound-foolish.



The motor guy did

[rhandwor]

The motor speed is based on the number of poles and has nothing to do with how well its built. Its true an older motor which is rewound is a better motor as the they used extra and were not computer designed.
2pole 3600rpm
4pole 1800rpm
6pole 1200rpm
8pole 900rpm
10pole 720rpm
12pole 600rpm
This is using 60 cycle power

[krooser]

The motor speed is based on the number of poles and has nothing to do with how well its built. Its true an older motor which is rewound is a better motor as the they used extra and were not computer designed.
2pole 3600rpm
4pole 1800rpm
6pole 1200rpm
8pole 900rpm
10pole 720rpm
12pole 600rpm

I didn't know how motor speed was controlled...and I forgot to ask him today.

Thanks for that info.

[TheToolMan]

We have a baldor in stock that i think would fit your application. I can give you 10% off the price so it would be about $295

http://www.plccenter.com/buy/BALDOR/M3613T

If that dosent work out for you let me know or look around we probly have something pretty cheap. I repair motors for a living and i have never found a motor under $500 worth rewinding unless it is a special motor. I takes ALOT of work to do a rewind

[krooser]

We have a baldor in stock that i think would fit your application. I can give you 10% off the price so it would be about $295

http://www.plccenter.com/buy/BALDOR/M3613T

If that dosent work out for you let me know or look around we probly have something pretty cheap. I repair motors for a living and i have never found a motor under $500 worth rewinding unless it is a special motor. I takes ALOT of work to do a rewind

I'll let you know what the guy sez next week when I get back to him.

Thanks.

[nissan_crawler]

I agree that 3450 is not a problem to achieve. But everytime the motor starts it starts under load when on a compressor. It's like having a load on a grinder/lathe/drill press, etc. everytime it starts... or having your car in gear when you hit the key. It takes a split second for a motor to spool up to speed... the compressor motor is always under load. There are "compressor duty" specs for these motors... I have a hunch that's why.

Many high quality compressor manufacturers run their pumps at lower speeds to help the unit live (and make it quieter). My friend's Curtis uses a 1725 motor... maybe I can swap the 5HP for a 7.5 and keep it at 3450. That might help with handling the load at start-up.

I'll take a ride over to the local air compressor shop tomorrow... Midwest Air Compressors. they do every possible commercial and industrial application. I'll get his feedback. I would have bought my unit from them but I got a good credit deal from NAPA when I bought mine. And "saved" a couple hundred bucks. That may turn out to be a case of penny-wise... pound-foolish.



The motor guy did

First off, if your compressor motor is starting under load, you need to fix your pressure switch, or install one with an unloader.

Second, high quality compressors have pumps DESIGNED to run at that lower speed. It's not just the motor running slower, the pumps do too. Have you looked at the cost of pulleys? you might as well buy a new compressor if you're going to replace the motor and pulleys.

Personally, I think this guy doesn't sound all that bright, but that's just me.

You could also check weco-group on ebay, they have good baldor prices.

We have a baldor in stock that i think would fit your application. I can give you 10% off the price so it would be about $295

http://www.plccenter.com/buy/BALDOR/M3613T

If that dosent work out for you let me know or look around we probly have something pretty cheap. I repair motors for a living and i have never found a motor under $500 worth rewinding unless it is a special motor. I takes ALOT of work to do a rewind

I agree. Then you're trusting the rewinder and their warranty also, which may or may not be good.

[Major Ramifications]

I agree with everything Nissan Crawler said. Compressors do not start under load.

[Chris Adams]

If you cut the speed of the motor in half, you would need a pulley twice as big on the motor. 6" in your example.

I have 2 Quincy 5HP compressors, One with a 3600 RPM motor and a 4" Pulley, the other with a 1800 RPM motor and a 8" Pulley. Both start about the same.

Of course would. I just had to gear my motor down on my 3450 WEG to power a much slower compressor, that maxes at 1200.
My point was that once you change the pulley you are starting at exactly the same load with the 1725 as a 3450 with twice the gearing advantage.
My point is, that slowing the motor down and then changing the pulley size to equal the same RPM is a wash, does NOTHING.

As has been posted by two others, the unloader valve would negate any difference anyway.

What we are trying to be too polite about is the guy that is telling him the 3450 needs to be replaced by a 1725 is full of it.

[krooser]

The guy that owns the shop has been in business since 1970... I would imagine he has SOME credibility! He's an authorized service center for many major motor manufacturers... I'm not arguing just stating a few facts. he seems pretty knowledgeable.

If my switch should have an unloader why can I build up pressure in my tank simply by manually turning the pump? I can spin it ten times, open the valve, and I have pressure?

The only valve I've ever seen on one of my compressors is the one on the discharge side of the tank... and a pressure switch which only senses pressure and shuts off the motor at a pre-set pressure...

[Chris Adams]

There may be communications errors creeping in.
To be clear, a motor turning at 1725 and at 3450, with the same size pulley the slow motor would be under less load, because it is only gonna put half as much on the job. If you swap the pulley till it makes the same CFM, it takes the same horsepower/load at startup. This is simple physics.
My reference to a slower motor 'possibly' working better just means the guy may have/prefer a slow motor because he happens to sell a slow motor he believes in.
But the load is the same. Period. If you cut the speed in half, you must double the size of the pulley, thus the load is the same.


OK, the other thing is the unloader valve. Not all compressors have an unloader valve, just any unit you can't carry in your arms...
The unloader valve (and there is a mile of posts/descriptions/pictures on Google) is what lets the pressure out of the compressor head. It's a leak, built into the line where the air goes into the tank. The 'leak' is plugged with a Schrader valve, like a tire core.
The way it works is air is compressed by your compressor. That air goes down a tube into the tank, past a one way check valve. That valve keeps the air in the tank. If your tank leaks down it's usually time for a check valve.
Anyway, the unloader/built in leak is so that when the motor shuts off the pressure in the line between the compressor and the tank leaks out. That's the 'hissing' sigh sound right after your compressor shuts down. It can be as simple as an actual leak in the line (poor design) that lets a tiny bit of air escape as you run it up, or it can be a nice valve with a switch in your D-Switch box, most common.
Without it, a compressor might be starting on the compression stroke, with air compressed in there. Motor ain't gonna hack it.

Even with an unloader valve you are asking a compressor to start hard if the piston is on at the bottom with the valve closed when it starts. Piston on the top starting to pull air in would be easy to spin the first split second.
So Compressor motors may have to take an extra load on startup even with an unloader valve.
But without an unloader valve, about one start in two or three would be straight up hill for the motor.

[krooser]

There may be communications errors creeping in.
To be clear, a motor turning at 1725 and at 3450, with the same size pulley the slow motor would be under less load, because it is only gonna put half as much on the job. If you swap the pulley till it makes the same CFM, it takes the same horsepower/load at startup. This is simple physics.
My reference to a slower motor 'possibly' working better just means the guy may have/prefer a slow motor because he happens to sell a slow motor he believes in.
But the load is the same. Period. If you cut the speed in half, you must double the size of the pulley, thus the load is the same.


OK, the other thing is the unloader valve. Not all compressors have an unloader valve, just any unit you can't carry in your arms...
The unloader valve (and there is a mile of posts/descriptions/pictures on Google) is what lets the pressure out of the compressor head. It's a leak, built into the line where the air goes into the tank. The 'leak' is plugged with a Schrader valve, like a tire core.
The way it works is air is compressed by your compressor. That air goes down a tube into the tank, past a one way check valve. That valve keeps the air in the tank. If your tank leaks down it's usually time for a check valve.
Anyway, the unloader/built in leak is so that when the motor shuts off the pressure in the line between the compressor and the tank leaks out. That's the 'hissing' sigh sound right after your compressor shuts down. It can be as simple as an actual leak in the line (poor design) that lets a tiny bit of air escape as you run it up, or it can be a nice valve with a switch in your D-Switch box, most common.
Without it, a compressor might be starting on the compression stroke, with air compressed in there. Motor ain't gonna hack it.

Even with an unloader valve you are asking a compressor to start hard if the piston is on at the bottom with the valve closed when it starts. Piston on the top starting to pull air in would be easy to spin the first split second.
So Compressor motors may have to take an extra load on startup even with an unloader valve.
But without an unloader valve, about one start in two or three would be straight up hill for the motor.

You did a good job of explaining the "unloader" valve.

I looked at my original owners papers I got with the compressor. The parts breakdown clearly shows the unloader tube running from the exhaust side of the pump at the check valve.

I agree with you that you still can have a high load start even with the unloader. My shop has only been heated continuously for three out of the 8 years I've owned the compressor. Starting a cold compressor at +10F with 30 weight oil in the pump would only add to the load.

I also read where the manufacturer, American IMC, recommends a mag starter on all motors over 3HP.... but the ad for my compressor (from the catalog) clearly said "no mag starter required"... so much for honesty in advertising.

I've got an idea that the many cold starts and not having a mag starter has contributed to the motor failure.

[Chris Adams]

I think you have it surrounded.
8 years, with 5 years in the cold could easily have worn that motor down.
Sure the guy is right about vibration but you know, 8 years is a long time.
I think you should just put a used/rebuilt motor on it and don't worry about it.

Changing a pulley is not expensive, by the way, but it is a pain to do. Mostly a pain to locate a pulley just like you want, in just the size and shaft.
Took me two days of going around small shops here. Everyone wants to sell you a fractional horsepower pulley. Or one with too narrow or too wide a groove.
Got a good used one for about 15 bucks. A heavy steel pulley is good for about 100 motor's life time so used works fine.



You may want to check that unloader valve to make sure it’s working properly. That could also contribute to early motor death.

[krooser]

I figured it was time to change the pump oil while I was fiddling with this thing. I looked up an old thread about pump oils and got some info saying to use 30 wt. non-detergent and some say to use automotive oils... even Mobil 1.

After I found my owner's papers I looked it up and IMC sez use 30 wt non-detergent compressor rated oils... automotive oil will carbon up the valves.

Luckily my local farm and barn store carries the stuff... at least I have clean oil in the pump!...

[mkdive]

You did a good job of explaining the "unloader" valve.



No BS on that. I learned something new today! :thumbup:

[Major Ramifications]

Changing a pulley is not expensive, by the way, but it is a pain to do. Mostly a pain to locate a pulley just like you want, in just the size and shaft.
Took me two days of going around small shops here. Everyone wants to sell you a fractional horsepower pulley. Or one with too narrow or too wide a groove.
Got a good used one for about 15 bucks. A heavy steel pulley is good for about 100 motor's life time so used works fine.

Grainger is the place I go to for pulleys (sheaves). They always have the one I need. Also, I have seen compressor pulleys wear out. They get loose on the shaft, and then the bore becomes "wallowed out" as the pulley rotates.

[krooser]

Well my motor was too far gone to repair so I bought a reconditioned motor for $150.00 with my trade-in.

I also took my drill press motor in to have him explain the wiring. I bought this on Fleabay several years ago and the wires were cut. I never even looked at it just put it in the corner. Now I want to get it working... found out it's a 3 phase motor... was sold to me a a single phase.

I wanted to keep this motor as it's got a 3 speed gearbox as well as 6 speeds on the belts. Now I'm going to use a new single phase motor that I had in stock... forgot i even bought it. the good news is i got it at Horrible Freight and it's American made... wonders never cease!

[krooser]

Got the compressor fired up today. The re-built motor I bought looked familiar... it was exactly the same motor as my old one except for the name...IR instead of AO Smith/Century.. hopefully I'll get another 8 years or more outta this one.

[Packard V8]

I wanted to keep this motor as it's got a 3 speed gearbox as well as 6 speeds on the belts.

FWIW, much preferable to keep the 3-phase motor with the gearbox and use a Variable Frequency Driver/Device (VFD) to run it on the drill press. Once you get up to speed, so to speak, on VFD, you'll want 3-phase on any machine which needs to vary output speed and most which don't.

thnx, jack vines

thnx, jack vines.

[krooser]

FWIW, much preferable to keep the 3-phase motor with the gearbox and use a Variable Frequency Driver/Device (VFD) to run it on the drill press. Once you get up to speed, so to speak, on VFD, you'll want 3-phase on any machine which needs to vary output speed and most which don't.

thnx, jack vines

thnx, jack vines.

The power company wanted $1100.00 to run 3 phase to my shop... poverty kept me from doing it.

Now I may buy a rotary phase converter but I already have single phase motors for the three pices of shop equipment that originally came with 3 phase... verticle mill, drill press and belt sander.