Choosing an electric motor for air compressor build

[misterfixit]

Because I'm a glutton for punishment and aggravation, I'm building an air compressor. Short version is I got a great deal on a new 80 gallon vertical tank, to which I added a 5hp rated compressor pump. Since this was a complete from scratch build, I bought mag switch and pressure switch as well as other parts and pieces to get it done. I've build an intercooler between the compressor pump and the tank to try to get some moisture out. I had acquired a 5hp Baldor single phase motor, but it turned out it was shorted and needed to be rewound. I have a friend who has a motor shop who was going to do it at no cost, but I've been waiting over a year and I really need to complete my project.

So now I'm looking at buying a motor. I can make any mount work, and I can buy whatever size pully with the correct shaft size needed. I'm not tied into replacing anything since I'm starting from scratch. I can use any RPM, I'll just buy the correctly sized pully to make the pump RPM right.

I see smaller motors on Ebay for about $275, I think they are 56 frame size and 3450rpm. Is there any reason not to go with one of those? They are listed as pulling 20+ amps.
I guess I'm trying to figure out if I need a larger frame size for any reason. Are the smaller frame sized motors more prone to breakdown? Do they seem to last as long?

I'm old enough to remember when the rule of thumb to electric motors was to buy the biggest sized in the correct horsepower so it would last. Is that still valid?

 

[misterfixit]

I guess I should also mention an option of buying a 3 phase motor and using a VFD on it. I have no idea if that is a good idea with a compressor or not. The system will have a large volume being unloaded each time the motor turns off, so the motor won't have to start up with pressure against it.

 

[marinusdees]

Because I'm a glutton for punishment and aggravation, I'm building an air compressor. Short version is I got a great deal on a new 80 gallon vertical tank, to which I added a 5hp rated compressor pump. Since this was a complete from scratch build, I bought mag switch and pressure switch as well as other parts and pieces to get it done. I've build an intercooler between the compressor pump and the tank to try to get some moisture out. I had acquired a 5hp Baldor single phase motor, but it turned out it was shorted and needed to be rewound. I have a friend who has a motor shop who was going to do it at no cost, but I've been waiting over a year and I really need to complete my project.

So now I'm looking at buying a motor. I can make any mount work, and I can buy whatever size pully with the correct shaft size needed. I'm not tied into replacing anything since I'm starting from scratch. I can use any RPM, I'll just buy the correctly sized pully to make the pump RPM right.

I see smaller motors on Ebay for about $275, I think they are 56 frame size and 3450rpm. Is there any reason not to go with one of those? They are listed as pulling 20+ amps.
I guess I'm trying to figure out if I need a larger frame size for any reason. Are the smaller frame sized motors more prone to breakdown? Do they seem to last as long?

I'm old enough to remember when the rule of thumb to electric motors was to buy the biggest sized in the correct horsepower so it would last. Is that still valid?

If you have a friend who is a motor winder, ask his advice. It doesn't get any better than that. IMHO I would prefer a full 5 hp single phase motor, but they ain't cheap. If you're talking used, likely there are more three phase out there. I would use a 1750 rpm, three phase with a VFD or single phase with likely a mag switch. Whichever is cheaper. Probly the cheapest new motor will be a WEG. Ask your buddy his opinion, they vary. Opinions, and motors.

 

[KenC]

IME, the 3450rpm, 5hpSPL rated motors are noisy and short-lived. Not knowing what pump you're using makes it difficult to make a recommendation, but I wound up using a 184 frame real 5hp. Quiet and has run for years. It hasf since been upgraded to a 3ph version, but same rpm and frame.

The WEG mentioned above is a good choice. Had one years ago and it served me well.

 

[wyliesdiesels]

I guess I should also mention an option of buying a 3 phase motor and using a VFD on it. I have no idea if that is a good idea with a compressor or not. The system will have a large volume being unloaded each time the motor turns off, so the motor won't have to start up with pressure against it.

Compressors have unloader valves that drains the air pressure in the head so the motor does not have to start under load. So the unloading of the tank after the motor shuts off is inconsequential

 

[dave*99]

I would use a single phase motor.

 

[fitter30]

Need to look at service factor SF on name plate. 1 is a cheap motor fla can't be over loaded. 1.15 15% over name plate a very good motor 1.35 35% over name plate. Compressors have a max rpm run hotter= more moisture. Compressors set up for pneumatic controls run towards the minimum rpm less moisture run cooler. 3450 rpm pulley might be to large to fit between pulley and tank.

 

[misterfixit]

Compressors have unloader valves that drains the air pressure in the head so the motor does not have to start under load. So the unloading of the tank after the motor shuts off is inconsequential

Guess I worded it weird, this is what I was trying to say. my unloader drains the head and about 24" of 3/4" piping so the motor starts with no load other than turning the pump.

 

[misterfixit]

If you have a friend who is a motor winder, ask his advice. It doesn't get any better than that. IMHO I would prefer a full 5 hp single phase motor, but they ain't cheap. If you're talking used, likely there are more three phase out there. I would use a 1750 rpm, three phase with a VFD or single phase with likely a mag switch. Whichever is cheaper. Probly the cheapest new motor will be a WEG. Ask your buddy his opinion, they vary. Opinions, and motors.

The problem is we are friends, and he offered to rewind the motor I had for free. But that was over a year ago, and I've asked politely about it several times. He's busy and I cant really insist since he's not charging, even after i offered to pay and he refuses. I'm kinda stuck.

 

[wyliesdiesels]

The problem is we are friends, and he offered to rewind the motor I had for free. But that was over a year ago, and I've asked politely about it several times. He's busy and I cant really insist since he's not charging, even after i offered to pay and he refuses. I'm kinda stuck.

find another shop

 

[marinusdees]

find another shop

Every good shop is gonna be busy. But, a good shop should be willing to give good advice, even if they don't sell anything to you. Even though your burned up motor is a Baldor (read, probably the best there is) it is cheaper to buy a new (expensive) motor than rewind the Baldor. Best prices I've seen, and I haven't looked in quite a while, are were on ebay. And, I'm not a particular fan of ebay. It's OK.

 

[jblnut]

This thread hits close to home as I'm also looking for a different motor. I cobbled this compressor setup together with stuff I had been gathering for a while and the 2hp 1750rpm motor with a 3-1/2" pulley and IR2475 pump takes a loooooong time to completely fill lol. I found a couple 7.5hp motors on Surplus Center but they're a little pricey.

I have a couple 5hp 3ph Leeson motors and a 10hp Vevor VFD but can't get it all to work right. The VFD isn't really putting out enough power/torque to do anything but spin the motor with nothing but the pulley. I've been meaning to start a thread on it but it just hasn't happened yet ....

Sorry to maybe hijack

I have to put the guard back on yet .......

 

[marinusdees]

This thread hits close to home as I'm also looking for a different motor. I cobbled this compressor setup together with stuff I had been gathering for a while and the 2hp 1750rpm motor with a 3-1/2" pulley and IR2475 pump takes a loooooong time to completely fill lol. I found a couple 7.5hp motors on Surplus Center but they're a little pricey.

I have a couple 5hp 3ph Leeson motors and a 10hp Vevor VFD but can't get it all to work right. The VFD isn't really putting out enough power/torque to do anything but spin the motor with nothing but the pulley. I've been meaning to start a thread on it but it just hasn't happened yet ....

Sorry to maybe hijack

I have to put the guard back on yet .......

Sounds like your VFD is undersized??

 

[wyliesdiesels]

This thread hits close to home as I'm also looking for a different motor. I cobbled this compressor setup together with stuff I had been gathering for a while and the 2hp 1750rpm motor with a 3-1/2" pulley and IR2475 pump takes a loooooong time to completely fill lol. I found a couple 7.5hp motors on Surplus Center but they're a little pricey.

I have a couple 5hp 3ph Leeson motors and a 10hp Vevor VFD but can't get it all to work right. The VFD isn't really putting out enough power/torque to do anything but spin the motor with nothing but the pulley. I've been meaning to start a thread on it but it just hasn't happened yet ....

Sorry to maybe hijack

I have to put the guard back on yet .......

whats the model number on the VFD?

 

[luvtheheat]

Because I'm a glutton for punishment and aggravation...

After reading this thread, I'd have to agree with you.
Good luck!

 

[jblnut]

Sounds like your VFD is undersized??

whats the model number on the VFD?

Supposedly a 10hp VFD.....


I wanted it to run a 10" grinder but it'll only spin this really slowly as well. I've messed with some of the settings and can't seem to get it to do anything but spin the motor slowly.


This is the 5hp motor I was trying it on initially.

 

[marinusdees]

Supposedly a 10hp VFD.....


I wanted it to run a 10" grinder but it'll only spin this really slowly as well. I've messed with some of the settings and can't seem to get it to do anything but spin the motor slowly.


This is the 5hp motor I was trying it on initially.

In my (limited ) experience, VFD manuals are written in Chinglish Tech, and I can't wrap my brain around the parameters and adjustments. That being said, I have a lathe, a mill-drill, a bandsaw, and a drill press all running on VFDs. Motors to run on VFDs are supposed to be "inverter rated" but mine are not. The downside is, running the motor at slower than nameplate rating may result in insufficient cooling due to fan insufficiency. I have not had a problem, but I am a short time user. If your motor runs too slowly, you are feeding it too few cycles per second, aka, frequency, because, in an AC motor, frequency controls the rpm.
Since both motors run too slowly, I suspect you have not altered the frequency setting. The best I can offer is, read the manual. Your VFD is more than ample to do what you want. If you purchased it from an English speaking source, call them. If it's Chinese, maybe YouTube??
Good luck!!

 

[dave*99]

^^ This is correct - output frequency determines motor speed.
But before it works properly, it probably has some configuration settings that need to be set properly.
Things like single phase input
Input Line frequency
Output Line voltage

 

[Norcal]

^^ This is correct - output frequency determines motor speed.
But before it works properly, it probably has some configuration settings that need to be set properly.
Things like single phase input
Input Line frequency
Output Line voltage

I have a couple of machines with 200 volt 3Ø motors, the ability to adjust the output voltage of the drive when the supply is 240V, made my life much simpler, another parameter is being able to have 3-wire control with the drive. Avoid Huanyang and other garbage drives and life will be much better, they have limited support & poorly written manuals, which is part of the reason they are cheap.

 

[oldmachinenut]

This thread hits close to home as I'm also looking for a different motor. I cobbled this compressor setup together with stuff I had been gathering for a while and the 2hp 1750rpm motor with a 3-1/2" pulley and IR2475 pump takes a loooooong time to completely fill lol. I found a couple 7.5hp motors on Surplus Center but they're a little pricey.

I have a couple 5hp 3ph Leeson motors and a 10hp Vevor VFD but can't get it all to work right. The VFD isn't really putting out enough power/torque to do anything but spin the motor with nothing but the pulley. I've been meaning to start a thread on it but it just hasn't happened yet ....

Sorry to maybe hijack

I have to put the guard back on yet .......

I Love that picture! It reminds me of when my 4 kids were little and hung out in the shop with me.
The oldest is now 43 and owns his own shop, the youngest is 33. The time goes way too fast, cherish every minute.

 

[yyc_ranger_4x4]

I rebuilt an Ingersoll from late 80's. I went with that unit as the pump was setup for lower rpm....I put a Westinghouse 5hp Farm Duty 1 phase low speed (1750rpm) motor on it. That motor is a beast. I thought about using a VFD and a 3 phase motor, but that would put more parts into the system, and the need to slow the motor down with a VFD wasn't needed. I'd say if you have 1 phase power, stick to the 1 phase motor.....at least for the air compressor.

I did build a custom magnetic starter and intercooler setup, the setup includes a normally open valve on the drain of the intercooler that closes when the motor runs, a 1 second timer that drains the tank only when the motor stops, custom fans on the intercooler and a timer relay that runs them while the motor is running and 10 min after it stops. Included in the controller is a dry contact that will allow me to interlock it with the lights in the garage, no lights....no run.