Need help with air compressor

[kzitekmpls]

I have a 60 Gallon Sanborn air compressor. I recently installed a 125v 3HP motor. Compressor runs great with valve on bottom of the tank open.

When valve is closed compressor will build pressure to 50psi (according to gauge on compressor) then slow down until motor cannot turn and just hums.

I suspected a bad check valve and have replaced it with no improvement. I have double and triple checked wiring. Side note motor will not start if tank has more than 30psi in it. But starts when tank is empty.

I'm stuck, please help!

 

[930dreamer]

Is the pressure switch set point correct?

 

[kzitekmpls]

Is the pressure switch set point correct?

Pressure switch adjustment is hot glued so I can't mess with it. Used to stop at 120psi and kick back on around 50psi when I had the 220v motor on there.

 

[American Locomotive]

What horsepower was the old motor?

 

[kzitekmpls]

Old motor was 3hp as well. It's weird, it acts like it wants to keep running but motor can't turn over the pump once tank gets to 50psi

 

[American Locomotive]

Is the new motor the exact same speed, with the exact same size pulley?

 

[Jswain]

You're not running it with an extension cord? Or recently switched outlets? Do you have it wired on its own circuit? GFCI or regular?

I would check voltage at motor with it running.

 

[kzitekmpls]

Is the new motor the exact same speed, with the exact same size pulley?

Yes

 

[kzitekmpls]

You're not running it with an extension cord? Or recently switched outlets? Do you have it wired on its own circuit? GFCI or regular?

I would check voltage at motor with it running.

I can do that, I just don't understand why it would run fine without pressure in the tank if it was a voltage issue.

 

[Jswain]

Takes more power to make more pressure. 3hp is a lot to ask out of 120v, 18amps? If you have it plugged in to a standard lighting circuit it's probably not enjoying the light awg wire.

The motors no load draw(0 psi) would be much less, and as the pressure increases so does the amperage that it is drawing(or trying to draw) from the circuit. Ideally you just hit FLA or Fla x SF of the motor when your compressor reaches its cutout pressure. Hit that before cutout and you need to lower cutout or size down pulley on motor. If your wires are undersized, too long, crappy connections then the voltage will drop.

Would probably be happier wired to 240v if it's dual voltage and actually a 3hp motor

 

[micromind]

What was the amperage of the old 230 volt motor? Somewhere around 15 would be normal.

What is the amperage of the new 115 volt motor? Somewhere around 30 would be normal.

It's possible that the old motor is an actual 3HP while the new one is 3HP peak and 1 - 1.5 actual HP. Compressor motors are notorious for over-rating the HP.

 

[kzitekmpls]

Takes more power to make more pressure. 3hp is a lot to ask out of 120v, 18amps. If you have it plugged in to a standard lighting circuit it's probably not enjoying the light awg wire.

The motors no load draw would be much less, and as the pressure increases so does the amperage that it is drawing(or trying to draw) from the circuit. If your wires are undersized, too long, crappy connections then the voltage will drop.

Would probably be happier wired to 240v if it's dual voltage and actually a 3hp motor

I don't have 220v and that's why I threw this motor on it.

 

[kzitekmpls]

What was the amperage of the old 230 volt motor? Somewhere around 15 would be normal.

What is the amperage of the new 115 volt motor? Somewhere around 30 would be normal.

It's possible that the old motor is an actual 3HP while the new one is 3HP peak and 1 - 1.5 actual HP. Compressor motors are notorious for over-rating the HP.

New motor is 18amp. Hooked up to a 20amp circuit.

 

[micromind]

New motor is 18amp. Hooked up to a 20amp circuit.

18 amps is roughly 1.5 - 2HP. Certainly not 3.

If the old motor was an actual 3HP, then the new one is too small.

 

[kzitekmpls]

18 amps is roughly 1.5 - 2HP. Certainly not 3.

If the old motor was an actual 3HP, then the new one is too small.

So you think the motor just isnt strong enough for the pump? Anyway I can remedy this? Also why does it run fine until 50psi?

 

[micromind]

So you think the motor just isnt strong enough for the pump? Anyway I can remedy this? Also why does it run fine until 50psi?

Yes, I think the new motor is less powerful that the old one. Could easily be wrong though........

A compressor pump needs far less HP at low pressure than high.

It's possible that a smaller motor pulley would work but all compressor pumps have a minimum RPM, usually around 400 or so. Going slower would result in inadequate lubrication.

Are you positive the new motor is 115 volts? It sounds a lot like a 230 volt motor being supplied with 120 volts.

 

[kzitekmpls]

Positive that it is 120v

 

[micromind]

Positive that it is 120v

Ok, then it's simply too small for the compressor pump.

 

[The Cobbler]

new motor or used?
18 amp @120v motor is not 3 HP , maybe 2 at the most .
what size cord do you have on it, how long is it?
I suspect not enough HP to turn the pump with pressure on it . as mentioned, you can run a smaller pulley on the motor to lower the rpm, but you need to be mindful of lubrication .

 

[Jswain]

Can you take a pic of the old and new motor nameplate?

 

[RTM]

So you think the motor just isnt strong enough for the pump? Anyway I can remedy this? Also why does it run fine until 50psi?

You might want to read this thread, guy had exact same symptoms, except he was running a 220 v motor on 110v, and didn’t have enough power to get above 50psi.

His motor was only working at half ish power due to wrong voltage.

Your motor may not have enough power, period.

Compressor 230v with 115v plug?

I recently picked up a free Craftsman 4HP compressor which I was told was working fine and it runs for about a minute before bogging down to a standstill and then trips the thermal switch on the motor. The guy I got it from said he never actually used it but the neighbor who gave it to him used...

www.garagejournal.com

 

[Jswain]

Sometimes wiring a 240v outlet isn't as hard as some people think either, not saying it's the case here, but sometimes you just need a new breaker/wire/outlet.(Cheaper then new motor).Pictures of your whole setup could eliminate a lot of questions here

 

[cannuck]

A 3 HP motor at 120v is indeed 18.75 or so amps - but no way you will start that on a 20A breaker. I have a 1HP vac pump that will trip a 15A breaker when starting so needs 20A. So: the OP's new motor is questionable as to actual power. I say that by pointing out the advertised "horsepower" of vacuum cleaners. Maybe the motor supplier has sunk to the questionable depths of every other Chinese supplier?

What IS happening is the motor is way below the power needed to fill the tank - obvious because is stalling. My guess it is actually a 3600RPM motor, not 1800 so WAY over geared. Also with it starting on the 20A breaker it is not likely an actual 3 HP anything.

 

[American Locomotive]

A 3HP motor is not 18A.

A 3HP motor generates 2238 "watts" of mechanical power. That means at the very minimum, it would be 18.65A of electrical power. Except that would require the motor being 100% efficient. There is no such thing as a 100% efficient motor. A 3HP 120v motor will be more like 80% efficient. I would expect a current rating of no less than 23A.

Where did you get this motor OP? Can you post a picture of the nameplate.

 

[Hohn]

Sometimes wiring a 240v outlet isn't as hard as some people think either, not saying it's the case here, but sometimes you just need a new breaker/wire/outlet.(Cheaper then new motor).Pictures of your whole setup could eliminate a lot of questions here

I think I spent all of 60 minutes adding my NEMA 6-50R. Definitely one of those "why didn't I do this before?" kind of things.
50a of 240V is a big upgrade in ability.

If you have ready access to the main panel, room for a double-pole breaker and are putting the receptacle near the panel, it's pretty easy to do and not terribly expensive. The $80 of 6awg was the most expensive part.

 

[cannuck]

A 3HP motor is not 18A.

A 3HP motor generates 2238 "watts" of mechanical power. That means at the very minimum, it would be 18.65A of electrical power. Except that would require the motor being 100% efficient. There is no such thing as a 100% efficient motor. A 3HP 120v motor will be more like 80% efficient. I would expect a current rating of no less than 23A.

Where did you get this motor OP? Can you post a picture of the nameplate.

Totally agreed. That is why I mentioned a 1HP motor not being able to run on 15A (and I have a few of them). I should have been more careful about how I stated the amperage at 120v (plus pointing out that a lot of 120V circuits are no where near 120 - especially while starting - thus drawing a lot more amperage.

Nice to know we have more than a few people around here paying attention and keeping us honest.

 

[MovingAlong]

I have a 60 Gallon Sanborn air compressor. I recently installed a 125v 3HP motor. Compressor runs great with valve on bottom of the tank open.

When valve is closed compressor will build pressure to 50psi (according to gauge on compressor) then slow down until motor cannot turn and just hums.

I suspected a bad check valve and have replaced it with no improvement. I have double and triple checked wiring. Side note motor will not start if tank has more than 30psi in it. But starts when tank is empty.

I'm stuck, please help!

That's a sign of a weak motor...

So you think the motor just isnt strong enough for the pump? Anyway I can remedy this? Also why does it run fine until 50psi?

Yes, you need the proper motor for the application.

Because compressing air to 50 psi is the best your current motor can do.

Unless there are variables you haven't mentioned, your problem seems to summarize as: old 220v motor worked great, new 110v motor doesn't. Well, there you go...

 

[cannuck]

So you think the motor just isnt strong enough for the pump? Anyway I can remedy this? Also why does it run fine until 50psi?

What I think you are missing is that the motor load is not a constant 3HP. It draws current (i.e. makes power) relative to the load placed on it by the compressor head, and that load increases in proportion to the tank pressure. 50 psi is a LONG way from where a 3 HP motor would stall.

As others have asked: please post a pic of the data plate on your motor and we might be able to tell you a lot more accurately why it doesn't work.

Still placing my bet on it being a 3600 rpm motor.

 

[kzitekmpls]

I think I spent all of 60 minutes adding my NEMA 6-50R. Definitely one of those "why didn't I do this before?" kind of things.
50a of 240V is a big upgrade in ability.

If you have ready access to the main panel, room for a double-pole breaker and are putting the receptacle near the panel, it's pretty easy to do and not terribly expensive. The $80 of 6awg was the most expensive part.

Cant add a 220v circuit or I would have years ago. Old fuse panel in basement and compressor is in detached garage. When I have 10k for an upgrade I'll switch this over to 220v

 

[kzitekmpls]

Here is the motor tag as requested

 

[Jswain]

And how many FLA and rpm was the old motor?

 

[kzitekmpls]

FLA 17.2 and same RPM

 

[Jswain]

But FLA was at 240v or 120v? Was the other motor dual voltage?

 

[Jswain]

The one you just posted for example is FLA 18a @ 120v, or FLA 9a @ 240v

 

[kzitekmpls]

The other motor was not dual voltage. I moved and no longer have 220v. I tried to run 220v motor on generator and blew the motor. Threw a 120v motor on to use at current house.

 

[kzitekmpls]

The other motor was not dual voltage. I moved and no longer have 220v. I tried to run 220v motor on generator and blew the motor. Threw a 120v motor on to use at current house.

I tried to match current motor as close as I could to 220v motor except in 120v.

 

[DerekV]

That is maybe a 2hp motor. A real 3hp motor would be rated for 25-30 amps on 120v, and for that reason, they don’t really exist.

Regardless, I’m surprised it taps out that fast. It’s certainly not a heavy duty unit, but I bet something’s wrong with it aka faulty. Put an amp clamp on it to confirm.

 

[cannuck]

I tried to match current motor as close as I could to 220v motor except in 120v.

Did you check the wiring to see if it was set up low (125) or high (250)? If you are feeding it a half voltage into high wiring arrangement that could explain your problem.

 

[kzitekmpls]

Did you check the wiring to see if it was set up low (125) or high (250)? If you are feeding it a half voltage into high wiring arrangement that could explain your problem.

Triple checked it's wired for 120v

 

[The Cobbler]

Did you check the wiring to see if it was set up low (125) or high (250)? If you are feeding it a half voltage into high wiring arrangement that could explain your problem.

OP is positive it is wired for 120 , but I think he should double check/ tripled check even
edit op posted as I was writing...