Small air compressor

[ptonsparky]

Checking out a samll 5hp 3ph 480v air compressor today. Overload was tripped. Motor pulls 17 amps with compressor, 3 without. Motor megs fine, bearings a little dry but not bad. Compressor turns easy by hand. Will a dirty air filter cause an overload on a reciprocating compressor? Never thought about it until I was gone. Other option is that someone has increased drive pulley size at some time or another. It hasn't ran for about 1 1/2 years and new manager wants things fixed.

 

[augie47]

If the nameplate is actually 5 HP, 17 amps at 480 is considerbly high.
Was it 17 on all 3 phases ?

The ampere draw increases as the pressure does. You might check to see if someone has significantly increased the pressure switch setting, but even then 17 would be way out of line.

 

[cadpoint]

Checking out a samll 5hp 3ph 480v air compressor today. Overload was tripped. Motor pulls 17 amps with compressor, 3 without. Motor megs fine, bearings a little dry but not bad. Compressor turns easy by hand. Will a dirty air filter cause an overload on a reciprocating compressor? Never thought about it until I was gone. Other option is that someone has increased drive pulley size at some time or another. It hasn't ran for about 1 1/2 years and new manager wants things fixed.

MO
They can't just change the size, its a cause and effect relationship. This while effect the ratio relationship to the output, you need to find the orginal design specification. Go do some belt gear ratio problems, besides the fact that it's been sitting there for a while, and find out what it was doing as opposed to what they thought they were doing!

 

[ptonsparky]

If the nameplate is actually 5 HP, 17 amps at 480 is considerbly high.
Was it 17 on all 3 phases ?

The ampere draw increases as the pressure does. You might check to see if someone has significantly increased the pressure switch setting, but even then 17 would be way out of line.

This compressor is paralleled with another when it's disconnect is ON. No pressure switch at this tank location. Yep, amps goes up as pressure goes up. Question is does amperage go up if the inlet is blocked?

 

[ptonsparky]

MO
They can't just change the size, its a cause and effect relationship. This while effect the ratio relationship to the output, you need to find the orginal design specification. Go do some belt gear ratio problems, besides the fact that it's been sitting there for a while, and find out what it was doing as opposed to what they thought they were doing!

I have been thru ratios and what happens with two different managers and I don't know how many employees at this plant. A new manager started a couple weeks ago. I suggested that he research the original design of compressor. As to what they thought they were doing if and when they made changes, I can only guess.

 

[iwire]

This compressor is paralleled with another when it's disconnect is ON. No pressure switch at this tank location. Yep, amps goes up as pressure goes up. Question is does amperage go up if the inlet is blocked?

If the inlet was blocked the amperage would not increase.

Like Augie said, 17 amps unloaded for a 5 HP at 480 is out of line.

A fully loaded 5 hp 460 volt 3 phase motor should not draw more than 7.6 amps, unloaded it would be much less than that.

IMO your motor is toast or it is wired incorrectly.

 

[augie47]

Bob, I would be 99% "motor" other than it pulling 3 amps w/o the compressor. Still won't rule the motor out, but that reading sounds about normal.

 

[One-eyed Jack]

I would bet on a sheave change at the motor. Been on plenty of trouble calls and after 5-10 min of asking what had been changed. Uh,we needed more air so we upped the motor sheave by 1/2 inch. Air does not weigh anything so why does it take more hp to move more??????:-?

 

[GeorgeB]

Yep, amps goes up as pressure goes up. Question is does amperage go up if the inlet is blocked?

I hope someone wrote 17 down meaning 7 ... perhaps a random mark on the paper? That motor wouldn't last long, even if force air cooled.

With most air compressor designs, blocking the inlet will unload the compressor. I believe that there are some designs that you can damage the compressor by doing this, however; check with the manufacturer.

 

[ptonsparky]

Air compressor requires a 15hp motor, not the 5 they have on it now. Duh. No wonder it overloads. No way it has ever worked.

 

[augie47]

If they had set the pressure low enough the 5 might have worked :grin:

Is this a case of "nobody changed anything!" ?

 

[ptonsparky]

If they had set the pressure low enough the 5 might have worked :grin:

Is this a case of "nobody changed anything!" ?

Not sure what to call it. Anyone that knew has been gone for at least a couple years.

 

[hunt4679]

I was gonna sy under sized motor, or what happened on my compressor is the backflow preventer to the tank was bad and letting all the pressure back to the pistons there fore tripping the overload on start up

 

[btharmy]

It's not, by any chance, a 480v compressor connected to 208v. Ive seen it happen before. Pulls too much current. Slow starts too. Gee, wonder why. Re-wired the peckerhead and all was well.

 

[sii]

If they had set the pressure low enough the 5 might have worked :grin:

Is this a case of "nobody changed anything!" ?

"It was like that when I got here."

 

[Cow]

Since I do a lot of farmwork I come across this all the time. Usually I have to tell them to contact the manufacturer and find out the specifics (HP/Rpm/Pulley size) before we spend any more time trying to figure out why it's pulling so much current. Seems nobody knows anything when I show up and ask to see the old motor/pulley/pump etc.