Compressors

[FamilyFarm]

I found an Emglo unit only 3 years old for about $100. It is $800 new and puts out a ton of air volume, and is very quiet also. It is a small tank unit (twin tanks), but also more portable than some - if you consider 195lbs portable - wheelbarrow style.
Learn how to compare the ratings, most are exaggerated. Large tanks are nice, but true CFM displacement is more important for heavy-duty use.

 

[Anonymous Poster]

When you convert to single phase, keep in mind that the running current for the same HP is slightly higher in the single phase units. For example, a 5 HP three phase motor is 15.2 AMPs, and the same HP on single phase is 28 AMPs. The inrush AMPs....that's starting load......is not much different percentage wise than three phase, but of course it too has to be higher. If you are thinking of going single phase, a smaller motor may be necessary if you have limited electrical power. I JUST FINISHED installing an electrical service to my "new" shop and it is 100 AMPs. If I get a 5 HP single phase compressor, I'm covered. There ARE several ways to run three phase motors on single phase, but aside from the expense of actually changing the motor, it actually doesn't save you any money. If you DO go single phase, BE SURE that the unloader is working perfectly and that the compressor has ample time to get up to speed before the load gets on it. That is a big help in prolonging the life of both single and three-phase motors. And another thing....take my word for it as an Infrared Thermographer......compressors want YOUR BEST electrical. If you think one particular size wire is sufficient, jump it up TWO sizes. You'll be glad you did.

 

[Charlie_in_TX]

Go to the forum http://www.practicalmachinist.com/c...opics&forum=Phase+Converters+and+VFD&number=3. You can find used/ask question about variable frequency drives. That is the best way to drive 3 phase motors. There are people there that have BTDT.

 

[Anonymous Poster]

Hmmmmm....VFD.....not a bad way to go, since the drive can be set up to accept single phase input. It might be a bit spendy, and they do have to have a fan and an air filter, but they ramp up slowly and that reduces the inrush current. What is BTDT?

 

[jima1]

BTDT = "Been There Done That" /w3tcompact/icons/smile.gif

 

[kevinro]

I have been considering putting my compressor outdoors and running the pipes indoors. One concern I had was the temperature of the air. As you might already know, when air is compressed, the air gets hot because you squeeze the heat energy into a smaller space. The inverse is also true. When the air expands, it gets colder.

My thought, and I was hoping someone who has an outdoor compressor could comment, is that if the outside air is cold (30 degrees for example), and has time to cool, is the expanding air too cold to use?

Kevin

 

[Anonymous Poster]

Since this thread began and since we are in the process of putting a shop together, I have been noticing air compressors that are offered for sale in farm supply stores. I recently looked at one that had "6 HP" on the label, and when I looked at the motor it was quite small. The nameplate on the motor did not give the HP, but instead it said "Spl Duty". What a complete crock! Farther down on the nameplate was the "truth", and where it listed running AMPs it had 15.6, which is................about THREE HP on 230Volts. If that isn't false advertising, I don't know what would be. 1 HP is 16 AMPs on 120V, 1-1/2 HP is 10 AMPs on 230V, 2 HP is 12 AMPs on 230V, 3 HP is 17 AMPs on 230V, and 5 HP is 28 AMPs on 230V. Where they can possibly get SIX HP is completely beyond me.

 

[Charlie_in_TX]

CJDave, yes it is a crock, lie, whatever adjective you want to discribe.

volts X amps X power factor = Power
240v X 20a X .8 = 3840watts
3840 X 80% effiecency = 3072
3072 / 750 = 4.1 HP at the output shaft

That same motor is rated as a 10 HP motor because, under lab conditions, with a locked rotor, the motor pulls 31 amps @ 240 volt. Air compressor manufacture rate there motor by how much power they can consume (just before they blow up) not how much they produce.

 

[Soundguy]

Any gear reductions taking place.. or were they really stretcing the limits.. like car amplifiers advertising peak to peak measurements, rather than rms...


Soundguy

 

[Anonymous Poster]

Hmmmmm.......Root Mean Square? I'll have to try and remember how to use that. Gear reductions will actually only reduce rpm, they can't literally make more HP. Oh yeah, and one more troubling thing...the motor was 3450 RPM instead of 1760, so everything is turning wayyyyy faster than what I would like to see. I like the throb-throb-throb of an ancient , slow speed Kellog anyday to the frantic pace of these modern high-attrition, high-speeders.