Hi.
These cheap compressors claim 2.5 HP. Do you know if they have universal motors ? Seen some naked and have a start capacitor; integral compressor and motor, no belt but direct drive; so it may be a no. Pictured is $150.
Comparatively, a 2.5 HP motor only is triple the size found in these and are sold at more $
If you have one similar, please give some light ? What is the magic ? Now I see " running horsepower" ratings. What is that crap ?

 

It shows what is apparently a split phase induction motor, hence the capacitor.
The size appears to be about right for 2.5HP.
Max.

 

Do you know the conversion from watts to HP? It is 746 watts per Horsepower. Motor should draw about 1865 watts plus. If not then claim might be suspect. You should also be able to compute the work involved in compressing 21 gallons of air to a pressure of 125 psi at room temperature. Raising the pressure of a constant volume will raise the temperature a bit, but you should be able to calculate that as well.

 

Without a CFM (cubic foot per minute) rating one cannot calculate the horsepower. Simply put, that compressor may take a week to pump 21 gallons of air to 125 psi.

Here's a discussion of how to do that calculation: http://www.truetex.com/aircompressors.htm

EDIT: Of course, a purchase decision should include whether it works for your purposes, not just whether the labels are accurate or not. Such compressors may be fine for occasionally used air tools, such as drills, impact wrenches, or a blow gun. For a floor nailer, that is probably OK, but they often require about 100 psi, and it will run a good bit of the time to keep that up. (My home compressor is 150 psi for that reason.) For spray painting, it is probably marginal. For sandblasting, it would be like spraying 20 acres of cropland with a 5 gallon sprayer.

 

If it's anything like the crap they pull with vacuum cleaners, it's probably a "peak" horsepower rating rather than an actual representation of the motor's continuous power output. (Seen those "6.5 horsepower" shop vacs around? Yeah right.)

Most compressors (except the 1/3HP 'pancake' and DC automotive styles) use induction motors. A lot of compressor-duty induction motors are smaller than others of the same power rating. Less emphasis on energy efficiency and an open drip-proof enclosure rather than a totally enclosed fan cooled design means they can be made to run hotter in a smaller form factor. Some of them are probably only rated for intermittent duty.

If you're just doing home-gamer stuff then direct drive pumps are generally okay for the money. Just don't expect them to last until the next ice age like the old 30's~70's cast iron pumps. If you genuinely need it to be indestructible at high duty cycles then you're looking at something more in line with a pressure lubricated dual-control Quincy or a screw compressor, etc.

 

I agree with Papabravo; if you have access to a wattmeter that would be the best way to verify the rating.

Bear in mind that intermittent duty motors are very inefficient, perhaps 60% or so. Take that into account when calculating the actual shaft power.

 

Bear in mind that intermittent duty motors are very inefficient, perhaps 60% or so. Take that into account when calculating the actual shaft power.

Heck, I've got a three phase, half horsepower industrial TEFC motor rated for continuous duty on one of my machines and it's nameplate is marked at something like 54% nominal efficiency with a power factor not too far off. An intermittent duty motor is basically going to be a space heater that incidentally produces a little bit of torque as it heats up your garage... and that's *before* factoring the 20-40% losses found in a single stage pump head.

...Not that it really matters if it's only running 40 minutes out of the week - but it's the principle of the thing!

You don't really start seeing nominal efficiencies above 80% in induction motors until you get above 2 horsepower or so - and even then that's looking in high efficiency product lines. It's an economy of scale of sorts. That, and premium efficiency motors come with a premium cost - what with brazed copper squirrel cages instead of cast aluminum, higher grade steel and more of it, etc, etc.

 

One thing to be aware of, if you intend purchasing a unit that is Chinese in origin and has Chinese made capacitors on the motor, pick up N.A. spares, because the odds are you are going to need them in the future.
They are notorious for early failure.
Max.