Originally Posted by Dugg
Look at the motor.
If it's an Emerson Electric Motor (assembled in mexico) and the data Plate says HP, SPL, then its's the same (we'd like you to believe it's) 5HP motor used by all the cheap-crap compressor assemblers.
Compare the size of this motor to the size of a real 5 HP electric motor from Grainger and tell me you think the skimpy little motor on the compressor is actually 5 HP. In fluid power there's a simple formula that is PSI X GPM divided by 1500 that is the general rule for horse power requirements.
When the Big Slox (Yes Paris, I intended it to be a slur of big box and sluts) stores advertise 7.5 peak horse power, it's when no work is being done. If you want a compressor that's rated for a, no-work-being-done situation, why would you need a compressor?
You are right that the quality of the motor used on the compressor is important.
The peak HP of the motor is the least of your considerations. That is a meaningless figure based on the motor being under absolutely no load. A better figure to look at is the service factor or SF. If it's 1.15, it means it can operate at 115% of horsepower rating. An electric motor, whether AC or DC, produces 3 ft lbs of torque per horsepower at 1750 RPMs. If the motor is running at 3500 RPMs, it is producing half that, or 1.5 ft lbs of torque per horsepower. All you need that motor to do is turn the compressor. Naturally, the more actual horsepower the motor produces will make it work less while it's turning the compressor, but if the motor is sized correctly for the load, turning the compressor is no problem for the motor. The motor should not burn up unless it is being overloaded because the compressor has locked up.