Is Porter Cable lying about the HP on their compressors?

[SOG38]

Compressor HP

Compressor HP

I understand about the law of conservation of energy.
The Hp is calculated
Horsepower = Torque X RPM/5252 What I explained poorly is that is how someone states (incorectly) the HP for thier advertising.

 

[kwired]

IMHO
As I recall from investagating long ago the running HP is the HP at the compressor to deliver the cfm to replinish the tank. The motor RPM, say 3600 is stepped down to say 600 rpm.
A 1 HP motor would be delivering 6 HP to the compressor. This is of course for the belt driven compressors
Then the motor is slightly larger to ensure it isn't running at full load with the losses. I always measure to see what is needed for a power circuit. It is misleading.

I understand about the law of conservation of energy.
The Hp is calculated
Horsepower = Torque X RPM/5252 What I explained poorly is that is how someone states (incorectly) the HP for thier advertising.

By changing speed beyond the motor output shaft the input power (at motor full load) remains the same. By reducing speed either by gears, pulleys, or whatever your final drive speed is reduced but the torque of the final drive will be greater than that of the input motor. The horsepower of the final drive will also be the same as the input motor less any losses in the method of speed reduction.

 

[kbsparky]

We connected up a large compressor just yesterday. I measured 52 Amps on A and C phases, and 58 Amps on the B phase (240 Volts 3 phase Delta-high leg supply).

That dang thing took 365 Amps to start, however. I could hear the breaker box groan when we kicked that thing on ....

 

[david luchini]

Conservation of energy dictates that you cannot get more pneumatic power out of a compressor than the electrical power you put into it. A compressor simply converts electrical power into pneumatic power, less losses. A motor is the same in that it converts electrical power into mechanical power, less losses.

I must not have made my point clearly. I don't believe that the compressor manufacturer's are listing a "horsepower" that is an actual expression of real "power," either mechanical or electrical power.

Instead, I believe that they are using the term "horsepower" as an indication of the "time" that it would take to charge or recover the compressor tank. For instance, if you had two air compressors with 25gal tanks, 150psi cutoff pressures, and both with a 2HP motor: If due to differences in the compressors and construction of the units, the first unit charges the 25gal tank to 150psi in 40 seconds, and the second unit charges in in 50 seconds, then by the formula HP=GPMxPSI/1714 the two units would have a different "horsepower" despite having the same size motor.

 

[kwired]

I must not have made my point clearly. I don't believe that the compressor manufacturer's are listing a "horsepower" that is an actual expression of real "power," either mechanical or electrical power.

Instead, I believe that they are using the term "horsepower" as an indication of the "time" that it would take to charge or recover the compressor tank. For instance, if you had two air compressors with 25gal tanks, 150psi cutoff pressures, and both with a 2HP motor: If due to differences in the compressors and construction of the units, the first unit charges the 25gal tank to 150psi in 40 seconds, and the second unit charges in in 50 seconds, then by the formula HP=GPMxPSI/1714 the two units would have a different "horsepower" despite having the same size motor.

So how can a unit that is marked 6.25 hp have a motor with a full load rating in the range of what a 2 hp motor normally would draw at full load?

I believe the 6.25 hp rating is not the continuous rating of the motor but rather the amount of power the motor can output for a short period of time before winding temperatures start getting too high. More of a duty cycle rating although they don't call it that.

I suppose it is also possible for the driven compressor to have a similar rating and not just the motor.

You still can't get more work output than you have energy input.

 

[One-eyed Jack]

An interesting comment on this is at http://www.truetex.com/aircompressors.htm where there are other links.

That is a great article which completely substantiates the "lie" by mfg. of home style air compressors.