Compressor horsepower clarified by Kelvin

A little tech info for members considering a compressor.
I have two compressors and was going to sell one of them. A friend of Dwight's came to look at it and saw that on the outside it quoted 3 running hp, whereas the one I am keeping quotes 7 hp. He felt that 3 hp was not enough for him and I wondered where the difference lies. Upon checking it appears manufacturers put on false info to attract buyers, so here is the truth.

To get a real comparison you need to look at the amps rated on the motor.  
Horsepower = volts x amps   divided by  746
This means my compressor that has a rating of 3 running hp is actually consuming 17.5 amps
Therefore the horsepower is really  240 x 17.5  divided by  746  which comes out at  5.6 HP
The 3 running hp is what the motor consumes at running speed which means it runs at  3  x  746  divided by 240 volts which = 9.3 amps, almost half its quoted max amps.

The other compressor quotes  7 hp which is a joke.
This motor is rated at only 15 amps so its horsepower is really  240 x  15  divided by  746 which  =  4.8 HP

So you can see that the smaller rating is in fact the more powerful machine, The 7 hp is a gross exaggeration at the least and the smaller rated unit is in fact truthful. It is rated at 5.6 HP whilst only using 3 HP when running at a constant speed.
What looks like the bigger and better unit at 7HP does in reality only give you a max. of 4.8 HP.

Hope this helps those who are considering investing in a compressor, don't believe what it says on the tank but go by what it says on the motor and work out the HP yourself.

PS: The 3 hp is by Porter Cable and the 7 hp is from Husky

Kelvin