Gotta Luv HP Ratings

I think the ridiculous claims for vacuum hp stem from the fact that power is equal to work/time and work can be expressed as pressure x the volume. If you figure that a vacuum can suck x cu ft/min at a maximum flow rate and is capable of producing a maximum negative pressure and multiply the two, you get the peak hp. Unfortunately, that only occurs at the instant the conditions are changing from maximum vacuum to maximum flow rate but that's good enough for marketing.
 
For a while motors were sold with frankly false claims, using stall current draw to describe 'peak horsepower'. A marketing person's wet dream. In the 1950s motors were rated in horsepower by checking sustained ft-lbs/sec which is more accurate. In those days motors were far less efficient (between 70 and 80%) but I still use my Dad's old repulsion motor from his table saw, made in the 1930s, and it still works well.

For air compressors, someone invents some kind of criteria for putting a bigger number than everyone else on their ads. I only check the current draw/voltage on a compressor and totally ignore the rest of it. Some of these guys are paid to mislead, and they do it very well. Even the output air claims can be grossly inflated.

From an engineering perspective, If you are putting 746 sustained watts into a motor with a piston compressor, the mechanical conversion factor is something like 40% so a 1 HP motor can only produce a little less than a half that in work using the output air. I am very reluctant to describe any compressor output in HP. FWIW, the silent compressors that use double pistons and 'oiless' compressors are less efficient at converting motor energy, but I couldn't find examples of their conversion efficiency easily.

I could give a serious rant about vacuums, but it would be less civil
 
Looking at motor current draw is a better way of evaluating hp. Figure 6 amps @ 120volts or 3 amps at 240 volts per hp. This is the input hp. The actual maximum usable hp will be 50% to 70% of that number.
 
Hi Guys,

A good rule of thumb is to take the plate rating and divide by 745 to get HP and then divide by two to get an approximate output.
 
I don't know if current draw, is a great comparison either. All they have to do, is make a very inefficient motor.

I have a very cheap, and under powered 6" grinder that my father had. It does not take much at all to slow it down, and at the old garage, you could see by the lights, that it was sucking down some amps. Just running with no load, it would get too hot to touch.
 
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