Horse power


What kind of games are they playing with HP now days?
My old air compressor had developed a rust hole in the tank and I am looking for another one.
The old one (probably 40 years old) is 2 HP and pulls about 20 amp, 230 as best as I can remember without looking. Saw one that was rated at 4 HP and ran on a standard 120 volt cord. In the vehicular, bull shit. Any real 4 HP motor should pull at least 30 amp, 230. Pulled out my old W W Grainger cat and didn't see a 4 HP but the 3 HP I saw pulled 21 amp 230. That's 42 amp 115 and they are getting 4 HP out of a 20 amp 115?
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If you get a magnifying glass and get a good, close look at the HP rating, you'll see it says "Peak HP". Somewhere, sometime, during the operation of the motor, there is a slight possibility of a chance that 4 HP might be generated in some tiny location within the motor. This, of course, is perfectly useless to anybody, hence the fine print.
I've heard several well-intentioned samaritans suggest going strictly by the amps and volts in determining the actual power output, however, an inefficient motor will draw plenty of amps without doing all that much.
Other helpful folks suggest going strictly by CFM ratings. Personally, I feel it's naive to believe these figures aren't just as inflated as the HP ratings.
My method for buying tools such as air compressors, is to pick the brains of folks who use them to make a living. If they buy junk that falls apart in a hurry, they lose money, so they know what works. This, of course, doesn't apply to slugs who don't finish the job. (Yes, I'm talking about YOU Nick! Finish my damn bathroom!)
Oh yes, the point. If I were in your place, I'd try to match up the specs (volts and amps) of what's being replaced. Not only will this ensure you'll have the power to run the new rig without popping breakers, but you'll get at least as much, probably significantly more, CFM than with the old. Motors have come a long way in 40 years in terms of efficiency.
CS
<DIV>What kind of games are they playing with HP now days?</DIV> <DIV>&nbsp;</DIV> <DIV>My old air compressor had developed a rust hole in the tank and I am looking for another one.</DIV> <DIV>&nbsp;</DIV> <DIV>The old one (probably 40 years old) is 2 HP and pulls about&nbsp;20 amp, 230 as best as I can remember without looking.&nbsp; Saw one that was rated at 4 HP and ran on a <STRONG>standard 120 volt cord.</STRONG>&nbsp; In the vehicular, bull shit.&nbsp; Any <STRONG>real</STRONG> 4 HP motor should pull at least&nbsp;30 amp, 230.&nbsp; Pulled out my old W W Grainger cat and didn't see a 4 HP but the 3 HP I saw pulled 21 amp 230.&nbsp; That's 42 amp 115 and they are getting 4 HP out of a 20 amp 115?</DIV></BLOCKQUOTE></BODY></HTML>
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Glenn wrote:

That is because they use the "peak horsepower" rating or they say it "develops" that horsepower. It is indeed a useless rating. Whenever you are running a circular saw and it binds in the wood and the blade stops, the saw develops its "peak" horsepower. If you will notice, when this happens, the lights may dim when the motor stops. For a brief moment, the motor is pulling way more amps than what it was intended to do and it has pulled its peak power.
Notice that while this is happening, the blade is stopped, so it is incapable of delivering any power to help you work, but it is pulling alot. Which is the main reason that the rating is bullshit. Continuous horsepower is the only rating to use and it works like this:
In perfect conditions, 746 watts is one horsepower. In real time conditions, due to inefficiencies in both electrical power delivery and the efficiency of the motor, a good number to use is 1250 watts is one horsepower.
So,...if you have a machine made to plug into a 15 amp circuit, 15 amps times 120 volts equals 1800 watts. Divide 1800 by 1250 and you have roughly 1.5 horsepower. The amperage number is the one to use to select your next machine.
Here is an excellent article on motors in general, and compressors in particular:
http://truetex.com/aircompressors.htm
--
Robert Allison
Rimshot, Inc.
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Got curious and looked on line at WW Grainger. (http://www.grainger.com/)
1st 3- HP capacitor start motor I looked at drew 33.8/16.9 amps, 115/230 volts (full load rating, service factor 1.15). Next one was 32.0/16.0 amps. Looking at 3- HP capacitor start, capacitor run, the draw was 30.0/15.0 a. (service factor 1.00), next one was 33.6/16.8 a.
Things have come a long way in 40 years.
<DIV>What kind of games are they playing with HP now days?</DIV> <DIV>&nbsp;</DIV> <DIV>My old air compressor had developed a rust hole in the tank and I am looking for another one.</DIV> <DIV>&nbsp;</DIV> <DIV>The old one (probably 40 years old) is 2 HP and pulls about&nbsp;20 amp, 230 as best as I can remember without looking.&nbsp; Saw one that was rated at 4 HP and ran on a <STRONG>standard 120 volt cord.</STRONG>&nbsp; In the vehicular, bull shit.&nbsp; Any <STRONG>real</STRONG> 4 HP motor should pull at least&nbsp;30 amp, 230.&nbsp; Pulled out my old W W Grainger cat and didn't see a 4 HP but the 3 HP I saw pulled 21 amp 230.&nbsp; That's 42 amp 115 and they are getting 4 HP out of a 20 amp 115?</DIV></BLOCKQUOTE></BODY></HTML>
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