Old motors: Why so different than today's

markba633csi

Mark Silva
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I've heard folks say stuff like "that old 2 HP motor would be like 20 HP today"
What does that mean exactly? Are current motor ratings exaggerated that much? Or is it certain manufacturers only?
Mark S.
 
There are lots of different ways to look at horsepower. An honest way would be to look at the maximum sustained load. in terms of work /time. Some manufactures use the input power of volts x amps = watts and 746 watts/hp to calculate their horsepower rating. This neglects losses due to electrical resistance and mechanical friction. and gives an inflated value for the useful output. Even worse is when they use the stall current instead of running current as that is in no way a sustainable value. The final straw is when they talk about peak horsepower. In no way relevent to the actual operation.

I used to have a Hoover vacuum cleaner that had a rating of 12 hp. It would be lucky to actually be generating a 1/2 hp. Marketing!
 
i think a lot of the old motors were rated without the efficiency factored in.
for example the same motor factored 2 different ways,
v=120
I=8
Hp=746
without the efficiency factored in this motor would have 1.286 theoretical hp

let's say the motor was 82% efficient and we then multiplied V x I x EFF then divided by 746
we'd get a theoretical hp of 1.056
the difference is .23 hp on a 1 hp motor dependent on the calculation you based the motor on
 
An exerpt from Ritzenbatt Vacuum (http://www.ristenbatt.com/xcart/Power-of-the-Vacuum-Cleaner-Suction-Motor.html)
Peak Horse Power
is an exaggerated rating of the output of a vacuum cleaner suction motor. Most motors used in power tools are rated in actual Horse Power in which one horse power equals 746 watts. Horsepower can also be derived from the formula in which one horse power equals 550 foot-pounds of work per second. The rating for Peak Horse Power is found by taking the suction motor without its fans and adding as muchload as possible without burning it out and then measuring the horse power using the formula which is based on torque per second. This results in a rating that is many times higher than the actual horse power of the motor. Keep in mind also that this motor rating does not take into account the number of fans which the motor is driving. An example of this is that a 4.0 PHP motor with two fans produces about 33% more suction than a 4.0 PHP motor with only one fan.
 
Wasn't there some kind of a law suit recently where someone was suing compressor manufacturers for falsly advertising hp ratings?
 
Wasn't there some kind of a law suit recently where someone was suing compressor manufacturers for falsly advertising hp ratings?

Yeah it was a while ago though, well the one I remember. Sometime in the early 2000s. There was a class action suit against many of the major air compressor manufacturers. I remember Sears, Devilbiss (they made some of the Craftsman models back then), & Campbell Hausfield were a few. I think they tried to add cfm ratings & tank sizes as well.

Not sure what ever happened though. After that the air compressor ratings changed & got conservative but they're still a lie. I remember my old 20 gal Craftsman/Devilbiss 110v air compressor said 5HP on it. Haha, that would mean that little air compressor is as powerfull as both my lathe & mill together! My current 220v 60 gal compressor says 3.5HP on it. My 7x12 bandsaw has a motor bigger than my lathe, mill, or air compressor & it only says 1HP on it
 
Awhile back we built a new drive station for a smaller ski lift. When the new 125HP motor arrived I tried to send it back. No way was it a 125 HP motor. They had to fax drawings with dimensions before I finally accepted it. (the trucker was p*ssed - I wouldn't let him take it off his truck until I got it straightened out - cost me a case of beer and a crow sandwich.)
The old 110HP (AC wound rotor, slip ring) motor was about the size of a 55 gallon drum. The new one was the size of a round kitchen waste can. It was so small I was sure it was a 12.5 HP motor and the paperwork just got fouled up.
We had to add a big flywheel to the output shaft because the lift would stop too suddenly without the mass of the old rotor keeping it turning.
 
A quick check is to multiply amps x volts = watts. Then divide by 746 to get hp. The output mechanical hp will always be less than this number.

One catch is this should be running amps, not peak amps. A clamp-on ammeter will verify this.
 
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