# circular saws exceed 100% efficiency?

• posted on July 22, 2004, 1:08 pm
I just found that some circular have logic-defying efficiency.
For example, dewalt DW368K specification says 15 Amps motor, and the max output is 2200 Watt. However, the input power is 15 Amps x 120V = 1800 Watt. How could it consume 1800W of electricity and produce 2200W of power? It must have an effiiciency exceeding 100%!
Another example: Milwaukee 6391 has a 15 Amp motor and claim the saw has 3 1/4 horsepower. Since 1 horsepower = 746 W, 3.25 HP = 2424.5 Watt. It is even more efficient than the dewalt!!
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• posted on July 22, 2004, 1:17 pm
Those are run of the mill efficiancies for woodworking tools. When they crack the 200 percent effieciency mark it will make news.
John
peter wrote:

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• posted on July 22, 2004, 2:14 pm

Ho hum, those 3 hp saws and routers have all been pushed to the back pages by the 5 and 6HP shop vacuums.
--

Larry Wasserman Baltimore, Maryland
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• posted on July 22, 2004, 2:17 pm

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• posted on July 22, 2004, 2:24 pm

"max", or "peak"? The peak of a 120V sine wave is 177 volts, it's only at that at the top of the graph, but it is the peak. It's a meaningless thing to use for anything other than marketing, because the RMS is what matters. Marketing weenies, of course, aren't into engineering truths.
I would suspect that they're equating "peak" with "max" now, for the same reason.

heh...
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• posted on July 22, 2004, 2:30 pm
I must admit that although I did well in my EE classes, that was years ago and even then I always viewed electricity as being something close to black magic and voodoo.
Does anybody have an explanation of how they get away with this?
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• posted on July 22, 2004, 3:47 pm
GigaNews wrote:

No or poorly defined standards. Different ratings for different purposes. "Amps" for how big a circuit it needs. "Watts" for marketing. "Horsepower" the same.
--
--John
Reply to jclarke at ae tee tee global dot net
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• posted on July 22, 2004, 4:56 pm

"Industry Practice" with universal motor-powered devices is to quote the instantaneous peak before self destruction iirc, if it's regarded as relevant, as in power tools. Nameplate amps is a better indication of "motor power" than "quoted HP"
"Sears Practice" with Induction Powered devices is to use words like "1-1/2 HP Induction Motor develops 3HP maximum" which is, iirc, based on locked-rotor current draw. You wanna hit that with an oak not, but not too often. Again, compare nameplates or just read carefully.
Vacuum manufacturers typically quote "Peak vacuum HP" which is PURE drivel as far as I'm concerned.
Aircompressors have similar meaningless numbers.
Another hitch is that electric motors will HAPPILY supply "peak HP" for short periods. Wheras gasoline engines simply cannot. So a 5HP Unisaw electric would need a 10HP gas or diesel engine to operate it.
Compare air compressors with similar pressure and CFM ratings, gas vs electric and you'll quickly see what I mean.
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• posted on July 23, 2004, 12:21 am
Regarding your comparison of electric and gasoline horsepowers. Keep in mind there are more than one method of creating horsepower rating for gas engines (and possibly electric). I can remember the days when the Province of Ontario required the horsepower rating of engines on car ownerships. People would get upset when they compared the licence horsepower with the brochure horsepower. The license would use "brake-horsepower" the brochure would use another system. For example the VW bug was rated 26 hp on the license while the showroom would claim, something like 60 hp for the engine.
wrote:

ago
purposes.
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• posted on July 22, 2004, 4:34 pm
Black (magic) and Decker

was
black
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• posted on July 22, 2004, 8:34 pm
wrote:

they just lie....
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• posted on July 23, 2004, 5:37 pm
I think you may be onto something there...

was
ago
black
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• posted on July 22, 2004, 4:44 pm

If you take the electrical energy input, and add to that the rotational energy stored in the blade, etc (all of which have momentum), then you can easily get 2200 watts. In fact, depending on how fast you extract the rotational energy you can get pretty much any wattage value you like.
Of course, none of that is of any value whatsoever in the real world.
John
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• posted on July 24, 2004, 12:54 am
Physics 101, not!

Watt.
efficient
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• posted on July 24, 2004, 1:28 am
On Sat, 24 Jul 2004 00:54:49 GMT, "Jim Giblin"
|Physics 101, not!
Marketing 101, Yes!
|
| |> I just found that some circular have logic-defying efficiency. |> |> For example, dewalt DW368K specification says 15 Amps motor, and the max |> output is 2200 Watt. However, the input power is 15 Amps x 120V = 1800 |Watt. |> How could it consume 1800W of electricity and produce 2200W of power? It |> must have an effiiciency exceeding 100%! |> |> Another example: Milwaukee 6391 has a 15 Amp motor and claim the saw has 3 |> 1/4 horsepower. |> Since 1 horsepower = 746 W, 3.25 HP = 2424.5 Watt. It is even more |efficient |> than the dewalt!! |> |> Why hasn't these more than 100% efficient machines made the headlines? |> |> |