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Baffled
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Join Date: May 2010
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Electrical vs Mechanical Horsepower

01/19/2012 2:58 PM

Inspired by ronseto's question re the price per Watt for heaters, I ask a similar question with respect to routers. I am looking at an advertisement for a router that turns at 27,000 RPM no-load and is rated at 7 Amps/120V. Ignoring inductive effects this is 7 X 120 = 840 W. However, the device is rated at 1-1/2 HP. I have been told that the horsepower in this case is calculated on the basis of the torque developed.

Still, the power output shouldn't exceed the power input. Here I am assuming that if I could calculate the reactive component, that would reduce the output power below the DC Watts the I calculated?

If my assumption is correct then how to explain the difference between the mechanical and electrical HP?
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Tornado
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#1

Re: Electrical vs Mechanical Horsepower

01/19/2012 3:38 PM

Shop appliances are often given artificial ratings such as "peak hp" or "develops x hp". For brief moments, such ratings may be true, but they do not represent continuous performance.

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SolarEagle
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#2

Re: Electrical vs Mechanical Horsepower

01/19/2012 3:56 PM

Have you tested the amp draw under full load vs no load? Also you must take into account the efficiency of the motor, no motor is going to be 100% efficient...

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JWthetech
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#3

Re: Electrical vs Mechanical Horsepower

01/19/2012 4:36 PM

Just because the motor is capable of developing 1.5 HP, doesn't mean it developes 1.5 HP. My car engine is rated at 120 HP at 5400 RPM. I'm certain that vast amounts of damage will occur at 5400 RPM.
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JRaef
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#4

Re: Electrical vs Mechanical Horsepower

01/19/2012 5:53 PM

Tornado hit the mark. Appliances and mass marketed home shop tools are being allowed to get away with marketing the term "HP" very generously. What they do is combine essentially Break Down Torque, which technically happens at maximum slip just befor a motor stalls, and maximum RPM. In reality the two values never meet, but because electric motor "HP" is not a standard unit of measurement regulated by the Dept of Weights and Measures, the Federal Trade Commission has done nothing to stop them from saying it. If the same principal were to be applied in the industrial world, where engineers exist, then the perpetrators would be laughed out of business.

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Baffled
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#7
In reply to #4

Re: Electrical vs Mechanical Horsepower

01/19/2012 8:00 PM

You have offered an explanation and thus GA. It is a bit peculiar that the HP rating should be a compound of two states that can never exist simultaneously. But now that I know the answer it is at least good for a chuckle.
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lyn
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#8
In reply to #7

Re: Electrical vs Mechanical Horsepower

01/19/2012 8:06 PM

You've obviously never been around marketeers. This verbal artistry is commonplace.
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JRaef
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#9
In reply to #7

Re: Electrical vs Mechanical Horsepower

01/19/2012 11:27 PM

I spent 11 years as a liaison between Engineering and Marketing in a small electrical equipment mfg company. The company had been founded by an Engineer and the son who was running it was an Engineer, but they both had a difficult time talking to "regular" people like marketing types (and accounting types as I later discovered). As an Engineer, I was able to stop the Marketing people from diverging as far off the path of reality as they wanted to, but I also had a gift of being able to explain the core engineering concepts in understandable terms so that they could grasp them. Sort of...

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SimpleMind
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#5

Re: Electrical vs Mechanical Horsepower

01/19/2012 5:54 PM

What's wrong about being optimistic? S.M.

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lyn
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#6

Re: Electrical vs Mechanical Horsepower

01/19/2012 6:04 PM

I have a router, too. It's called a 1 HP, I think. Compact and does a great job. It's a respected name brand.

I think that if you buy a top quality router it won't matter what the specs say. It should give you good service.

Relax, go buy one and carve some wood!
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perry
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#10

Re: Electrical vs Mechanical Horsepower

01/20/2012 11:35 AM

baffled; you said no load was 7 amps, that is a series wound motor, will operate on A.C. or D.C. when you load it down it will run slower than 27k speed thus the current will rise thus giving your horse power perry
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Yahlasit
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#11
In reply to #10

Re: Electrical vs Mechanical Horsepower

01/20/2012 12:17 PM

I think he specified just the RPM's as to be at no load condition, not the motor rated current; otherwise your answer would not only explain the increase, but even fall short.

I believe it is the eye-catching rating they stamp at the factory what causes confusion.

Regards

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Users who posted comments:

Baffled (1); JRaef (2); JWthetech (1); lyn (2); perry (1); SimpleMind (1); SolarEagle (1); Tornado (1); Yahlasit (1)

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