Re: relay help req. (horsepower definitions)

Doug wrote:

>Hello Terry;
>Horspower ratings for consumer appliances are grossly inflated. This
>is an accepted (albeit dishonest) practice.

Doug,Terry,list,

Horsepower is a measure of power, or work-ability (the ability to do
work in a time frame; in this case the work is related to what a
horse can do). James Watt (needing a way to relate steam engine
output to the common man) defined horsepower as the ability to raise
550 pounds a distance of one foot in one second. Or 33000 pounds one
foot in one minute. Now, even a Draft Horse is going to find it tough
going raising 33000 pounds, period! BUT, the same horse could easily
lift 550 pounds 60 feet in one minute.

The horsepower claimed for rotational tools like routers and vacuums
is not dishonest, merely the same sort of "equation twisting" shown
above. The dishonesty, if any, is leaving out the word "developed"
(as in 3-1/4 DEVELOPED horsepower)

(more Snips,inserts below)

>One horsepower is equal to 746 watts. The efficiency of various
>motors runs from say 50% to about 85%. For purposes of simplifying
>things, you can assume it takes 1000 watts to get one horsepower
>from an average motor. 7 amps times 120 volts gives you 840 watts,
>or a little less than one HP.

First, the 'equating of HP to Watts' was done at a time when electric
motors functioned at a certain level! The magnets, bearings,winding
methods used were very different when this 'equation' was derived.
More importantly, this is an 'input' way of looking at horsepower.
Meaning if I input 746 watts, I WILL 'get' one HP.

This is a nice way to simplify things, but it always has been
incorrect and becomes more so as time goes on... (just look above
at the motor efficiencies you listed, how can I get the same output
from 746watts AT 50 PERCENT,that I get from 746 watts at 85 percent?)
BTW, I DO know how :-) see below...

>PS: Sears started this shameless advertising technique about 25
>years ago. 15 years ago they had a 3HP, 4HP and 5HP compressor, all
>of which were identical except for the size of the motor pulley.
><snip> Go look at any Home Depot compressor and you will notice
>something similar.

I don't know if Sears is to blame or not, But...
Remember, horsepower is a measure of the ability to DO work! Power is
the RATE of doing work. Using the equations shown at:

http://www.uark.edu/depts/aeedhp/agscience/MEASROTR.htm

We can easily see how the changing of a pulley size WILL result in
different horsepower being DEVELOPED... If the compressor parameters
were also changed (stroke length, cylinder diameter; i.e.,volume) the
HP would again be equal across the three models...

We all know and use this fact in our CNC calcs: a servo turning at a
higher speed, with less torque is EQUAL in POWER to a stepper(or
servo) turning more slowly, with higher torque.

What we have here is a new, and actually MORE CORRECT way of looking
at Horsepower. Problem is, we're back to the same sort of thing as I
mentioned above; NO horse can lift 33000 lbs.! (mechanical advantage
systems aside, of course) But mathematically, it is correct!

So what REALLY happened is that manufacturers (or Salespeople)
figured out that if we looked at the 'output' side of the horsepower
equation, and looked at the work capable of being DONE, as opposed to
the work being input, the number derived is better suited to SELLING
PRODUCT...

Finally, if we have a spindle turning at 20,000rpm and we imagine
wrapping a string around it and lifting some reasonable small weight
a certain distance in a second: then we 'simplify' the numbers using
550 ft.-lbs./second as the horsepower definition; we discover there
is no dishonesty involved only an update of the way horsepower is
determined...

Hope this helps.

Ballendo


P.S. Another useful link is:

http://www.eng-
tips.com/gviewthread.cfm/lev2/16/lev3/58/pid/406/qid/2964