Re: [Phys-l] backwards units : entrenched usage

[Jeff Radtke <jr@cloudchambers.com>]

John,

Writing as an engineer, I suspect that the nameplate is the way it is  
to save on resources used to convey the relevant specifications.   
There may also have been a historical tendency to make specs look  
better than actual performance.  For example, these ratings may not  
all be achieved at the same time, since  the rotational speed at  
maximum power output is usually much less than the rated RPM. A torque  
curve tells a more complete story, but won't fit on a name plate.   
Colons are not equal signs, but shorthand for the following:

Maximum Power Output (see torque curve for speed) = 1.333 HP
No-Load Rotational Speed = 1725 RPM
Maximum Current Draw (probably with motor stalled) = 12.3 A
Optimal Power Source Voltage = 115 V
C Frame Design
Service Factor Rating = 1
Single Phase Power Source
Optimal Power Source Frequency = 60 Hz

--
Jeff Radtke
jr@cloudchambers.com
www.cloudchambers.com


Quoting John Denker <jsd@av8n.com>:

> Hi --
>
> The other day I was reading the data plate on an electric motor:
>
>        HP  :   1 1/3                Type : C
>        RPM :   1725                 SF   : 1.0
>        A   :   12.3                 PH   : 1
>        V   :   115                  Hz   : 60
>
> This seems backwards relative to the way we would write things in the
> physics lab:
>
>       1.333 HP
>       1725 RPM
>       12.3 A
>       115 V
>       60 Hz
>
> A Philadelphia lawyer might argue that the A and V on the data plate
> stand for amperage and voltage ... but that only goes so far.  I'm
> not going to buy horsepowerage or Hertzage or RPMage.
>
> I'm not a historian, but I conjecture that the idea of "unit analysis"
> (where the units are algebraic quantities, with their modern meaning)
> is relatively new.  I've seen the "backwards style" in old engineering
> books.  There's no doubt that the colons on the data plate are the
> equivalent of equals signs.  In old books I've seen formulas of the form
>
>         RPM = 28.75 * Hz
>
> which is utterly backwards from a modern unit-analysis point of view.
> We would write the conversion factor as 28.75 RPM per Hz.
>
> =========
>
> So, why mention this?  Three reasons:
>
>  a) The weakest reason is that I'm mildly curious about the history.
>   Does anybody know when/where/how units came to be considered algebraic
>   quantities, in the modern fashion?   There's probably a paper on this
>   somewhere in the history-of-science literature.
>
>  b) We should add this to the endless list of possible student   
> misconceptions.
>   Life is hard enough when they show up with no idea how to use units, but
>   it is so very much harder when they show up with entrenched diametrically
>   backwards notions.
>
>  c) Even if the students have never encountered this before, at some point
>   in the game (not too early) we should warn them that they might encounter
>   backwards usage here and there.
>
> See also next message.
>
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