From: scot@fc.hp.com (Scot Heath)
Newsgroups: rec.woodworking
Subject: Re: developed vs rated hp
Date: 29 Sep 92 16:17:22 GMT
Article-I.D.: fc.BvCL8y.EGI
Distribution: na
Organization: Hewlett-Packard Fort Collins Site
X-Newsreader: TIN [version 1.1.4 PL6]

Well, you're right.  When the talk is of developed horsepower, the meaning
is "peak" which for an induction motor is typically the local peak of the 
torque curve near synchronous speed.  A typical induction motor torque
curve is:

          |
          |
          .                                            
          |.
          |  .                                        
   Dev.   _    .                                           .
          |      .                                    .      .
          |         .                               .         .
          |              .                        .            .
          |                       .             .
   Rated  _                              .                      .
          |
          |                                                      .
          |                                       
          |                                                       .
          |                                  
  Torque  |                                                        .
          |
          |
          |                                                         .
          |__________________________________________________________._
                                                                     |
          0                   RPM                              1800 or 3600


As you can see, the curve is very steep in the operating region and in
fact, the observed operation is typically that once you load the motor past
the local maximum torque, the speed jumps to the corresponding point on the
initial portion of the curve or simply stops.  The actual operation depends
upon the shape of the curve near 0 RPM.

The Rated HP is typically the torque level at which the motor can be run
continuously without exceeding the temperature at which the winding
insulation beaks down.  Since there is thermal mass involved, you can operate
the motor at higher than rated torque for less than 100% of the time and not 
exceed this temperature if the motor is cool preceding the run etc. etc.
etc.

Typically, two motors with different rated HP develop different HP in a
ration close to the same as the difference in rating.


The story is somewhat different for a universal motor such as is used on
most hand held tools.  In these motors, for a given input voltage, the
torque goes up as the speed goes down.  The more you load them, the slower
they run until they stall, at which point their torque is a maximum.
In this case, the developed horsepower is a the point along the torque
curve where the speed X torque is a maximum.  As with the induction motor,
the rated horsepower means you can run the motor there at 100% duty cycle.
Again, you can load the motor more and it will produce more torque but you
may only do this on a limited basis.

The final word is heat.  If you exceed the winding insulation temperature
rating, you will fail the insulation and ruin the motor ( or pop the
thermal cutout if so equipped). 

-Scot