[TheForge] HP ratings

[theforge@mailman.qth.net]

The fact is, manufacturers tend to be more liberal in their ratings 
these days. An AC split phase  induction motor( the most common type) 
derives its speed from two things, the number of poles and the frequency 
of the input (60 cycles in the US of course)  This is why most motors 
have a speed of about 1800 or 3600 rpm,  multiples of 60. Typically, 
they are rated at  3450 and 1750.  The horsepower produced by a motor is 
a product of the "slip" or difference between the synchronous speed and 
the actual speed.   Generally, the  greater the slip, the greater the 
developed power.  Unfortuneately, the greater the slip, the more heat 
generated in the windings.  Many of todays motors are rated evidently at 
stall speed ( 0 rpm) or otherwise known as locked rotor.  They may 
produce the nameplate rating, but not for long.   The nameplate 
horsepower games are primarily played with power tools.   When comparing 
new power tools, a more reliable indicator is the amps.  Amps times 
voltage is watts.  750 watts is one horsepower, so about seven amps on 
110 voltage line is one horsepower. 

Older motors are more honestly rated, but they also have a downside.  
They are generally less efficient and heavier, since the steel used in 
the laminations for the rotor and stator were not as good as the alloys 
available today.   Their weight was also aided by the fondness for cast 
iron for even fractional horsepower motors.   Still, I like them because 
they will continue to be inefficient  for a lot of years to come, and 
are easier to hand rewind than modern motors.   The old motors used fish 
paper and wooden wedges to hold the windings in place.   New ones tend 
to use plastic, which after a motor burns up, melts into the stator and 
is a pain to remove.

A good buying guide for electric motors can be found at:

http://www.woodnet.net/toolreviews/el_motor.html

Charles


Dave Brown wrote:

> From discussions past, the details of which are foggy, I understand 
> that the HP ratings of new motors these days is something different 
> than it used to be.  A new 2hp motor isn't the same as an older 2hp 
> motor, right?  So, with our regard to the specifics of how HP ratings 
> are applied/assigned, how much difference is there between, for 
> example, an older (big ol' frame and weight type) 2hp motor and a new 
> 2hp motor?