Newer motors are smaller compared to the old ones of same HP. From "Audels
Electric Motors", 5th Ed:

"Electric motors can now operate at higher temperatures because of the
development of insulation systems capable of handling temperature extremes.
The trend in motors is for reduced size and weight. This causes more heat
to be generated by the operation of the motor. This higher heat must be
handled by the insulation system without allowing motor life to be reduced
significantly. The future may see a change inasmuch as aluminum may be used
as the winding wire and the motor size will have to be increased to handle
the larger (physical) size conductors. This will probably create a motor
with more durability.

The insulation system, once the weakest link in the moor, no longer limits
the designer of industrial motors. Newer materials may well be developed to
allow for even longer life and the reduction of heat generated by motor
operation."

Generally speaking, if you find a motor with a fluted case, it is one of the
newer motors with improved insulation. The flutes are there to increase the
radiating surface because the motor will run much hotter than an old one of
the same HP.

Bob Swinney
wrote in message
ups.com...
You might look at the spec plate on the motor. It could be very large
for several reasons. Old motors had much larger frames than modern
motors. A good many years ago, I built a ripping saw with a 5 hp three
phase motor. The motor was not new when I bought it and it was about
the size of a big watermelon ( and painted green to boot. ). I think
it was a 284 frame. Now you can get a 25 hp motor in a 284 frame.
The saw acted as if it had a 25 hp motor. Never saw it slow down even
when ripping wet cedar.

It might also be a low rpm motor which would mean a bigger frame.
Currently a 15 hp 1180 rpm and a 25 hp 1760 rpm are both 284 frame
motors.

Dan