On Thu, 2 Sep 2010 21:23:45 -0700, "Michael Koblic"
wrote:

I have been trying to work this out from basic principles but the answers
are at best foggy:

Consider two electric motors (commutator) with identical inputs, say 250W,
and identical RPM, say 15,000. What are the factors that determine the motor
efficiency and thus the output power?

For given input power, efficiency is determined by losses: friction,
windage, current*resistance, hysteresis and eddy current.

If I understand the physics, the output power is directly related to torque
if rpm are held constant. What then, in the nature of the motor
construction, will increase its torque? Is the diameter of the rotor one of
the factors?

Torque is the integral of vector products of magnetic field density,
armature current, # of armature conductors carrying that current, and
armature conductor radius.