"Michael Koblic" wrote in message
...
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?

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?

Thanks,

short answer "yes"

longer answer - you are looking for factors that affect losses, for your
example, they include resistive losses in the windings, magnetic losses due
to the air gap, bearing losses, air friction of the rotating parts,
hysteresis loss in the core, and so on. Armature diameter will affect these
factors in some way, so yes, it is a factor. It's just not a direct
relationship
--
Michael Koblic,
Campbell River, BC