On Sep 2, 9:23*pm, "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?

Output power == torque x RPM
... within a constant, because RPM (revolutions per minute)
is the traditional measure, but you really want radians per second

If I understand the physics, the output power is directly related to torque
if rpm are held constant.

Proportional, not just related, by definition.

What then, in the nature of the motor
construction, will increase its torque? Is the diameter of the rotor one of
the factors?

That's a completely unrelated issue. The torque results from magnetic
attraction, and your magnetized materials (the steel) have limits to
their magnetizability. You will need more steel, or steel capable
of higher saturation magnetization, to raise this torque limit, and
some very efficient flux coupling (tight fit of rotor/stator ) to
get the full benefit. There's also remagnetization of the materials
occurring, so that the motor doesn't just lock up but actually
continues to move... so the duty cycle makes a part of your steel
ineffective because its magnetization is swinging through zero.

Yes, also rotor diameter changes the torque (the force
times the radius is the torque, increase the radius and keep the
force constant... increase the torque).

The motor is likely to burn up if the saturation magnetization limits
are reached. It can also burn up because too much current
(in the resistive wiring) is required to reach the target
magnetization.
It can also burn up if the magnetization is changed too often (there's
heat generated due to remagnetization).