On 09/02/2010 11:23 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?

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?
Iron losses in the rotor are proportional to speed and field flux.
There is going to be an optimum for any particular output rating where
iron losses hit a minimum, and then gradually increase above that size.
Copper losses require a specific cross sectional area for minimal
resistance, but use too much copper and you increase eddy current
losses. These are probably the main factors. You need enough field
amp-turns to resist the armature field and maintain the field flux.
But, you want to keep the field flux low enough to keep the iron losses
down. So, there are a bunch of minima that are loosely related. You
want to juggle all these so that overall minimum hits near the bottom
of all the individual minima.

Jon