Goodness factor
The goodness factor is a metric developed by Eric Laithwaite to determine the 'goodness' of an electric motor. Using it he was able to develop efficient magnetic levitation induction motors.
where
From this he showed that the most efficient motors are likely to be relatively large. However, the equation only directly relates to non-permanent magnet motors.
Laithwaite showed that for a simple induction motor this gave:
where p is the pole pitch arc length, ρr is the surface resistivity of the rotor and g is the air gap.
...
Wikipedia