Goodness factor

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.

$G = \frac {\omega} {\mathrm{resistance} \times \mathrm{reluctance}} = \frac {\omega \mu \sigma A_\mathrm{m} A_\mathrm{e}} {l_\mathrm{m} l_\mathrm{e}}$

where

G is the goodness factor (factors above 1 are likely to be efficient)

A_m, A_e are the cross sections of the magnetic and electric circuit

l_m, l_e are the lengths of the magnetic and electric circuits

μ is the permeability of the core

ω is the angular frequency the motor is driven at

σ is the conductivity of the conductor

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:

$G \propto \frac {\omega \mu_0 p^2} {\rho_\mathrm{r} g}$

where p is the pole pitch arc length, ρ_r is the surface resistivity of the rotor and g is the air gap.