The goodness factor is a metric developed by

Eric Laithwaite Eric Roberts Laithwaite (14 June 1921 – 27 November 1997) was a British electrical engineer, known as the "Father of Maglev" for his development of the linear induction motor and maglev rail system. Biography Eric Roberts Laithwaite wa ...

to determine the 'goodness' of an

electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...

. Using it he was able to develop efficient

magnetic levitation Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic force is used to counteract the effects of the gravitational force and any other forces. The ...

induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction mot ...

s. :

G = \frac   = \frac

where : is the goodness factor (factors above 1 are likely to be efficient) :, are the

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

s of the magnetic and electric circuit :, are the lengths of the magnetic and electric circuits : is the permeability of the core : is the

angular frequency In physics, angular frequency "''ω''" (also referred to by the terms angular speed, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit tim ...

the motor is driven at : is the

conductivity Conductivity may refer to: *Electrical conductivity, a measure of a material's ability to conduct an electric current **Conductivity (electrolytic), the electrical conductivity of an electrolyte in solution **Ionic conductivity (solid state), elec ...

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

where is the pole pitch

arc length ARC may refer to: Business * Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s * Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services * ...

, is the surface

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

of the

rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...

and is the air gap.

References

{{reflist Induction motors Dimensionless numbers Magnetism