The goodness factor is a metric developed by
Eric Laithwaite to determine the 'goodness' of an
electric motor
An electric motor is a 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 electromagnetic coil, wire winding to gene ...
. Using it he was able to develop efficient
magnetic levitation
Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation (physics), suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitation ...
induction motor
An induction motor or asynchronous motor is an AC motor, AC electric motor in which the electric current in the rotor (electric), rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding ...
s.
[
]
:
where
: is the goodness factor (factors above 1 are likely to be efficient)
:, are the
cross sections of the electric and magnetic circuits
:, are the lengths of the electric and magnetic circuits
: is the
permeability of the core
: is the
angular frequency
In physics, angular frequency (symbol ''ω''), also called angular speed and angular rate, is a scalar measure of the angle rate (the angle per unit time) or the temporal rate of change of the phase argument of a sinusoidal waveform or sine ...
the motor is driven at
: 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:
:
where is the pole pitch
arc length
Arc length is the distance between two points along a section of a curve. Development of a formulation of arc length suitable for applications to mathematics and the sciences is a problem in vector calculus and in differential geometry. In the ...
, is the surface
resistivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity i ...
of the
rotor
ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...
and is the air gap.
References
{{reflist
Induction motors
Dimensionless numbers of physics
Magnetism