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Goodness factor

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Metric for determining the efficiency of an electric motor

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.

G = ω r e s i s t a n c e × r e l u c t a n c e = ω μ σ A e A m l e l m {\displaystyle G={\frac {\omega }{\mathrm {resistance} \times \mathrm {reluctance} }}={\frac {\omega \mu \sigma A_{\mathrm {e} }A_{\mathrm {m} }}{l_{\mathrm {e} }l_{\mathrm {m} }}}}

where

G is the goodness factor (factors above 1 are likely to be efficient)
Ae, Am are the cross sections of the electric and magnetic circuits
le, lm are the lengths of the electric and magnetic 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 ∝ ω μ 0 p 2 ρ r g {\displaystyle G\propto {\frac {\omega \mu _{0}p^{2}}{\rho _{\mathrm {r} }g}}}

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

References

  1. ER Laithwaite (1965). "The Goodness of a Machine". Electronics and Power. 11 (3): 101–103. doi:10.1049/ep.1965.0071.
  2. DJ Patterson; CW Brice; RA Dougal; D Kovuri (2003). "The "goodness" of small contemporary permanent magnet electric machines" (PDF). IEEE International Electric Machines and Drives Conference, 2003. IEMDC'03. Vol. 2. pp. 1195–1200. doi:10.1109/IEMDC.2003.1210392. ISBN 0-7803-7817-2. S2CID 14563810.
  3. ER Laithwaite (1965). "Electromagnetic levitation". Electronics and Power. 11 (12): 408–410. doi:10.1049/ep.1965.0312.
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