Motor Effect; the St. Louis Motor is a terrible design

From: Nicholas Bodley (
Date: Wed Mar 28 2001 - 15:53:14 PST

Two notable devices that make use of this principle are loudspeakers
and the head positioners in computer rigid-disk* drives. The latter
have coils wound on forms and are made with wire that bonds to
itself**, so the shaping form can be removed at the factory.

*Technically-correct term (actually used by Hewlett-Packard) for a
"hard" disk drive. Hardness and rigidity are two different

**Probably an outer layer of a thermoplastic (one that can be melted,
as contrasted with a thermosetting plastic, which doesn't melt) over
the electrical insulating layer.

I'm really glad you included this one; it is quite fundamental.

I learned about motors as a child, and even built a rough equivalent
of the St. Louis motor. It was horribly inefficient; current hog, and
not much more torque than what was needed to overcome bearing

To keep it reasonably brief, the St. Louis motor has huge air gaps, a
holdover from the 1850s or thereabouts before the E.E.s of the time
learned how to make practical electric motors. It is so misleading,
un-representative, and such a poor design that it really ought never
to be used in classrooms. (Science fair project, sure to raise a few
hackles: Measure the efficiency of a St. Louis motor and explain the

It's easy when learning about electromagnets (and permanent magnets)
to be over-impressed by the way ferromagnetic materials attract and
repel. The fatal flaw in the St. Louis motor and related designs is
that the idea of attraction and repulsion was the evident basis, not
the phenomenon shown so nicely by this experiment.

Long air gaps have an utterly disastrous degrading effect on
efficiency, and until the contemporary topology and geometry
developed, motors with useful efficiency simply could not be made. A
25-cent surplus Mabuchi motor is a vastly superior design,
technically. (It has to be! Battery life counts, folks!)

I was fooling with miserably-inefficient motors as a child, and knew
well about commutators. However, it was decades before I realized
that a commutator is no more than a special type of rotary switch
that can function well over many billions of switching cycles.

Nicholas Bodley |@| Waltham, Mass.
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