IEEE Power & Energy Magazine - July/August 2019 - 95
hill, he would have no series field component, with the resultant large armature current damaging the commutator
and brushes.
Thus, it's clear that Sprague used a
differential compound motor. What's
not so evident is his reason for not using a
series motor. There are several possibilities. Although the series motor was well
known at that time, it was almost entirely
used on constant-current arc lamp circuits. That series motor was actually
a shunt motor converted to run on a
constant-current circuit by connecting
what was really a shunt field in series with
the armature. Therefore, the pole windings would have been a similar (small)
gauge like any ordinary shunt field.
When used in a constant-current circuit,
it would act like an arc lamp. As the load
increased, more street lights turned on.
As more torque was required from the
motor, the generator voltage increased
to hold the current constant. This type
of series motor was
intended to allow motors to be installed in
manufacturing plants
with series lighting.
Its operation was not
at all like the constant
potential series motor, and it was considered by most to be a
dead technology.
Also, Sprague had
expended a considerable amount of intellectual capital patenting and promoting
his differential compound motor. The design parameters for
the motor field proportions were known as
Sprague's law, and he
may have been reluctant to drop a design
for which he was famous. A third factor
may have been the contemporary mind-set
regarding the governed steam engine that
blinded Sprague to
the need for a series
motor. With his differential compound
motor, its speed was
self-regulating: no governor was requi red.
From this standpoint,
it was better than the
straight shunt motor.
However, for traction
applications, Sprague's
differential compound
motor was worse than
the shunt motor. It was
the worst motor he
could have chosen. This
high current not only
overheated the brushes and commutator, it also skewed the magnetic field,
The design
parameters for
the motor field
proportions
were known
as Sprague's
law, and he
may have been
reluctant to
drop a design
for which he
was famous.
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july/august 2019
ieee power & energy magazine
95
http://www.pr-tech.com
IEEE Power & Energy Magazine - July/August 2019
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2019
Contents
IEEE Power & Energy Magazine - July/August 2019 - Cover1
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IEEE Power & Energy Magazine - July/August 2019 - Cover3
IEEE Power & Energy Magazine - July/August 2019 - Cover4
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