IEEE Power & Energy Magazine - March/April 2016 - 89

to as "Sprague laws." Since his
Key Sprague dc
years at Annapolis, Sprague had
Motor Patents
repeatedly argued that almost
Of his some 30 electric motor
any action in the electrical field,
patents, the following are some of
or any other technical field for
the most important ones related to
that matter, could be explained
this motor. U.S. Patent 295,454,
using scientific first principles;
"Electro-Dynamic Motor," was
hence the beginning of his laws.
filed 2 May 1883 and issued 18
Reviewing his patent noteMarch 1884. Following steam
books provides fascinating inengine practice, speed control is
sight into how his mind worked
maintained "using a mechanical
and the process involved in the
centrifugal governor attached to
development of his self-regulatthe axle of the armature which,
ing dc electric motor, a process
as speed of rotation changes with
that took place almost entirely in
varying loads, through a second1884. In one notebook entry he
ary apparatus, changes the posidescribes the derivation of a series
tion of the switch that controls the
of "Sprague laws" that interrelate
relation of the armature and field
E (initial emf), e (counter emf),
circuits in such a way as to comm (magnetic moment of the main
pensate for changes in the axle
shunt coil), and u (magnetic morotation" (see Figure 3). Sprague
ment of the differential series coil).
was never very happy with it beSeveral examples of specific laws
cause it was mechanical, crude,
are shown in the patent discussions
and above all, inelegant. But it
that follow. In a notebook discusworked and was a start.
sion of self-regulation of one of his
U.S. Patent 313,247, "Regumotors, one finds an entry headed
lator for Electro-Dynamic Mo"Suggestion," which proposes
tor," was filed 21 February 1884
We know that to get a conas an improvement on U.S. Patent
stant current with a variable
295,454. The mechanical regulator
external resistance (R or load)
is still present, but the motor configure 5. A drawing for the Sprague electric motor
in a dynamo-electric gentrol is improved by adding a small
and generator patent U.S. Patent 428,732, issued to
erator, and to get a constant
auxiliary motor between the govSprague on 27 May 1890. (Photo courtesy of John
speed with constant potential
ernor and the commutator switch.
L. Sprague.)
in a motor with varying load,
Also, for the first time there is a
it is necessary to have identi"Sprague law" describing how the
modified the theory as necessary. As control system works. An excerpt of the
cally the same windings (in each).
we shall see, development of his self- law is shown in Figure 4. It theorizes that
If so, it is probable that if a dynamo
regulating nonsparking dc motor was in a motor with armature and field coils in
generator can be wound to give a
an iterative process covering a little shunt relationship, power, economy, and
constant potential at its terminals
over a year.
under a varying external load, if it is
efficiency depend upon the initial and
then used as a motor it will maintain
a constant speed under constant current with varying loads. So we'll first
try the generator ....
Pages of calculations, curves, circuit
diagrams, and measurements follow.
This is a perfect example of how
his mind worked in solving complex
problems such as creating a constant
speed motor. Using first principles and
prior experience, he developed a theory or law. He then went about making
dynamos or motors according to the figure 6. A group of constant-speed motors manufactured by the Sprague Electric
law, measured their performance, and Railroad and Motor company, circa 1884. (Photo courtesy of John L. Sprague.)
88

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