IEEE Power & Energy Magazine - July/August 2019 - 93

To understand
why Sprague's
motor missed
the mark,
we need to
look at the
technological
focus of the late
19th century.
unstable. If large surges of current occur,
the series field can overpower the shunt
field and cause the armature to rotate
backward. Its one salient feature, however, is its very flat speed-torque curve.
However, the downside is that, when
heavily overloaded, it will draw a very
large armature current in its attempt to
maintain constant speed.

ent description is very similar to how
Sprague describes his motor in literature of that time.
In Sprague's 1888 article in The
Electrician, he wrote about his design,
"Their fields were wound with three
sets of coils, varying in the number of
turns, the cross section of the wire, and

the resistance; these are connected with a
switch, so that when the motor is thrown
into circuit the field coils are all in series
and have a definite high resistance. The
coils are progressively commutated till
they are finally thrown into a multiple
(i.e., parallel) circuit and have a resistance one-ninth that first existing."

hughesbros.com

quality forged daily

Richmond, Virginia,
1887-1888
Sprague's Richmond motors were very
similar to the motors he had been
manufacturing for several years. Figure 6 shows one of his motors installed
in a trolley car. They were differential
compound motors. Sprague's U.S. Patent 315180, issued 7 April 1885, details his design. Figure 7 is a diagram
showing the connections for the fields.
The letter A denotes the armature windings, and B indicates the windings of
the field poles. In this diagram, which
accompanied the patent application,
Sprague said this is "an arrangement
whereby the simple shunt motor may
be made a differential one ... the relation between the shunt and series fieldcoil may be changed." The commutator used by the operator for switching
the field connections is shown at D. In
this case, the word commutator was
used in its general sense: a device that
switches. The commutator shown at
A is what switches the armature coils
and is still referred to as a commutator in modern terminology. This patjuly/august 2019

testing
services

storm kits

ieee power & energy magazine

93



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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
IEEE Power & Energy Magazine - July/August 2019 - Cover2
IEEE Power & Energy Magazine - July/August 2019 - Contents
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IEEE Power & Energy Magazine - July/August 2019 - Cover3
IEEE Power & Energy Magazine - July/August 2019 - Cover4
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