IEEE Power & Energy Magazine - July/August 2019 - 89

Virtually all steam engines were used
to drive mills or machine tools whose
owners made their living on how much
work could be done in a unit of time. As
business improved, more machines were
added to a steam engine's line shaft.
Each driven production machine would
require a certain amount of torque to
produce parts at a certain speed. With
more machines, more torque would be
required from the steam engine. This
increased load torque would cause the
steam engine to slow down and the entire production line with it. If some device was not devised to automatically
increase the output torque of the engine,
production levels would be reduced.
In 1788, James Watt invented the
steam engine governor for just that
purpose. Within the control limits
of its governor (it was often a simple
proportional controller), the steam engine was a constant-speed machine. Its
torque/speed curve was (mostly) flat.
Because of the electric motor's poor
regulation, it was thought that some
governing means, such as that of the
steam engine, would be needed to keep
the motor speed as close to a constant
speed as possible. In 1889, Francis B.
Crocker wrote "Electric Motor Regulation," in which various luminaries
put forth ideas regarding the best way
to hold constant the speed of an electric motor.
Sprague was no different. In Figure 2, one of his early patents shows
control of a motor by means of a flyball governor. Later, as theoretical
knowledge advanced, the motor would
be redesigned to reduce armature resistance and magnetic losses and, thus,
improve regulation. However, this was
a slow process. Moreover, the incentive
to design a better motor would have
to wait until the industrial market required it.
For street railway motors, however,
the market was different. The streetcar
of that time used horses as power. But
horses were very expensive to maintain, and their exhaust products were
becoming difficult to manage. The person who was able to install an electric
motor on a horse car could become
july/august 2019

wealthy. Several individuals were trying, and Sprague was one of them.
Streetcars did not require a large
amount of horsepower. This lined up
well with the 10-20-hp capacity of contemporary motors. But streetcars were
not like the machinery driven by steam
engines: they did not require a constant-

speed drive. They needed a motor that
acted like a horse: when the going got
tough, the horse slowed down. Horses
didn't try to maintain a constant speed.
Instead, they just pulled harder. Ultimately, the successful streetcar motor
would do the same: pull harder. In other
words, they would produce more torque.

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ieee power & energy magazine

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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
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IEEE Power & Energy Magazine - July/August 2019 - Cover3
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