IEEE Power & Energy Magazine - July/August 2019 - 88

were "cut and try." Sprague was one
of the few that used some theoretical
basis for his work. Although the electric motor had many advantages over
the stationary reciprocating steam engine, the demand for motors to replace
steam engines was not yet compelling.
In 1888, when Tesla gave his induction
motor paper to the American Institute
of Electrical Engineers (as described in
J.J. Cunningham's article "89 Liberty
Street: The Birthplace of Commercial
Polyphase Power"), the reciprocating stationary steam engine was the
motor used for virtually all industrial
power requirements. Steam engines

Sprague was a
major contributor
to modern
technology and
is considered
the father of
electric traction.

had become a highly
sophisticated, mature
technology, as they
had been developed
more than a century
earlier. The 1890 U.S.
Census showed that
there was fewer than
3,000 hp in electric
motors in New York
State but more than
397,000 hp in stationary steam engines.
And these were the engines in the iron
and steel industry only. For every 1 hp
produced by electric motors, at least

figure 2. A drawing for a Sprague motor using a fly-ball governor to produce
constant motor speed. (Source: U.S. Patent 295454, 18 March 1884.)
88

ieee power & energy magazine

300 hp was produced
by steam. In many
industries, electric motors did not replace
steam engines until
well into the 1920s.
These motors had
a physical appearance
very different from the
motors we know today.
Figure 1 shows one of
Sprague's machines
of the mid-1880s. The
fundamentals were the same as today:
two magnetic fields (from the field poles
and armature) interacted to produce
torque. Also, voltage drop followed Ohm's
law. However, the effects of changing
magnetic fields were poorly understood
by virtually everyone. This gave rise to
some strange machines that reacted to
their electric circuit much differently
than machines of today. The electric
motor's takeover of the steam engine
would have to wait until the processes
by which motors operated were better
understood, not just by researchers but
also by practicing engineers.
Another problem was the high armature resistance. It was significantly
higher than in a modern machine. There
were several reasons for this. Poorly designed armature cooling was one factor.
Heat was generated both by current flow
in the armature conductors and by eddy
currents in the unlaminated armature
core. This heating further increased the
resistance of the armature conductors.
Also, the conductors were often connected in series around the armature
rather than in a number of parallel paths.
These series-or lap windings, as they
would later be called-allowed the use
of only two brushes, but they increased
the armature resistance.
In 1887, the year the Richmond project
began, the most common motor was a dc
machine with the field poles connected in
parallel with the armature. This type of
machine came to be called a shunt motor.
Because of high armature resistance, a
motor's speed could drop 5% from no load
to full load. This speed regulation was not
considered acceptable for an electric motor to replace a steam engine.
july/august 2019
IEEE Power & Energy Magazine - July/August 2019

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