IEEE Power & Energy Magazine - July/August 2019 - 96

By the turn
of the 20th
century, the
fundamental
concepts
of traction
technology
were well
established.

changing the position
of the neutral axis. This
axis change caused the
armature coils to be
commutated when there
was a significant voltage
between segments.
Outside of motors
for Sprague's own usage, the differential
compound motor did
not find widespread acceptance. In Dynamo
Electric Machinery,
Hausmann said, "A
large starting torque
cannot be obtained from
this motor" because "the series winding
greatly decreases the field strength."
He continued, "These motors are rarely
used in practice, as improvements in
the design of shunt-wound motors have
given the latter sufficiently good speed

regulation." In The Making of the Electrical
Age: From the Telegraph to Automation,
Sharlin stated that
"the first stage in the
development of the
steel rolling mill motors was on the streets
of Richmond" is not
correct. Steel rolling
mill motors were and,
in many cases, still
are series or cumulative compound motors. They bear virtually no resemblance
to Sprague's Richmond motors.

Standard Traction
Motor Design
By the turn of the 20th century, the fundamental concepts of traction technolo-

gy were well established. Power resistors
were switched in and out of the circuit
to control acceleration. Series motors became the standard form of traction motor control. This was a departure from
Sprague's differential compound motor
that used no resistance coils whatsoever.
Figure 8 is a typical series-parallel motor control scheme. On the first step of
the controller, the motors and their
accelerating resistors are connected in
series. This maximum resistance limits the inrush to the motor to approximately 1.5 times the full load current
while developing an adequate amount
of starting torque. As the motors come
up to speed and the counter EMF reduces motor current, the starting resistance is progressively removed, going through several steps from R1 to
R7. This is shown Figure 8(a) and (b).
In Figure 8(c), the two armatures are
connected in parallel with part of the

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IEEE Power & Energy Magazine - July/August 2019

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