IEEE Power & Energy Magazine - July/August 2019 - 90

FP
FP
A

figure 3. A field pole and armature
flux interact to produce motor torque.
(Source: L.W. Gant, Elements of Electric
Traction for Motormen and Others,
New York: Van Nostrand, 1907, p. 81.)

However, because of the preoccupation
with replacing the steam engine, this
was not recognized at that time. As a result, the ordinary dc motor was applied
to the streetcar.
In 1888, the ordinary motor was, with
few exceptions, a shunt motor. This was

90

ieee power & energy magazine

FS
A

FS
A

VS

VS

VS

(a)

(b)

(c)

figure 4. Three types of motors: (a) a shunt motor, with the field connected in
parallel with the armature; (b) a series motor, with the field connected in series
with the armature; and (c) a compound motor, with one field connected in parallel with the armature and the other in series.

not the best choice for streetcar work. To
appreciate what the problems were, we
need a brief review of motor operation.

Motor Operation
Motor operation is best understood in
terms of torque produced by the in-

teraction of two magnetic fields: one
field from the field poles (Bp) and one
from the armature (Ba) (see Figure 3).
The torque equation also involves the
sine of the angle between the fields;
but for a dc motor, the commutator
ensures that this angle is very nearly
90°. Therefore, strengthening either
(or both) of these fields will increase
motor torque.
There are three types of dc motors:
shunt, series, and compound, as illustrated in Figure 4. In a shunt motor,
the field windings (F) are connected
in shunt or in parallel with the armature windings (A). This shunt field
is made from many turns of a small
diameter wire. Figure  4(a) shows this
connection. The motor is operated from
a constant potential supply (Vs). With a
streetcar system, the potential is never
constant. It varies with the number of
cars on the line and the distance from
the powerhouse. However, for our purposes, the assumption of constant potential is valid.
In the series motor, the field is connected in series with the armature, as
shown in Figure 4(b). The series field is
wound with a small number of turns of
a large diameter wire. This configuration has been used for more than 100
years. As with the shunt motor, it is also
operated from a constant potential supply. The compound motor, shown in
Figure  4(c), has both series and shunt
fields. The series field can be connected in two ways: to aid the shunt field or
buck it.
july/august 2019



http://trakkasystems.com/

IEEE Power & Energy Magazine - July/August 2019

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Contents
IEEE Power & Energy Magazine - July/August 2019 - Cover1
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