Electrical Power Systems Technology

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360 Electrical Power Systems Technology


small amount of current flows through the field. A strong electromagnetic
field is produced because of the many turns of wire that form the field
windings.
A large majority (about 95 percent) of the current drawn by the shunt
motor flows in the armature circuit. Since the field current has little ef-
fect on the strength of the field, motor speed is not affected appreciably
by variations in load current. The relationship of the currents that flow
through a DC shunt motor is as follows:


IT = IA + IF
where:
IT = the total current drawn from the power source,
IA = the armature current, and IF = the field current.


The field current may be varied by placing a variable resistance in
series with the field windings. Since the current in the field circuit is low, a
low-wattage rheostat may be used to vary the speed of the motor in accor-
dance with the variation in field resistance. As field resistance increases,
field current will decrease. A decrease in field current reduces the strength
of the electromagnetic field. When the field flux is decreased, the armature
will rotate faster, because of reduced magnetic-field interaction. Thus, the
speed of a DC shunt motor may be easily varied by using a field rheostat.
The shunt-wound DC motor has very good speed regulation. The
speed does decrease slightly when the load increases, as the result of the
increase in voltage drop across the armature. Because of its good speed
regulation, and its ease of speed control, the DC shunt motor is commonly
used for industrial applications. Many types of variable-speed machine
tools are driven by DC shunt motors.
Compound-wound DC Motors—The compound-wound DC motor,
shown in Figure 14-7, has two sets of field windings, one in series with the
armature and one in parallel. This motor combines the desirable charac-
teristics of the series- and shunt-wound motors. It has high torque similar
to that of a series-wound motor, along with good speed regulation simi-
lar to that of a shunt motor. Therefore, when good torque and good speed
regulation are needed, the compound-wound DC motor can be used. A
major disadvantage of a compound-wound motor is its expense.
Comparison of DC Motor Characteristics—The characteristics of DC
motors should be considered when motors for particular applications are
selected. Figure 14-8 shows comparative graphs that illustrate the relative

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