13.4 DIRECT-CURRENT MACHINES 601DC Motor Characteristics
We gain an understanding of the speed–torque characteristics of a dc motor from Equations
(13.4.2) through (13.4.4). In shunt motors, the field current can be simply controlled by the
use of a variable resistance in series with the field winding; the load current influences the
flux only through armature reaction, and its effect is therefore relatively small. In series mo-
tors, the flux is largely determined by the armature current, which is also the field current;
it is somewhat difficult to control the armature and field currents independently. In the com-
pound motor, the effect of the armature current on the flux depends on the degree of com-
pounding. Most motors are designed to develop a given horsepower at a specified speed,
and it follows from Equations (13.4.2) and (13.4.3) that the angular velocityωmcan be ex-
pressed as
ωm=Vt−IaRa
Kaφ(13.4.8)Thus, the speed of a dc motor depends on the values of the applied voltageVt, the armature current
Ia, the resistanceRa, and the field flux per poleφ.
RaIaIf IL = Ia + If
Ia = ILEa
−++−VtSeries fieldSeries fieldShunt fieldShunt fieldField rheostat(a)
Diverter resistanceRaEa
−++−Vt(b)If IL = Ia + IfRaIaEa
−++−Vt(c)Figure 13.4.8Schematic diagrams of dc motors.(a)Shunt motor.(b)Series motor.(c)Cumulatively
compounded motor.