Direct Current Power Systems 181
power systems for large earth-moving equipment, and DC motor-drive
units for precise equipment control.DC CONVERSION SYSTEMS
Most of the electrical power produced is 60-Hz three-phase alternating
current. However, the use of direct current is necessary for many applica-
tions. For instance, DC motors have more desirable speed control and torque
characteristics than AC motors. We have already discussed two methods of
supplying direct current-batteries and DC generators. The fact that batteries
and DC generators have been used as DC power sources for many years has
been discussed. In many cases today, where large amounts of DC power are
required, it is more economical to use a system for converting AC to DC. DC
load devices may be powered by systems, called either rectifiers or convert-
ers, that change AC into a suitable form of DC.Figure 7-19. Illustration
of armature reaction:
(A) Main magnetic flux
with no current flow in
the armature windings,
(B) Distortion of main
magnetic flux with cur-
rent flow in the armature
windings