Page 647 - Basic Electrical Engineering

7.14 APPLICATIONS OF DC MACHINES

7.14.1 DC Generators

In earlier days dc generators were used to generate electricity and the power
was supplied to consumers through dc distribution networks. At present, use

of dc generators for generation and distribution of electricity is rare. All
commercial generators are ac generators which are also called alternators.

Generation and transmission of alternating current has a number of
advantages. The use of dc generators is confined to supplying excitation
current to ac generators and to convert ac to dc for industrial applications.

7.14.2 DC Motors

DC motors are available as dc shunt motors, dc series motors, and dc

compound motors. DC shunt motors are more or less constant-speed motors.
They may be used in driving a line shaft where speed has to be kept fairly
constant between no load and full load. In situations where variable load has

to be driven but at constant speed, such as driving a lathe, the speed change
can be obtained using a shunt field regulator.

7.14.3 DC Series Motors

In applications where high starting torque is required, such as in driving
hoists, cranes, electric trains, etc. series motors are used. Series motors are

also used where the motor can be permanently coupled to the load, such as
fans, where the torque requirement increases with speed.

Series motors attain very high speed at light load. That is why series
motors should never be run on no load.