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Direct current devices are used frequently in today’s technology. Before the
construction and operation of these devices can be introduced, a few common
terms must be understood.


Terminal Voltage

Terminal voltage, as applied to DC generators, is defined as the voltage that can be measured at
the output of the generator.

Counter-Electromotive Force (CEMF)

In a generator using a rotating armature, the conductors cut the magnetic lines of force in the
magnetic field. Voltage is induced in the armature conductors. This induced voltage opposes
the applied voltage; it counteracts some of the applied voltage, which reduces the current flow
through the armature. This induced voltage acts counter to applied voltage; therefore, it is called
counter-electromotive force (CEMF).

Applied Voltage

Applied voltage is defined as the voltage that is delivered across the load. This voltage should
be the same as terminal voltage; however, various circuit faults and losses may reduce the
terminal voltage.


Commutation is the positioning of the DC generator brushes so that the commutator segments
change brushes at the same time the armature current changes direction. More simply stated,
commutation is the mechanical conversion from AC to DC at the brushes of a DC machine, as
shown in Figure 1.



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