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.

EO 1.1DEFINE terminal voltage as it applies to DC generators.

EO 1.2DEFINE counter-electromotive force (CEMF) as it applies to a DC machine.

EO 1.3DESCRIBE the effects of commutation in a DC generator.

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

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.

Figure I AC to DC Conversion with a Commutator

In a DC generator, commutation provides for the conversion of AC to a DC output that is generated in the armature windings. Commutation will be discussed in greater detail in subsequent chapters.

Summary

DC equipment terms are summarized below.

DC Equipment Terminology Summary

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) is defined as the induced voltage that acts to counter the applied voltage in a DC motor or a DC generator.

Applied voltage is defined as the voltage that is delivered across the load.

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.

In a DC generator, commutation provides for the conversion of AC to a DC output that is generated in the armature windings.