DC GeneratorsDC EQUIPMENT TERMINOLOGYDC EQUIPMENT TERMINOLOGYDirect current devices are used frequently in today’s technology. Before theconstruction and operation of these devices can be introduced, a few commonterms must be understood.EO 1.1 DEFINE terminal voltage as it applies to DCgenerators.EO 1.2 DEFINE counter-electromotive force (CEMF) asit applies to a DC machine.EO 1.3 DESCRIBE the effects of commutation in a DC generator.TerminalVoltageTerminal voltage, as applied to DC generators, is defined as the voltage that can be measured atthe output of the generator.Counter-ElectromotiveForce(CEMF)In a generator using a rotating armature, the conductors cut the magnetic lines of force in themagnetic field. Voltage is induced in the armature conductors. This induced voltage opposesthe applied voltage; it counteracts some of the applied voltage, which reduces the current flowthrough the armature. This induced voltage acts counter to applied voltage; therefore, it is calledcounter-electromotive force (CEMF).AppliedVoltageApplied voltage is defined as the voltage that is delivered across the load. This voltage shouldbe the same as terminal voltage; however, various circuit faults and losses may reduce theterminal voltage.CommutationCommutation is the positioning of the DC generator brushes so that the commutator segmentschange 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, asshown in Figure 1.Rev. 0 Page 1 ES-05
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