Principles of naval engineering - Historic Naval Ships Association

GENERATING A VOLTAGEChapter 20. -SHIPBOARD ELECTRICAL SYSTEMSThe field \windings of ad-c generator receivecurrent either from an external d-c source ordirectly across the armature, thus becomingelectromagnets. They are connected sothattheyproduce alternate north and south poles and, whenenergized, they establish magnetic flux in thefield yoke, pole pieces, air gap, and armaturecore, as shown in figure 20-11.The armature is mounted on a shaft and isrotated through the field by an outside energysource (prime mover). Thus we have a magneticfield, a conductor, and relative motion betweenthe two— which, it will be remembered, are thethree essentials for producing a voltage by magnetism.If the output of the armature is connectedacross the field windings, the voltageand the field current at start will be smallbecause of the small residual flux in the fieldpoles. However, as the generator continues torun, the small voltage across the armature willcirculate a small current through the field coilsand the field will become stronger. In a selfexcitedgenerator, this action causes the generatorvoltage to rise quickly to the proper valueand the machine is said to "buildup" its voltage.The simplest generator armature winding is aloop or single coil. Rotating this loop in a magneticfield will induce an emf whose strengthis dependent upon the strength of the magneticfield and the speed of rotation of the conductor,A single-coil generator with each coil terminalconnected to a bar of a two-segmentmetal ring is shown in figure 20-12. The two41.10Figure 20-12.— Single-coil generatorwith commutator.segments of the split ring are insulated fromeach other and the shaft, thus forming a simplecommutator which mechanically reverses thearmature coil connections tothe external circuitat the same instant that the direction of generatedvoltage reverses in the armature coil.The emf developed across the brushes ispulsating and unidirectional. Figure 20-13 is agraph of the pulsating emf for one revolution ofa single-loop armature in a 2-pole generator.A pulsating direct voltage of this characteristic(called ripple ) is unsuitable for most applications.In practical generators, more coils andmore commutator bars are used to produce anoutput voltage waveform with less ripple. FigureFIELDWINDING27.248.1 41.10Figure 20-11.— Magnetic circuit of a 2-pole Figure 20-13.— Pulsating voltage from a singlegenerator,coil armature.501