Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



GENERATING A VOLTAGE 



The field \windings of ad-c generator receive 

 current either from an external d-c source or 

 directly across the armature, thus becoming 

 electromagnets. They are connected sothatthey 

 produce alternate north and south poles and, when 

 energized, they establish magnetic flux in the 

 field yoke, pole pieces, air gap, and armature 

 core, as shown in figure 20-11. 



The armature is mounted on a shaft and is 

 rotated through the field by an outside energy 

 source (prime mover). Thus we have a magnetic 

 field, a conductor, and relative motion between 

 the two— which, it will be remembered, are the 

 three essentials for producing a voltage by mag- 

 netism. If the output of the armature is con- 

 nected across the field windings, the voltage 

 and the field current at start will be small 

 because of the small residual flux in the field 

 poles. However, as the generator continues to 

 run, the small voltage across the armature will 

 circulate a small current through the field coils 

 and the field will become stronger. In a self- 

 excited generator, this action causes the gen- 

 erator voltage to rise quickly to the proper value 

 and the machine is said to "buildup" its voltage. 



The simplest generator armature winding is a 

 loop or single coil. Rotating this loop in a mag- 

 netic field will induce an emf whose strength 

 is dependent upon the strength of the magnetic 

 field and the speed of rotation of the conductor, 



A single-coil generator with each coil ter- 

 minal connected to a bar of a two-segment 

 metal ring is shown in figure 20-12. The two 



41.10 

 Figure 20-12.— Single-coil generator 

 with commutator. 



segments of the split ring are insulated from 

 each other and the shaft, thus forming a simple 

 commutator which mechanically reverses the 

 armature coil connections tothe external circuit 

 at the same instant that the direction of gener- 

 ated voltage reverses in the armature coil. 



The emf developed across the brushes is 

 pulsating and unidirectional. Figure 20-13 is a 

 graph of the pulsating emf for one revolution of 

 a single-loop armature in a 2-pole generator. 

 A pulsating direct voltage of this characteristic 

 (called ripple ) is unsuitable for most applica- 

 tions. In practical generators, more coils and 

 more commutator bars are used to produce an 

 output voltage waveform with less ripple. Figure 



FIELD 

 WINDING 



27.248.1 41.10 



Figure 20-11.— Magnetic circuit of a 2-pole Figure 20-13.— Pulsating voltage from a single- 

 generator, coil armature. 



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