Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGin which the output voltage is generated, and(2) a group of conductors through which directcurrent is passed to obtain an electromagneticfield of fixed direction. The conductors in whichthe output voltage is generated are always referredto as the armature windings . The conductorsin which the electromagnetic fieldoriginates are always referred to as the fieldwindings .In addition to the armature and field, theremust also be relative motion between the two.To provide this relative motion, a-c generatorsare built in two major assemblies— the statorand the rotor. The rotor rotates inside thestator. The rotor may be driven by any one ofa number of commonly used prime movers,including steam turbines, gas turbines, and internalcombustion engines.TYPES OF A-C GENERATORSIn the revolving-armature a-c generator,the stator provides a stationary electromagneticfield. The rotor, acting as the armature, revolvesin the field, cutting the lines of force,producing the desired output voltage. In thisgenerator, the armature output is taken throughsliprings and thus retains its alternating characteristics.For a number of reasons, the revolvingarmaturea-c generator is seldom used. Itsprimary limitation is the fact that its outputpower is conducted through sliding contacts(sliprings and brushes). These contacts aresubject to frictional wear and sparking. Inaddition, they are exposed, and thus liable toarc-over at high voltages. Consequently, revolving-armaturegenerators are limited toapplications of low power and low voltage.The revolving-field a-c generator (fig. 20-19) is by far the most commonly used type. Inthis type of generator, direct current from aseparate source is passed through windings onthe rotor by means of sliprings and brushes.This maintains a rotating electromagnetic fieldof fixed polarity (similar to a rotating bar magnet).The rotating magnetic field, following therotor, extends outward and cuts through thearmature windings embedded in the surroundingstator. As the rotor turns, alternating voltagesare induced in the windings, since magneticfields of first one polarity and then the othercut through them. Since the output power istaken from stationary windings, the output maybe connected through fixed terminals directlyARMATURE WINDINGSSTATORft-COUTPUT TERMINALS147.124Figure 20-19.— Essential parts of a rotatingfielda-c generator.to the external loads, as through terminals Tland T2 in figure 20-19. This is advantageousbecause there are no sliding contacts and thewhole output circuit is continuously insulated,thus minimizing the danger of arc-over.Sliprings and brushes are still used on therotor to supply direct current to the field; theyare adequate for this purpose because the powerlevel in the field is much lower than in the armaturecircuit.THREE-PHASE GENERATORSThe three-phase a-c generator has threesingle-phase windings spaced so that the voltageinduced in each winding is 120° out of phasewith the voltages in the other two windings. Aschematic diagram of a three-phase statorshowing all the coils becomes complex and isdifficult to understand. A simplified schematicdiagram, showing all the windings of a singlephase as one winding, is given in figure 20-20.The rotor is omitted for the sake of simplicity.The waveforms of voltage are shown to theright of the schematic. The three voltages are120° apart and are similar to the voltages thatwould be generated by three single-phase a-cgenerators whose voltages are out of phase byangles of 120°. The three phases are independentof each other.Rather than have six leads come out of thethree-phase alternator, one of the leads from506