PRINCIPLES OF NAVAL ENGINEERING 



in which the output voltage is generated, and 

 (2) a group of conductors through which direct 

 current is passed to obtain an electromagnetic 

 field of fixed direction. The conductors in which 

 the output voltage is generated are always re- 

 ferred to as the armature windings . The con- 

 ductors in which the electromagnetic field 

 originates are always referred to as the field 

 windings . 



In addition to the armature and field, there 

 must also be relative motion between the two. 

 To provide this relative motion, a-c generators 

 are built in two major assemblies— the stator 

 and the rotor. The rotor rotates inside the 

 stator. The rotor may be driven by any one of 

 a number of commonly used prime movers, 

 including steam turbines, gas turbines, and in- 

 ternal combustion engines. 



TYPES OF A-C GENERATORS 



In the revolving-armature a-c generator, 

 the stator provides a stationary electromagnetic 

 field. The rotor, acting as the armature, re- 

 volves in the field, cutting the lines of force, 

 producing the desired output voltage. In this 

 generator, the armature output is taken through 

 sliprings and thus retains its alternating char- 

 acteristics. 



For a number of reasons, the revolving- 

 armature a-c generator is seldom used. Its 

 primary limitation is the fact that its output 

 power is conducted through sliding contacts 

 (sliprings and brushes). These contacts are 

 subject to frictional wear and sparking. In 

 addition, they are exposed, and thus liable to 

 arc-over at high voltages. Consequently, re- 

 volving-armature generators are limited to 

 applications of low power and low voltage. 



The revolving-field a-c generator (fig. 20- 

 19) is by far the most commonly used type. In 

 this type of generator, direct current from a 

 separate source is passed through windings on 

 the rotor by means of sliprings and brushes. 

 This maintains a rotating electromagnetic field 

 of fixed polarity (similar to a rotating bar mag- 

 net). The rotating magnetic field, following the 

 rotor, extends outward and cuts through the 

 armature windings embedded in the surrounding 

 stator. As the rotor turns, alternating voltages 

 are induced in the windings, since magnetic 

 fields of first one polarity and then the other 

 cut through them. Since the output power is 

 taken from stationary windings, the output may 

 be connected through fixed terminals directly 



STATOR 



ft-C 

 OUTPUT TERMINALS 



ARMATURE WINDINGS 



147.124 

 Figure 20-19.— Essential parts of a rotating- 

 field a-c generator. 



to the external loads, as through terminals Tl 

 and T2 in figure 20-19. This is advantageous 

 because there are no sliding contacts and the 

 whole output circuit is continuously insulated, 

 thus minimizing the danger of arc-over. 



Sliprings and brushes are still used on the 

 rotor to supply direct current to the field; they 

 are adequate for this purpose because the power 

 level in the field is much lower than in the ar- 

 mature circuit. 



THREE-PHASE GENERATORS 



The three-phase a-c generator has three 

 single-phase windings spaced so that the voltage 

 induced in each winding is 120° out of phase 

 with the voltages in the other two windings. A 

 schematic diagram of a three-phase stator 

 showing all the coils becomes complex and is 

 difficult to understand. A simplified schematic 

 diagram, showing all the windings of a single 

 phase 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 the 

 right of the schematic. The three voltages are 

 120° apart and are similar to the voltages that 

 would be generated by three single-phase a-c 

 generators whose voltages are out of phase by 

 angles of 120°. The three phases are independ- 

 ent of each other. 



Rather than have six leads come out of the 

 three-phase alternator, one of the leads from 



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