AC  GENERATORS Alternating-current   generators   produce   most electric power used today. Ac generators are also used in  aircraft  and  automobiles. Ac  generators  come  in  many  different  sizes, depending on their intended use. For example, any one of the huge generators at Boulder Dam can produce millions of volt-amperes, while the small generators used  on  aircraft  produce  only  a  few  thousand volt-amperes. Regardless  of  their  size,  all  generators  operate  on the  same  basic  principle-a  magnetic  field  cutting through conductors, or conductors passing through a magnetic field. All generators have at least two distinct sets of conductors: A  group  of  conductors  in  which  the  output voltage is generated known as the  armature winding. A second group of conductors through which direct   current   is   passed   to   obtain   an electromagnetic field of fixed polarity known as the field  winding. Since  relative  motion  is  needed  between  the armature and field flux, ac generators are built in two major assemblies—the stator and the rotor (fig. 3-11). Figure 3-11.-An ac generator and schematic. The rotor rotates inside the stator. It is driven by several commonly  used  power  sources:  gas  or  steam  turbines, electric motors, and internal-combustion engines. THREE-PHASE GENERATORS A three-phase ac generator, as the name implies, 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  it  is  difficult  to  see  what  is actually happening. A simplified schematic diagram showing all the windings of a single phase lumped together as one winding is illustrated in figure 3-12, view  A. The  rotor  is  omitted  for  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 ac generators whose voltages are out of phase  by  angles  of  120°.  The  three  phases  are independent  of  each  other. Wye  Connection Rather  than  have  six  leads  come  out  of  the three-phase ac generator, one of the leads from each phase may be connected to form a common junction. The stator is then said to be wye, or star, connected. The common lead may or may not be brought out of the machine. If it is brought out, it is called the neutral. The simplified  schematic  (fig.  3-12,  view  B)  shows  a wye-connected stator with the common lead not brought out. Each load is connected across two phases in series. is connected across phases A and B in series; is connected across phases A and C in series; and Figure  3-12.—Three-phase  ac  generator:  A.  Simplified schematic and wave forms; B. Wye connection; C. Delta connection. 3-8


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