Figure 2-12.Sectional view of a dc generator

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Figure 2-12.—Sectional view of a dc generator. electromagnets  or  poles  magnetized  by  current flowing through the field coils. Soft iron pole pieces (or  pole  shoes)  are  contained  in  the  field  frame  that forms  the  magnetic  circuit  between  the  poles. Although generators may be designed to have any even number  of  poles,  two-  and  four-  pole  frames  are  the most common. The field coils are connected in series. In the two-pole type frame, the magnetic circuit flows through only a part of the armature core; therefore. the armature must be constructed according to the number of  field  poles  because  current  is  generated  when  the coil  (winding  on  the  armature)  moves  across  each magnetic circuit. The current is collected from the armature coils by brushes  (usually  made  of  carbon)  that  make  rubbing contact with a commutator. The commutator consists of a series of insulated copper segments mounted on one end of the armature, each segment connecting to one  or  more  armature  coils.  The  armature  coils  are connected  to  the  external  circuits  (battery,  lights,  or ignition) through the commutator and brushes. Current induced in the armature coils thus is able to flow to the external  circuits. There are two types of field circuits, determined by the point at which the field circuit is grounded, which are as follows: One circuit, referred to as the "A" circuit, shunts the  field  current  from  the  insulated  brushes through the field winding grounding externally at the regulator. In the other, the "B" circuit, the field current is shunted from the armature series winding in the regulator  to  the  generator  field  windings, grounding internally within the generator. The  three  basic  design  factors  that  determine generator output are (1) the speed of armature rotation, (2)  the  number  of  armature  conductors,  and  (3)  the strength  of  the  magnetic  field.  Any  of  these  design factors could be used to control the generator voltage and  current.  However,  the  simplest  method  is  to determine the strength of the magnetic field and thus limit the voltage and current output of the generator. 2-12

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