CHAPTER V 

 THE ALTERNATOR 



50. Rotating -field Type. The generation of an electromotive 

 force in a conductor may take place with the magnetic field 

 stationary and the conductor moving through this field, as in a 

 direct-current generator, or with the conductor stationary and 

 the field moving past the conductor. It is merely necessary that 

 then- be rdtitive motion between the conductor and the field. In 

 direct -current machines, the commutator makes it necessary 

 that either the armature be the rotating member, or that the 

 brushes revolve with the field. 



A- alternators have no commutator, it is not necessary that 

 t he armature be the rotating member. Most commercial alterna- 

 tors have stationary armatures, inside of which the field poles 

 rotate, as shown in Figs. 102, 103, etc. This construction ha.s 

 two distinct advantages. A rotating armature requires two or 

 more slip-rings for carrying the current from the armature to the 

 external circuit. Such rings must be more or less exposed, and 

 arediflicult to insulate, particularly for the higher voltages of 6,600 

 and IM.'JOO volts at which alternators are commonly o|x>rated. 

 These rings may l>ecome a frequent source of trouble, due toarc- 

 . -hort-circu A stationary armature requires no 



sli|>-ring-, and the armature leads can be cont inuously insulated 

 conductors from the armature coils to the bus-bars. It is more 

 difficult to insulate the conductors in a rotating armature than 

 tationary one, because of cent ril u. and the vibration 



MI rotation. 



When the field is the rotating member, the field current must 

 inducted to the field winding through slip-rings. A- ilie 

 field voltage seldom exceed- L'."() volts and the amount of power 

 i- -mall, no particular diflinil I in the ope; 



h >lip-r. 



dillicub -iillicieni copper on the 



an armature. This is particularly true with high-speed, high. 



90 



