CHAPTER VII. 



ALTERNATORS. 



Generation of Electromotive Force in the Armature. In 



an alternator electromotive force is generated in the 

 conductors by causing them to move across the lines of a 

 magnetic field.* 



The value of the electromotive force induced in each 



lines cut per second 

 conductor is equal to ^ - volts. The 



voltage induced in the armature at any instant = (the voltage 

 induced in each conductor) x (number of conductors in 

 series), if the voltage in each conductor is equal and similarly 

 directed to that of the others. 



In a direct-current generator the number of conductors 

 acting in series between the brushes is constant, and, owing 

 to the action of the commutator, the conductors supplying 

 current to the external circuit are always similarly situated 

 with respect to the magnets of the machine. Consequently 

 the voltage induced in the armature is constant, so long as 

 the speed and strength of the field do not vary. 



In an alternator, current is collected from the same and 

 not from similarly situated conductors. Consequently the 

 voltage varies with the strength of the field in which the 

 conductors are moving at any moment, and reverses as the 

 conductors move from a field of a given direction into an 

 oppositely directed field. 



Since the conductors move at a constant speed, the electro- 

 motive force induced in them will undergo exactly the same 

 variations as the field strength exhibits when traced round 

 the armature. 



With rectangular poles it is not usually possible to get a 

 field which varies exactly harmonically so as to give a true 

 sine curve as the wave form of the electromotive force induced 

 in the conductors. This may, however, be accomplished 

 by using specially shaped pole tips, or setting the pole faces 

 at an angle to the axis of the armature. 



* For simplicity in explanation it is assumed that the conductors move 

 and the field is fixed. The same reasoning exactly applies to machines in 

 which the armature is fixed and the field rotates. 



