80 ELEMENTS OF ELECTRICAL ENGINEERING. 



where a smaller wire attaches to a larger one, otherwise a short 

 circuit might occur in one of the small groups of lamps, taking 

 enough current to make the small copper wires red hot, but not 

 enough to open the circuit breaker BB, or to melt the main fuses 

 in the wires b and c, thus giving rise to a serious fire risk. 



The connections of a shunt generator are the same as in Fig. 

 69 except that there is no series field winding. 



44 The building up of a generator at starting. When a self- 

 excited generator is started, the slight residual magnetism in the 

 field structure induces a small electromotive force in the armature 

 windings. This electromotive force produces a small current in 

 the field winding which strengthens the residual magnetism. 

 This strengthened residual magnetism induces an increased elec- 

 tromotive force ; this produces an increased current in the field 

 winding which in turn still further increases the magnetism, and 

 so on until the machine is in full action. This process is called 

 building up ; it usually takes half a minute or more for this build- 

 ing up process to bring a machine up to its full voltage. 



If a generator has no residual magnetism it cannot begin the 

 building up process. To get such a machine into operation it is 

 necessary to excite its field magnet temporarily, by direct current 

 from an outside source. This generally leaves enough residual 

 magnetism to start the building up process. 



It frequently happens that the current produced in the arma- 

 ture by the residual magnetism flows through the field winding 

 in such a direction as to ^veaken the residual magnetism. In this 

 case the machine cannot build up at all. This failure to build 

 up does not depend upon the direction of the residual magnetism ; 

 for if the field current produced by given residual magnetism 

 strengthens (or weakens) the residual magnetism, then if the re- 

 sidual magnetism be reversed, it will produce a reversed current, 

 and this reversed current will strengthen (or weaken) the reversed 

 residual magnetism as before. 



Whether the current produced by the residual magnetism flows 

 through the field winding in the direction to strengthen or in the 



