OPERATION AND CARE OF ELECTRIC MACHINERY 443 



16. A self-exciting generator often fails to build up 

 when started but the cause generally can easily be located. 

 If the machine fails to build up, the first thing to examine 

 is the connection of the shunt field. It must of course be 

 properly connected to the armature circuit. It may be 

 that, even though the field circuit is properly connected 

 to the armature, there is an open circuit somewhere. One 

 of the connections between the various coils may be open. 

 A bell-ringing magneto may be used to see if the field cir- 

 cuit is open, or a test may be made with any c-c. power 

 line available; an incandescent lamp connected to a 110- 

 volt line through the field circuit to be tested, will light if 

 there is no open circuit but will not burn if the field is open 

 somewhere. When this test is made, the field must be 

 disconnected from the armature otherwise the armature 

 forms a short circuit for the field. 



Of course the generator cannot build up if the field cir- 

 cuit is open as its magnetic circuit cannot become excited. 



17. There is always a rheostat in the shunt field of a 

 generator and if this rheostat is turned to the " all in " 

 position the generator will likely refuse to build up. 



The current which flows through the shunt field circuit 

 is equal to E/Rf +r where 



E = the generated voltage 



Rf+ r = the resistance of the field coils plus the field 

 rheostat resistance. 



Now suppose the magnetization curve of the machine 

 is as given in Fig. 286, and tan (j>i = the resistance of the 

 field circuit. (The same scale is supposedly used for both 

 volts and amperes. If not, a proper change must be intro- 

 duced in the value of tan <j>.) With the resistance of the 

 field circuit equal to tan (]>, it is apparent that at no value 

 of the field current (except at very low values) does the 

 armature generate enough voltage to force through the 



