134^ ELECTRICAL MACHINERY 



The field coils can produce no magnetic field because they 

 have no current (the terminal voltage being zero) but 

 there is some residual magnetism to give the armature 

 e.m.f. which produces the short circuit current OE. At 

 this point the external circuit has zero resistance and the 

 machine is short circuited. 



Compound Generator. In the case of the compound 

 generator the curve of generated e.m.f. rises as the load 

 increases. This is due primarily to the fact that the series 

 turns on the field give a magnetizing force which increases 

 directly with the load while the magnetizing effect of the 

 shunt field remains nearly constant; the total magnetizing 

 force, therefore, increases somewhat with the load. 



Amount of Compounding Used. The amount of increase 

 in generated e.m.f. as the load increases, depends upon 

 the number of turns in the series windings. It is customary 

 to put in the series coils a sufficient number of turns to 

 cause the terminal 'voltage of the machine to rise as the 

 load increases; such a machine is said to be over-com- 

 pounded. The amount of over-compounding is generally 

 about 10%; for example a small lighting generator might 

 be rated as 110 volts no load, 125 volts full load, and a 

 railway generator might be rated as 550 volts no load, 

 600 volts full load. The amount of over-compounding 

 depends upon the service for which the machine is intended; 

 it may be none at all, in which the machine is said to be 

 flat compounded. If there are not enough turns in the 

 series field to keep the terminal voltage from dropping 

 as the load increases, the machine is said to be under- 

 compounded. Such machines are never used in practice. 



Use of the Compound Generator. Practically all machines 

 used in lighting or railway . service are over-compounded. 

 In such work the characteristic to be obtained is a constant 

 voltage at the load, not at the generator. The life and effi- 

 ciency of an incandescent lamp are both greatly affected if 

 the voltage of the line to which it is connected goes either 



