OPERATION OF THE DYNAMO AS A GENERATOR. 87 



of a series generator is proportional to the speed, and the ter- 

 minal voltage, E x , is very nearly proportional to the speed. 



Adaptability of the various methods of field excitation to special 

 purposes. Separate excitation is very often used in dynamo 

 testing. When it is desired to obtain a voltage which fluctuates 

 as little as possible with changes of speed, separate excitation is 

 used in commercial installations. 



Generators which are used for charging storage batteries, and 

 generators which are used for electroplating, are liable to have 

 their field magnetism reversed when the machine is stopped with- 

 out disconnecting it from the external receiving circuit. In such 

 service the reversal of field magnetism is very much less likely to 

 occur when the generator is separately excited. Therefore sep- 

 arately excited generators are sometimes used for charging stor- 

 age batteries, and usually for electroplating. 



Direct-current generators in central stations for lighting or 

 power are nearly always self-excited, inasmuch as modern engines 

 and water wheels give very nearly constant- speed driving, and 

 therefore the use of an auxiliary generator for supplying current 

 for exciting the field magnets of the main generators is not 

 usually warranted by the greater constancy of voltage obtainable 

 thereby. Alternating current generators are always separately 

 excited, since the alternating current delivered by an alter- 

 nating current generator is not suitable for field excitation, un- 

 less rectified by a special commutator as in the " composite field " 

 alternator. 



When electric current is to be supplied at a constant, or approxi- 

 mately constant, voltage, shunt or compound generators are used, 

 as has been previously pointed out. Series generators are not 

 suited to the delivery of a varying current at a constant voltage. 

 When a constant current is to be supplied at varying voltage, 

 series generators are generally used. Such generators are pro- 

 vided with special regulating devices to vary the voltage, so as to 

 keep the current output constant when the resistance of the 

 external circuit changes. 



