i-BJ COMPOUND GENERATOR. 13 



EXPERIMENT i-B. Characteristics of a Compound* Generator. 



i. Introductory. A compound generator is made for the 

 purpose of delivering current at constant potential either at the 

 terminals of the machine or at some distant receiving point on the 

 line. In the former case the machine is flat compounded, the 

 ideal being the same terminal voltage at full load as at no load, 

 giving a practically horizontal voltage characteristic. In the 

 latter case the machine is over compounded, giving a terminal 

 voltage which rises from no load to full load to compensate for 

 line drop, so that at the receiving end of the line the voltage 

 will be constant at all loads. Constant potential service is used 

 both for power and for lighting. Constant delivered voltage is 

 essential in lighting for steadiness of illumination and in power 

 for constant speed. 



2. For such service, the series generator is not at all adapted, 

 its voltage being exceedingly low at no load and, for a certain 

 range, increasing greatly with load. 



3. A shunt generator almost meets the conditions, generating 

 a voltage which is nearly constant but decreasing slightly with 

 load (Figs. 4 and 6). Obviously by increasing the field excita- 

 tion (field ampere-turns) when the machine is loaded, the voltage 

 can be increased to the desired value ; this is true, however, only in 

 case the iron is not saturated and it is accordingly possible for the 

 increase in field ampere-turns to produce a corresponding in- 

 crease in the magnetic flux. (Compare Fig. 2.) In a shunt 

 machine this increase in field excitation can be obtained by an 

 increase in field current produced either by an attendant who 

 adjusts trie field rheostat or by an automatic^ regulator. 



* (ia). This experiment can be applied to a Shunt generator by omit- 

 ting 20-25. 



t ( 3a). Tirrell Regulator. Many older forms of regulators, which oper- 

 ated by varying field resistance, are superseded by the Tirrell Regulator. 

 This regulator operates through a relay as follows: (i) When the volt- 

 age is too low, it momentarily short circuits the field rheostat, causing the 



