CHAPTER XII 

 CHARACTERISTICS 



Note. In plotting always use the same scale for abscissae and 

 ordinates. 



Unless otherwise stated " armature current" means the com- 

 bined current of all the parallel paths, and " armature resistance" 

 means the resistance from brush to brush. 



1. Assuming that a certain magneto generates a constant 

 e.m.f . of 60 volts regardless of load, and has an armature resistance 

 of 120 ohms, what current will it give when short circuited? With 

 what current will it give 50 volts at the terminals? (J min.) 



2. Required the e.m.f. generated and the current in the arma- 

 ture in each of the following cases: 



(a) A 100-ohm magneto sends 0.1 ampere through 1000 ohms 

 resistance. 



(6) A shunt machine having field and armature resistances, 50 

 and 0.01 ohms, supplies 50 100-volt 40-watt lamps connected in 

 parallel. (1 min.) 



3. Required the e.m.f. generated and the current in the arma- 

 ture in the two following cases: 



(a) A series machine supplies ten 45-volt 10-ampere lamps 

 through a line having 5 ohms resistance; the field and armature 

 resistances are 0.2 and 0.3 ohms. 



(6) A long-shunt compound generator gives 500 volts and 50 

 amperes; the armature and series and shunt-field resistances are 

 0.03, 0.02 and 500 ohms. (1 min.) 



4. The curve of generated e.m.f. with current, i.e. the total 

 characteristic of a separately excited generator, is given by the 

 following points: 



Current 20 40 60 



E.m.f 110 109 107 104 



The armature resistance is 0.05 ohm. 



Construct the above curve and derive the external characteristic. 



(5 min.) . 



30 



