172 ALTERNATING CURRENTS 



95. Determination of Open -circuit and Short = 

 circuit Characteristics of Alternator 



The determination of the open-circuit and short-circuit charac- 

 teristics of an alternator presents no difficulty, and involves the use 

 of only a small fraction of the power required to drive the machine 

 when fully loaded. In finding the short-circuit characteristic, the 

 armature short-circuit current may be increased up to three or even 

 more times the normal full-load value of the current, provided care 

 is taken not to maintain it for too long a period. As the readings for 

 both characteristics are slightly different with ascending and descend- 

 ing values of the exciting current, it is advisable to determine both 

 branches of the curve, and to use the mean in calculations regarding 

 the regulation of the alternator. It will, of course, be understood 

 that in obtaining the short-circuit curve of a polyphase generator all 

 the phases must be simultaneously short-circuited. 



Potier's method (see 86 and 87) requires the determination, in 

 addition to the short-circuit and open-circuit curves, of at least one 

 point on the load curve corresponding to a highly inductive load. 

 The load being highly inductive, it follows that this test also can be 

 carried out with a small expenditure of power, suitable choking coils 

 being used as the load. 



96. Hopkinson Efficiency Test 



As in the case of dynamos and transformers, the efficiency test 

 of an alternator is most economically and conveniently carried out by 

 the use of an indirect method, the power lost in the alternator being 

 measured directly. 



Where two similar alternators are available, the Hopkinson test 

 may be used. The shafts of the alternators are coupled to each other 

 and to a continuous-current motor, which is large enough to supply 

 the power corresponding to the losses. This motor is first carefully 

 tested for efficiency, so that the exact amount of f mechanical power 

 transmitted by it to the alternators is known for any given values of 

 the p.d. and current supplied to the motor. By weakening the field 

 of one of the alternators, any desired current may be made to circulate 

 in the local circuit of the two armatures, which are short-circuited on 

 each other through an ammeter ; the machine with the stronger field 

 acting as a generator, while that with the weaker field acts as a motor. 

 If the relative positions of the alternator armatures are such that their 

 e.m.f.'s are in direct opposition, the power factor will be practically 



