198 



ALTERNATORS. 



appreciable residual magnetism, and the alternator gave no 

 voltage when unexcited, consequently the curve is made 

 to pass through zero, and there are not two distinct branches 

 of the curve as were obtained with the direct-current dynamo, 

 whose magnetisation curve is shown in Fig. 12, page 39, of 

 " Practical Testing of Dynamos and Motors." 



120 



110 



100 



90 

 8O 

 70 



o 60 

 > 



5O 



3O 

 2O 

 1O 



-2 -4 -6 -8 VO 1-2 14 1-6 1'8 2"O 2'2 2-4 2'6 



Amperes Excitation. 



PIG. 9-2. MAQNKTISATION CUBVE. V^) *>-4L^L 

 Speed 1,200 r.p.m. 



The no-load magnetisation curve of an alternator does 

 not differ in character from that of a direct-current dynamo. 

 A brief summary only of the chief points is given here. 



Hysteresis. The ascending and descending curves do not 

 usually coincide accurately on account of the hysteresis of the 

 magnetic circuit, which is caused by some kind of molecular 

 friction in the material, tending to maintain the particles 

 of the metal in their positions of magnetisation or demagnetisa- 

 tion, when these positions are once assumed. 



Soft, wrought iron shows very little hysteresis effect, and 

 the distance apart of the ascending and descending curves 



