126 



SYNCHRONOUS MOTORS 



and the curves in Fig. 60 enable its value in each particular case to 

 be calculated as a function of the data. These curves show that J 

 varies in the inverse ratio of y and in direct ratio of x, so long as z is 

 lower than a value near . The conditions that insure the best stability 

 in the case of sudden overloads are, therefore, substantially the same 

 as those which insure the maximum theoretical output. In particular, 

 in the case where E 2 =E\, we should have cos <=, whence mlr\ / '~^. 

 The experiments made on a Ganz motor (Experiments made by 

 the Frankfurt technical commission, Upperborn, Lumiere Eleclrique, 

 Vol. XXXVI, p. 315), showed that, starting from no-load, a sudden 



load equal to 150 per cent of the nominal 

 load may be put on the motor. This load 

 could be much exceeded for alternators 

 of lower inductance, especially if they are 

 provided with dampers on the pole- 

 pieces. 



Damping of Oscillations. The surest 

 way to prevent oscillations of both 

 kinds from attaining an excessive ampli- 

 tude consists in damping them rapidly. 

 It is an application of the general 

 principle laid down in a masterly man- 

 ner by Cornu (" On the synchroniza- 

 tion of oscillating systems," Journal 

 de Physique et Comptes Rendu de 

 VAcademie des Sciences, 31 Mai, 1887; 

 Bulletin de la Societe des Elcctriciens), 



who showed that perfect synchronization can be obtained only when 

 there is damping, i.e., when the speed-variations cause a supplementary 

 expenditure of energy. This supplementary expenditure of energy 

 can only occur to a slight extent in the armature-circuit, but it is pos- 

 sible to bring it about in the field-circuits by allowing eddy cur- 

 rents to be produced in the massive pole-pieces or, better still, in special 

 circuits wound on the fields, and short-circuited. These circuits 

 constitute the damper of Hutin and Leblanc. (C. F. Guilbert, Lumiere 

 Electrique, Vol. XL VI, 1892, p. 801; and Leblanc, Bull, de la Soc. 

 Int. des Electriciens, 1898). This damper consists either of bars 

 of copper surrounding the pole-pieces, as shown in Fig. 61, or, Ix-tkr 

 still, of a series of bars passing through the fields and united on each 

 side by copper rings, as shown in Fig. 62. 



FIG. 60. 



