OPERATION OF SYNCHRONOUS MOTORS. HUNTING 113 



arovided for that purpose, and which can be of small wire, inasmuch 

 the time required for -starting is so short. By sending into this aux- 

 liary winding a current which is more or less out of phase with respect 

 to that in the principal winding, the armature produces a revolving 

 ield which is more or less imperfect and pulsating, but which is, never- 

 icless, sufficient to start the motor. 



To produce the phase-difference between the two currents, recourse 

 lay be had to some one of the methods known for producing induction- 

 lotors by means of a single alternating force. For example, use may 

 made of the Tesla method of introducing a self-induction into one 

 ranch of the circuit, and a resistance in the other, or use may be made 

 the Leblanc method, which is more perfect, and which consists in 

 itroducing a self-induction in one side and a capacity or a polarizer 

 the other. 



But in order to reduce to a minimum the current taken from the 

 le, there is again a great advantage in inserting a reducing trans- 

 mitter, built according to the principle already explained, and supply- 

 ig current either for one phase or for both phases of the motor. One 

 rangement consists in supplying only the principal winding by means 

 of the reducing transformer and in connecting the secondary winding 

 lirectly to the switchboard bus-bars. In such a case, the secondary 

 /inding is made with a very high number of turns so as to have a very 

 high self-induction and to absorb only a weak current having much 

 lag with respect to the current of the other winding. 



The arrangement employed by Labour (Fig. 52) is the reverse 

 of this. . In this case, it is the supplementary winding of the armature, 

 made of fine wire, which is supplied by a reducing transformer, while 

 the principal winding receives current direct from the switchboard 

 through a rheostat. The double-throw switch C being moved to the 

 right (starting position) as shown in the diagram, the supplementary 

 rinding will then be in circuit with the secondary of the reducing trans- 

 former, and at the same time the rheostat is inserted in the principal 

 :ircuit, which is closed by the circuit-breaker 7. The motor starts as 

 in induction-motor. When the speed approaches synchronism, the 

 switch C is suddenly thrown to the left; by this operation the 

 current in the supplementary winding is broken, and the rheostat of 

 the principal winding is short-circuited, at the same time that the 

 fields are excited. The motor is then in step. 



In large motors the circuit-breaker/ is replaced by a starting rheostat, 

 the object being to avoid fluctuations in the lights supplied from the same 



