114 SYNCHRONOUS MOTORS 



distributing system. Sometimes an electromagnetic circuit-breaker 

 is interposed in the excitation-circuit, its function being to open the 

 main-circuit whenever the excitation-current fails. 



The operation of starting and synchronizing requires about 30 

 seconds. The maximum current taken from the mains during this 

 period does not exceed the normal current by more than 20 per cent; 

 and it falls down to ^ or the normal current when synchronism is 

 attained. 



Besides these general methods, there is still another, which can be 

 used only in the case of motors having laminated field-poles. The 

 method consists in sending alternating currents through the field- 

 coils and through the armature-coils, which are then provided with a 

 commutator. The latter can be adapted either to the principal wind- 

 ing, or to a supplementary winding, which serves only for starting and 

 for excitation. This arrangement has been used by the Fort Wayne 

 Company in the United States [Churchward, Edairage Electrique, 

 Vol. XVII, p. 41]. At the time of starting, the current is sent through 

 the armature and field-circuits connected in series; the machine then 

 operates like a series-motor having a commutator; the direction of 

 the current changes, both in the armature and in the field, and con- 

 sequently the torque is always in the same 

 direction and can thus cause the motor to 

 start. 



Both the field and armature of the 

 Fort Wayne motor are laminated; the 

 field has two windings, one being a fine 

 wire winding for synchronous operation, 

 the other being a coarse wire winding 

 which serves for starting. The armature 

 itself also has two circuits wound on a 

 FlG - 55- core composed of sheet-iron disks of the 



form indicated in Fig. 55. The prin- 

 cipal winding, whicn serves for synchronous operation, is placed in the 

 round holes, there being as many coils as there are field-poles, and the 

 winding is connected with two collector-rings. The other winding, 

 distributed in the slots, is similar to that of an ordinary D.C. drum 

 winding with its commutator. 



The operation of starting is accomplished by means of a single 

 lever controlling the switches. If it is first placed in the position 

 indicated in Fig. 56 the motor will be connected to the source of current 



