OPERATION OF SYNCHRONOUS MOTORS. HUNTING 109 



ictor, this method is no longer applicable and the phase-lamp (or else 

 several lamps in series) must be connected to the terminals of the field- 

 circuit, which is then subjected to the inductive action of the variable 

 flux of the armature. The lamp burns brightly at first, then its bright 

 ness diminishes gradually, and the extinction of the lamp shows that 

 synchronism is approaching; the direction which the E.M.F. will 

 have when the circuit is closed is not 

 determined, but this is generally of 

 very slight importance. 



With a well-constructed synchron- 

 ous motor there is no need of a phase- 

 indicator, because, when once it has 

 attained its highest speed as an induc- 

 tion-motor (which may sometimes re- 

 quire several minutes), the powerful 

 reactions which occur when the ex- 

 citation-circuit is closed are sufficient 

 to insure the machine falling into step. 



Current-controllers. It is necessary, Second -Step 

 in order to reduce the amount of current 

 taken from the line during the starting 

 period, to employ a current-controller, 

 owing to the absence of counter E.M.F. 

 in the armature. Use may be made 

 for this purpose of a simple rheostat 

 or a choke-coil. Rheostats are more 

 frequently used, on account of their 

 simplicity. Switching appliances are 

 used for the purpose of connecting them 

 into circuit and then short-circuiting 

 them, an armature circuit-breaker 

 being interposed to break the circuit in 

 the field -excitation fails. 



Fig. 51 shows, for example, the 

 diagram of the starting apparatus 



of a certain type of three-phase motors ranging from i to 22 H.P. When 

 the motor is at rest, the pulley is on the idler A, the triple-switch C 

 and the excitation circuit-breaker D are open. 



To start the motor the triple-pole switch C is closed on the first 

 :eps, and the motor then receives a current from the supply-trans- 



FIG. 51. 





