OPERATION OF SYNCHRONOUS MOTORS. HUNTING 111 



attained the rheostats are short-circuited. For these large motors, 

 separate exciters are generally used, as already seen (Fig. 52). 



The operation of starting without load does not require more than 

 20 to 30 seconds, and it is effected with a current not exceeding the nor- 

 mal full load current. 



FIG. 53. 



It will be observed that the process of starting involves the use of 

 ligh -tension switches on the primary circuit, to isolate the transformer; 

 also fuses on both the primary and secondary circuit. 



I The belt is shifted from the idler to the driving pulley in the ordinary 



way by a belt-shifter. Fig. 53 shows the manner in which this shifter 

 should be placed to correspond properly with the direction of rofation 

 and the direction of driving, for an 

 observer placed at the commutator end. 

 Figures A and B refer to the case where 

 the direction of rotation (indicated by 

 the arrows) is the same as the hands of 

 a watch ; and Figs. C and D refer to the 

 case where the rotation is in the opposite 

 direction. 



The use of a reactance-coil instead of 

 a rheostat has the objection of increas- 

 ing the lag of the current taken from the 

 supply-source, which current already has 

 too much lag. It is therefore preferable 

 to replace it by an auto-transformer 

 which enables the current of the motor 

 to be increased at low voltage while 



reducing the amount of current taken from the line. Fig. 54 shows 

 an example of this kind of arrangement which is in extensive use. 

 The diagram represents the simplest case, i.e., that of a single-phase 

 alternating current machine. It is a transformer with a single wind- 



FIG. 54. 



