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ELECTRICAL MACHINERY 



Low-voltage Taps for Starting. In starting a motor by 

 the induction method it is not feasible to connect it, when 

 stationary, to the line of normal voltage because the start- 

 ing current would be so excessive (from 5 to 10 times the 

 rated current) that the armature winding might be injured. 

 So the transformers feeding the motor are usually fitted 

 with half voltage taps and power is taken from these taps 

 to the lower sides of a double-throw switch, to the blades 

 of which the motor armature is connected as in Fig. 202. 

 The starting switch is thrown down at first and held there 

 until the motor approaches synchronous speed, when it is 



Primary 



Synchronous 

 Motor 



FIG. 202. Connection of a Synchronous Motor to Half-voltage Taps 

 for Starting. 



quickly thrown to its upper position, which is the running 

 position. 



Excitation. Of course the field circuit of a synchronous 

 motor requires continuous current. As the a-c. line feeding 

 the motor cannot furnish the current for excitation some 

 separate source of c-c. power is required. Sometimes a 

 small c-c. self-exciting generator is mounted on the armature 

 shaft of the synchronous motor as shown in Fig. 203; the 

 output of this small c-c. generator is just sufficient to sup- 

 ply the power for the field circuit of the synchronous motor, 

 perhaps 3% of the rating of the motor. 



