368 



ALTERNATING CURRENTS 



N 



A centrifugal device is often used to trip the circuit-breaker 

 when the speed exceeds the safe value. Another method, not 

 often used, is to have an exciter on the converter shaft. As the 

 speed increases, the exciter voltage increases and the converter 

 field is strengthened. This tends to check the increase of speed 

 of the converter. 



Inverted converters will operate satisfactorily in parallel on 

 the alternating-current side, any converter being made to take 

 more load by weakening its field. 



151. Starting the Synchronous Converter from the Alternating - 

 current Side. There are several methods of starting direct syn- 

 chronous converters, some of which are similar to the methods 

 used with the synchronous motor. 



If polyphase currents are supplied to the armature, a rotating 

 field is produced about the armature, Fig. 335. This is similar 

 to the rotating field of the induction motor, except that it is 



produced by a rotating 

 armature about itself. If the 

 armature speed is below syn- 

 chronism, this field cuts the 

 pole faces and the damper 

 windings (Fig. 339), and in- 

 duces currents. A reaction 

 results between the rotating 

 field and these induced cur- 

 rents, producing rotation. 



When starting the converter 

 in this manner, several precau- 

 tions are necessary. The 

 armature is the primary, and 

 the shunt field coils are the 

 secondary of a transformer, 

 the secondary having a very 

 large number of turns. The rotating armature field, there- 

 fore, induces very high voltages in the field coils on starting and 

 tends to puncture them. To reduce this voltage, the field is 

 usually split into sections by a field-splitting or sectionalizing 

 switch. Figure 336 shows the connections of a three-pole switch 

 used to sectionalize the field circuit of a four-pole converter into 



FIG. 335. Relative directions of rota- 

 tion of the armature and of the rotating 

 field produced by the armature. 



