344 



ALTERNATING CURRENTS 



manner, the machine is said to be an inverted synchronous con- 

 verter. This is not the usual method of operation. 



139. Polyphase Converters. The output of a converter 

 increases materially with the number of phases. For example, 

 the rating of a six-phase converter is more than twice its rating 

 when operated single-phase (see Par. 144). 



The connections of polyphase converters are comparatively 

 simple. For example, the four-phase converter shown in Fig. 

 315 requires four slip-rings. The points at which the slip-rings 

 are connected to the winding are 90 space-degrees apart in the 



-or 4-Phase 

 Supply 



FIG. 315. Two-pole, 4-ring, 4-phase synchronous converter. 



two-pole type. If the machine has four poles, two taps from each 

 ring to the winding are necessary. This is illustrated in Fig. 316, 

 in which a three-phase, four-pole converter is shown. Two taps 

 run from each ring to the winding; in this case the taps are diame- 

 trically opposite. For example, if the tap from one ring connects 

 to a portion of the winding which at some particular instant is 

 under the center of a north pole, then there must be similar taps 

 running from this same ring to every point of the winding which 

 lies at that instant under the center of a north pole. (See points 

 a,a,Fig. 316). 



