258 ALTERNATING CURRENTS 



alternating side (which is now the generator side), and the dangerously 

 high speed to which the converter armature may run up. In order 

 to prevent this " running away " of an inverted rotary, it is provided 

 with a separate exciter coupled direct to the converter shaft, and 

 designed so that under normal running conditions its field is 

 comparatively weak, and hence very sensitive to an increase in the 

 exciting current. Any tendency on the part of the converter 

 armature to race produces a very rapid increase in the exciting 

 current, which immediately checks any further increase of speed. 

 This arrangement is used by the Westinghouse Co. 



'53' Converters v. Motor-generators 



The relative merits and demerits of rotary converters as compared 

 with motor-generator sets that is, alternating-current motors coupled 

 to continuous-current generators have formed the subject of much 

 controversy. Considered by itself, a rotary converter is considerably 

 cheaper and more efficient than a motor generator. But while the 

 latter is self-contained and requires no auxiliary apparatus (such as 

 step-down transformers, for the alternating- current motor may be 

 wound for the high voltage of the line unless this is exceptionally 

 high), the rotary converter requires various accessories, which have 

 the double effect of increasing the total cost of the transforming 

 plant and reducing its efficiency. When the additional cost of the 

 step-down transformers, the induction regulator or the reactance coils 

 and compound winding, are taken into account, the difference in first 

 cost between a rotary converter and a motor-generator plant is very 

 slight not exceeding some 5 per cent. the rotary converter plant 

 being the cheaper of the two. Similarly, as regards efficiency, there 

 appears to be but little to choose between them, the rotary converter 

 plant being again slightly more efficient. On the other hand, the 

 motor-generator plant allows of a simpler mode of voltage control 

 over a wider range. A good deal depends on the frequency, and on 

 low-frequency (say, 25-cycle) circuits rotary converters would 

 probably be preferred. 



Two types of motor-generators are in use, depending on the type 

 of motor the synchronous and the asynchronous or induction motor 

 type. For capacities up to 300 k.w., the induction motor-generator 

 set would appear to be slightly cheaper, while for outputs exceeding 

 500 k.w., it would appear to be slightly more costly than the 

 synchronous set. The induction set is self-starting; on the other 

 hand, the losses with the synchronous set may be made somewhat 

 smaller by reason of the higher power factor. 



