254 ALTERNATING CURRENTS 



coils are not directly connected, each being joined to two diametrically 

 opposite points in the armature winding D and G, F and K, and 

 H and E respectively in Fig. 155 (a). 



149. Hunting of Rotary Converters 



The diminished heating of the armature of a converter as com- 

 pared with that of a continuous-current generator supplying an equal 

 current is due to the partial neutralization of the continuous by the 

 alternating current in the armature windings, and a further result 

 of this neutralization is reduced field distortion. For this reason, 

 a converter is less liable to spark, and is capable of standing a 

 much heavier momentary overload without sparking than a con- 

 tinuous-current generator, provided no hunting takes place. The 

 hunting or phase-swinging trouble has in some cases assumed a 

 very acute form, and has at times rendered the running of converters 

 impossible. The polyphase currents circulating in the armature 

 give rise to a rotating field, which, owing to the synchronous rota- 

 tion of the armature, is stationary in space, i.e. stationary with respect 

 to the field-magnets. If hunting takes place, however, this field 

 begins to oscillate, and its oscillations may cause vicious sparking 

 at the brushes. A converter which is likely to hunt is thus pecu- 

 liarly liable to the sparking trouble. 



Difficulties due to hunting have been experienced mainly in 

 cases where the angular velocity of the generators was not sufficiently 

 uniform, or where the converters were supplied through a very long 

 line of comparatively high resistance. Hunting may be to a large 

 extent prevented by the use of copper damping bridges placed so 

 as to connect neighbouring poles (the equivalent of a squirrel-cage 



being thereby obtained), 

 the poles themselves being 

 of solid metal, not lami- 

 nated. Any oscillation of 

 the field is rapidly damped 

 out by this equivalent of 

 FIG. 156. Damping Circuit of Botary Converter. a squirrel-cage winding. 



One such arrangement, as 

 used by the Union E. Gr. of Berlin, is shown in Fig. 156. 



Practical experience has shown that where a number of converters 

 have to be run in parallel, it is not advisable to run them from a 

 common bank of transformers ; each rotary should be supplied from 

 a separate transformer or bank of transformers. 



