374 



ALTERNATING (Tit HE NTS 



then tend to operate as a shunt motor, usually with a weakened 

 field, due to the differential action of the compound winding. 

 Under these conditions the converter may tend to race. There- 

 fore, converters are usually equipped with reverse-energy relays 

 on the direct-current side, or else the direct-current breakers are 



Series Field 



110 



folts 



olts 



110 Volts 



Neutral 



Fio. 338. 220-volt, 3-wire D. C. system obtained from a 220-volt synchronous 

 converter employing double-Y connection. 



interlocked with the alternating-current ones, so that the direct- 

 current side will be opened simultaneously with the alternating- 

 current side. 



The resultant current in the converter armature conductors 

 produces the torque which overcomes the stray-power losses of 

 the converter. This resultant current 

 is the difference of two nearly equal 

 currents, as has already been demon- 

 strated. A small percentage change in 

 either the motor current alone or in the 

 generator current alone produces a large 

 percentage change in this torque current. 

 Therefore, the converter is very sensitive 

 to line disturbances, such as fluctuations 

 of voltage or of frequency. Accord- 

 ingly, it has a much greater tendency to 

 "hunt" than has the synchronous motor. 



Fio. 339. Main pole with 

 damper winding. 



For this reason, converters always have amortisseur or damper 

 windings or grids built around and into the poles, as shown 

 in Fig. 339. The action of these windings is the same as in the 



