370 ALTERNATING CURRENTS 



As a rule, converters excite their own shunt fields. The arma- 

 ture rotates in a direction opposite to that of the rotating field 

 which is set up about it, Fig. 335. Therefore, as the armature 

 approaches synchronism, the rotation of this field becomes slower 

 and slower with respect to the field structure, as the rotating field 

 rotates in one direction and the armature in the opposite direc- 

 tion. The field poles themselves, which are magnetized 

 alternately north and south by this field, become more and more 

 slowly magnetized as the armature approaches synchronism. 

 Finally, due to hysteresis action, (see Chap. X, page 321, Par. 

 130), the poles themselves become permanently magnetized 

 through armature reaction, and the armature pulls into synchro- 

 nism in a manner similar to that of the salient-pole synchronous 

 motor when started in this manner. 



When the shunt-field switch is closed, the field produced by the 

 shunt winding may oppose the field built up in the field poles by 

 armature reaction. Consequently there' is a tendency for the 

 armature to slip a pole. Should the armature slip a pole, the 

 direct-current voltage at the brushes reverses. This reverses 

 the shunt-field current, which again causes the converter to slip 

 a pole. This action, unless checked, may continue indefinitely. 

 It may be stopped by reversing the shunt field current by means 

 of the field-reversing switch, Fig. 336. 



It often happens that the direct-current field is not strong 

 enough to cause the armature to slip a pole, because the field 

 voltage may be low, due to the alternating voltage being reduced 

 through the starting taps. However, the tendency exists, and 

 due to the resulting distortion of the pole flux, the brushes are no 

 longer in the commutating zone. The brush voltage is thereby 

 reduced, which again lowers the tendency to slip a pole. The 

 converter will continue to run under these conditions, but it will 

 take a large current at low power-factor, will spark at the 

 brushes, and its operation will be unsatisfactory. By reversing 

 the field current, however, normal operating conditions can be 

 obtained. 



152. Methods of Obtaining Correct Polarity. It is important 

 that the converter always come up with the same direct-current 

 polarity, as it may be operating in parallel with other apparatus. 

 As has just been pointed out, the converter may build up with 



