RECTIFIERS: THE SYNCHRONOUS CONVERTER 371 



either polarity. If this polarity happens to be wrong, there are 

 several methods of correcting it. 



Below are given some of these methods. The st a it ing compensa- 

 tor or transformer taps are assumed to be in the starting posit ions. 



(a) Open the shunt-field circuit and then open the line switch 

 long enough for the converter to slip one pole. This can be 

 determined very readily with a stroboscope. Close the field 

 switch and then throw the alternating-current switch quickly 

 into the running position. With a little practice this operation 

 can he readily performed. 



(6) Reverse the shunt field by means of the field-reversing 

 switch. This causes the machine to slip a pole and so reverses 

 the direct -current voltage, making it correct. If left this way 

 the machine will continue slipping, one pole at a time, as has just 

 been pointed out. Therefore, the shunt-field switch must be 

 thrown back immediately to its original position. 



(c) When the converter is first connected across the alternat- 

 ing-current line, the rotating field produced by the armature cuts 

 the armature conductors and generates alternating currents in 

 conductors, as has already been pointed out. The brushes are 

 stationary and the field rotating, so there is no commutating 

 action. Therefore, there is an alternating electromotive force 

 of line frequency across the brushes at the instant of starting. 

 The armature rotates in a direction opposite to that of its rotating 

 field, because of the reaction with the pole-face currents. This is 

 illust rated l>y Fig. 335. The rotating field about the armature is 

 shown as rotating clockwise. A conductor, such as the pole 

 . when placed in this field, would tend to rotate clockwise. 

 That ig, if the armature were held stationary, the field structure 

 would tend to rotate in the direction of the rotating flux produced 

 by the armature, or in a clockwise direction. Therefore, the 

 torque produced l.y this rotating llu\ is in such a direction that 

 rids to cause the field -tructure to rotate in a dockuise 

 direction, II ihe field structure i- Ji\,.l in position and 



rmature i- free to i Tin- trtictinn l.etween the i wo 



remains unchanged. Consequent ly the armat un- will rotate in 

 a coutitrr-rliirl.-iri*,- direction. Tin- relative motion between arm- 

 ature and field structure i- the same afl if the armature 

 stationary and the field e to rotate. 



