CONVERSION OF ALTERNATING CURRENT. 177 



87. The starting of the synchronous converter.* Synchronous 

 converters are often used in sets, one or more of the machines 

 being always in operation. When this is the case a converter not 

 in use may be started as a direct-current motor, taking direct cur- 

 rent from the other machines, and its speed may be adjusted until 

 it is in exact synchronism with the alternating-current supply by 

 which it is to be driven, and then, when its alternating electromo- 

 tive force is opposite in phase to the supply voltage, the alternat- 

 ing current switches may be closed. The procedure is exactly 

 the same as the starting of a synchronous motor as described in 

 Art. 73 and the same synchronizing devices are used. 



When direct current is not available for starting a converter, 

 an induction motor, is generally used to bring the machine up to 

 speed. Any polyphase alternator however contains the essential 

 features of an induction motor, and therefore a polyphase alter- 

 nator or synchronous converter can be started by its own induc- 

 tion motor action. In some cases this method of starting is em- 

 ployed. See Art. 132. 



88. The operation of the synchronous converter. Not only in 

 the matter of starting but also in certain matters of operation the 

 synchronous converter is like a synchronous motor. Consider, 

 for example, a four-ring converter. Such a converter is often 

 called a two-phase converter because it is supplied with two alter- 

 nating currents 90 apart in phase, each alternating current being 

 delivered to a pair of opposite collector rings, that is, to collector 

 rings which are connected to opposite points on the armature of 

 a two-pole machine. Let B be the effective value of the total 

 induced electromotive force between opposite collector rings, R 

 the resistance of the armature between opposite collector rings, 

 L the inductance of the armature between opposite collector rings 

 plus any outside inductance that may be connected in series in 

 each alternating- current circuit between the supply mains and 

 the machine, and let A be the effective value of the supply volt- 

 age (each phase) and o>/27r the frequency. 



* See a series of short practical papers on this subject by A. Wagner, Electric 

 Journal, Vol. II, pages 436, 494 and 572, 1905. 

 13 



