RECTIFYING DEVICES 379 



current machine it will have more or less capacity used as 

 a synchronous converter, depending on the number of 

 phases, as given below. 



Number of phases 1 3 4 6 12 



Safe capacity in kw 85 . 2 133 162 193 219 . 5 



If the number of a-c. taps were increased to a very 

 great number, the safe capacity of the armature would 

 increase to 230 kw. 



Converters always Polyphase. Single-phase converters 

 are practically never used because they have such a low 

 capacity and in addition they have a marked tendency 

 to " hunt," as explained in the chapter on synchronous 

 motors. Most converters are operated three-phase, although 

 in the larger sizes the tendency is to wind them six-phase 

 because of the increased capacity obtained. They are prac- 

 tically never used twelve-phase because the increase in 

 capacity is obtained only at the expense of complication 

 and increased cost of the brush rigging, rings, etc. 



117. Methods of Starting Synchronous Converters. 

 The synchronous converter is essentially a synchronous 

 motor when considered from the input side; some method 

 therefore must be used for starting and synchronizing it 

 with the line. There are three different methods in use 

 to-day; starting as a c-c. motor from the commutator end; 

 as an induction motor from the slip-ring end; or by having 

 some auxiliary starter, such as a small induction motor. 



Starting from the Continuous Current End. In order 

 to start a converter from the c-c. end it is necessary to 

 have a c-c. supply available of the same voltage as that 

 which the converter gives when in normal operation. If 

 for example, the converter is only one of several in a 

 sub-station and one or more are in operation, the c-c. 

 power for starting may be taken from the c-c. end of those 

 already operating. A starting rheostat is put in the arma- 

 ture circuit and gradually cut out as the machine (starting 



