GENERAL CONSIDERATIONS 279 



of synchronous converters, especially for 60 cycles and above, 

 the problems of commutation and commutator construction be- 

 come of importance. The pole pitch on the commutator, arma- 

 ture or field, is the space passed through in one alternation, and 

 it is thus seen that there is a natural tendency for higher per- 

 ipheral speeds at the higher frequencies, and it is the limitation 

 of peripheral speed which fixes the limits of design. 



With direct-current machines this occurs with turbine-driven 

 generators and in the commutators, which are necessarily mechan- 

 ical in construction, consisting, as they must, of a certain amount 

 of insulation. Direct-current generators are, therefore, more 

 limited in speeds than alternating-current, and the same holds 

 true when they are combined as in rotary converters. 



Improvements in design have made the 60-cycle synchronous 

 converter entirely satisfactory for the conditions under which 

 such machines operate. In efficiency 25-cycle converters are 

 slightly higher than the 60-cycle. 



Railroad Work. Twenty-five cycles has been recognized as 

 the standard frequency for railway systems in this country. 

 Until not long ago all systems were of the alternating-current- 

 direct-current type, alternating current being generated and 

 transmitted to the various substations, where it was changed to 

 direct current by means of synchronous converters. The choice 

 of this frequency was, therefore, chiefly caused u.y the less satis- 

 factory operation of the earlier types of 60-cycle converters. 



Even with the successful operation of the present 60-cycle 

 converters, there is no reason for changing the standard 25-cycle 

 frequency. While 60 cycles would be preferable as far as the 

 generators and transformers are concerned, this is offset, however, 

 by the advantages of the 25-cycle transmission system and the 

 lower cost of synchronous converters for larger capacities. Where, 

 the supply is 60 cycles, synchronous motor-generator sets are very 

 often used for the conversion. 



With the introduction of the alternating-current railway motor, 

 60 cycles is obviously entirely eliminated, due to the excessive 

 impedence drop and " skin effect " caused by the alternating 

 current flowing in the rails. The 25-cycle system, on the other 

 hand, is fully satisfactory f or this service, and, although the 15- 

 cycle system has been advocated, its advantages over the 25-cycle 

 system have not been proved to be of sufficient weight to neces- 



