392 ELECTRICAL ENGINEERING 



up to the positive brush by the path of least resistance. Since 

 the current in the neutral wire is a direct current the reactance of 

 the compensator does not oppose it and the only voltage drop is 

 that due to resistance. 



The actual amount of current carried by the various sections of 

 the armature winding is very difficult to calculate and it varies 

 from instant to instant due to the change in the relative positions 

 of the direct-current brushes and the compensator connections. 

 The average current carried by each half of the compensator 



winding is-)*' 



z 



The unbalancing of the currents in the sections of the armature 

 winding produces an unbalancing of the armature reactions and 

 results in a slight unbalancing of the voltages between the neutral 

 point and the brushes. With two or more compensators, Fig. 

 371, this unbalancing is reduced due to the more even distribution 

 of the current. 



Machines can be designed to give a regulation of 2 per cent or 

 less with an unbalanced load of 25 per cent. 



With this system the voltages on the two sides cannot be regu- 

 lated independently and the flexibility of the three-wire system 

 supplied by two generators in series is lost, but there is a corre- 

 sponding gain in space and cost of machines. 



The capacity required in the compensators is small. With 

 25 per cent unbalancing of the loads the required compensator 

 capacity is less than 10 per cent of the generator capacity. 



248. Frequency Converters. Frequency converters are used 

 where power is transmitted at 25 cycles and is required by the 

 consumer at 60 cycles. The most usual form of frequency con- 

 verter is a synchronous motor-generator set, a 25-cycle motor 

 direct connected to a 60-cycle alternator. The numbers of poles 

 on the two machines must be in the ratio of 25 to 60. When a 

 10-pole motor is used the alternator must have 24 poles and the 

 speed is fixed at 300 r. p. m. 



When frequency converters are to be operated in parallel they 

 must be synchronized on both the 25-cycle and 60-cycle ends. 

 If the motor is synchronized first there is only one chance in five 

 that the alternator is in synchronism, while if the alternator is 

 synchronized first there is only one chance in twelve that the 

 motor is in synchronism. 



