RECTIFYING DEVICES 381 



to cause bad sparking at the c.-c brushes while the machine 

 is speeding up. To prevent this, the brush holders are 

 equipped with a simple lever arrangement whereby the 

 brushes may easily be lifted off the commutator, while 

 the armature is accelerating, and then dropped back in place 

 on the commutator when synchronous speed has been 

 reached and the danger of serious sparking is over. 



Use of Auxiliary Starter. When the third method of 

 starting is used, a small induction motor is mounted on one 

 of the armature pedestals, its rotor being mounted directly 

 on the shaft of the converter. The stator is wound for 

 the same voltage and number of phases as the converter 

 armature and the motor has just sufficient capacity to run 

 the converter at synchronous speed when it is supplying 

 no load. The induction motor always has one pair of poles 

 less than the converter (why?) and has a capacity rating 

 of from 5 to 10% of that of the converter. 



Large Commutator Required on a Polyphase Converter. 

 A cut of a large six-phase converter is given in Fig. 249. 

 It is to be noticed that on this converter the commutator 

 is much larger than it would be on a c-c. generator of the 

 same size. This is because of the fact brought out in the 

 last paragraph; the capacity of the six-phase converter 

 is practically twice as much as that of a c-c. generator of the 

 same size (size not capacity) and hence the c-c. brush rigging 

 and the commutator must be of twice the current capacity 

 as that required for a c-c. generator of the same size. 



118. Compounding a Synchronous Converter by Series 

 Field and Line Inductance. We have proved that the 

 ratio of continuous voltage to alternating voltage is fixed 

 for a given synchronous converter. It is not therefore, 

 at once evident how a converter may be designed to give 

 a voltage on the c-c. end, increasing with load; yet this is 

 generally desired. For use in railway and lighting instal- 

 lations a converter is generally desired with about 10% 

 compounding; for example, the specifications for a rail- 



