CONVERTERS 389 



with taps on the secondaries to give from one third to one half of 

 full' voltage at start. When synchronous speed is reached the 

 field circuit is closed and full voltage is impressed. The load 

 circuit must not be connected at start since below synchronous 

 speed the voltage between the direct-current brushes is alternating 

 at the frequency of slip. 



By this method the trouble of synchronizing is eliminated and 

 the machine can be put in operation very quickly, but a large 

 current at low power factor is drawn from the lines. 



(2) Converters may be started from the direct-current end if 

 suitable power is available in the station, but a longer time is 

 required to put them in operation than when starting from the 

 alternating-current end and they must be synchronized. 



(3) If an induction motor with a smaller number of poles than 

 the converter and consequently a higher synchronous speed is 

 mounted on the same shaft it may be used to start the converter. 

 This method of starting requires synchronizing and thus takes 

 longer than (1) and the induction motor draws a large lagging 

 current. It has, therefore, the disadvantages of the two first 

 methods and in addition requires the extra starting motor. 



244. Frequencies and Voltages. Converters are built for both 

 25 and 60 cycles, but for large outputs the lower frequency is more 

 satisfactory since on account of the larger number of poles re- 

 quired the commutation in a 60-cycle converter is not so good as 

 in a 25-cycle machine. 



Any direct voltage up to 600 volts can be obtained and many 

 converters giving 1200 volts between brushes are in successful 

 operation. In the majority of cases, however, where a line 

 potential of 1200 volts is required two 600-volt machines are 

 operated in series. 



245. Inverted Converter. Where a small alternating-current 

 load is to be supplied from a direct-current system, a rotary con- 

 verter may be used as an inverted converter to transform direct 

 current to alternating current. 



The ratios of the voltages are the same as under normal operat- 

 ing conditions but the ratios of the currents vary since it is not 

 possible to eliminate or control the wattless components of the 

 alternating current. These components depend on the character 

 of the load and are not affected by varying the exciting current. 



When changing from alternating current to direct current the 



