380 ELECTRICAL MACHINERY 



as a shunt wound c-c. motor) speeds up; the speed is adjusted 

 by the field rheostat to bring the converter into synchronism 

 with the line and then the converter is switched to the line 

 after the indicating device (lamps or synchronoscope) shows 

 the proper phase. 



Starting as an Induction Motor. The induction motor 

 method of starting is becoming more and more common. 

 Half-voltage taps are supplied on the transformers which 

 normally supply the converter, and with the field circuit 

 of the machine open, the a-c. end of the converter is switched 

 to the half-voltage line. When it has accelerated to nearly 

 synchronous speed, the starting switch (which is double 

 throw) is thrown over to the normal voltage line and then 

 the field circuit is closed (connected to the c-c. end of the 

 converter), and the field current gradually increased to its 

 normal value. 



Field " break-up " Switch. In using this method of 

 starting a very high voltage may be induced in the field 

 coils, due to the field coils acting like the secondary of a 

 transformer, the armature being the primary; the field 

 circuit must be subdivided to avoid puncturing the insula- 

 tion of the field coils. This is done by a field " break-up 

 switch " generally mounted on the frame of the machine. 



Fig. 249 shows a 2000-kw. synchronous converter on 

 the frame of which may be seen the field " break-up " 

 switch ; on this machine a seven-pole switch is used to break 

 the field circuit in several places. The large single-pole 

 switch on the field frame is connected in the equalizer bus- 

 bar circuit. 



Reversed Polarity. The field current may start to build 

 up in the wrong direction, making the c-c. instruments 

 connected in the converter circuit deflect backward; to 

 remedy this difficulty it is necessary to make the converter 

 slip back one pole before putting on the field excitation. 



Sparking at the C-C. Brushes. If a converter is equipped 

 with commutating poles, this method of starting is likely 



