STARTING OF CONVERTERS 257 



i^sume that the voltage at which the stop-down transformer is 

 supplied is constant, and let its secondary voltage, also approximately 

 constant, be denoted by V. The excitation due to the shunt is such 

 that at light loads the converter takes a lagging current, represented 

 by I a in Fig. 158 (a). In order to obtain the slip-ring voltage V,, we 

 have to subtract (vectorially) from V the drop due to the reactance 

 coil VV< in Fig. 158 (a).* As the load increases, the field is 

 strengthened, and the current comes more nearly into phase with 

 V, ( 72), so that when, as in Fig. 158 (i), coincidence of phase 

 is reached, V, may, in spite of the increase in W,, remain the same 

 or even increase in value. This constancy or increase is maintained 

 when, as in Fig. 158 (c), the current has become a leading one. By 

 this method, an amount of over-compounding of from 10 to 15 per 

 cent, may be obtained without difficulty. 



151. Starting of Rotary Converters 



There are various methods of starting rotary converters. Where 

 possible they are run up to synchronous speed from the continuous- 

 current side, synchronized, and switched on to the alternating-current 

 mains like ordinary alternators. But in some cases a supply of 

 continuous current may not be available. The best method is, then, 

 to provide a small auxiliary induction motor coupled direct to the 

 converter shaft, and having a somewhat smaller number of poles, so 

 as to enable the converter to be run up to a speed slightly above 

 that of synchronism. The induction motor is then switched off, and 

 as the speed of the converter slowly diminishes it is synchronized 

 and thrown into circuit, f 



152. Racing of Inverted Rotaries 



When a converter is used as an "inverted rotary," to transform 

 continuous into alternating currents, difficulties may arise on account 

 of the excessive weakening of the field by lagging currents on the 



* VV, being, of course, perpendicular to the current vector. 



f A third method, which docs away with the auxiliary starting motor, consists in 

 i ijifiiing the field circuit preferably in several places and switching on the alternating 

 current to the armature, through suitable resistances or reactance coils. By the action 

 of eddy currents and hysteresis, the armature will run up to practically synchronous 

 speed, and the field winding is then closed (it may, however, be closed at the wrong 

 moment, so that in order to prevent this a polarized instrument must be provided on 

 the continuous-current side). This method is not to be recommended, as it throws a 

 severe strain on the insulation of the field coils, in which very high e.m.f.'s arc 

 induced at the instant of switching on. 



S 



