FIELD-EXCITATION OF ROTARY CONVERTERS 211 



If, on the other hand, it be required to have the voltage rise with 

 he load, the excitation must be compound. The shunt- winding will 

 still be such that it, alone, gives the no-load E.M.F. e, and the series- 

 winding will be such that it will produce the additional ampere-turns 

 necessary to give the E.M.F. e' required with the load I w . As already 

 stated, in a case like this, it is desirable to decrease the reactance X, 

 and the lag 0, to a minimum. 



Regulation of Voltage by Varying the Reactance X in the 

 Circuit. The two windings, in this case, will be determined in the same 

 way as in the preceding case, since the voltage of the alternating cur- 

 rent supplied to the converter is still regulated by outside means without 

 causing lag in the current inside the machine. The excitation will 

 be that produced by a simple shunt-winding when constant voltage 

 is wanted, and that produced by a compound-winding, when a rising 

 voltage is wanted. The shunt-winding will be determined by the 

 voltage required with no load, and the series-winding will be determined 

 by the voltage required with full load. 



Possibility of Suppressing the Shunt- Winding. It is not abso- 

 lutely necessary to use either a separate exciting winding, or else a 

 shunt winding, as has been supposed hitherto; because, by making 

 the number of armature-winding turns greater and decreasing the air- 

 gap, it is always possible to obtain the necessary no-load excitation, 

 merely by the effects of the armature-reaction. Eqs. (21) and (22), in 

 that case, reduce to the following forms: 



Ttkn . . . . 



7 =(I m -jo} = A m . ...... (2ia) 



V2 



KN' . 

 --=^o=A (220) 



V 2 



It is not possible to obtain constant voltage or rising voltage, in 

 the direct-current circuit with this method any more than when a shunt- 

 winding is used, without series-winding. This method has, to some 

 extent, the self-regulating properties of transformers, but to a degree 

 which is inadequate, owing to the greater reluctance of the magnetic 

 circuit. 



On the other hand, the high values which the reactive currents 

 must then be allowed to attain have the great objection of causing 

 increased heating of the armature by increasing the total current in 



