334 THE ROTARY CONVERTER. 



oil the frequency of the supply. It would at first appear 

 that the direct voltage generated in the armature would 

 consequently vary in the same proportion as the strength 

 of the field, 'since the conductors always rotate at a 

 constant speed in this field. This will be found from the 

 experiment which follows not to be the case, for reasons 

 given later in the discussion of the curve obtained from 

 the results. 



When driven from the alternating-current side, a 

 rotary converter behaves in most respects similarly to a 

 synchronous motor. It must be run up to speed and 

 synchronised in a similar manner ; and, generally, what 

 has already been stated as to the conditions of running 

 of a synchronous motor will apply in this case also. 

 Thus, variation in the excitation will produce changes in 

 the phase relations between the current and terminal 

 voltage of the machine. 



For the purpose of investigating the current taken 

 by the converter from the line, it will be convenient to 

 consider the direct current as being generated in a 

 separate armature coupled to the armature receiving the 

 alternating-current supply. Under these conditions we 

 may consider three voltages as being in equilibrium in 

 the alternating-current armature. These voltages are (1) : 

 The impressed voltage dependent only on the source of 

 supply ; (2) the induced back electro-motive force depend- 

 ing only on the strength of field for a given speed ; (3) the 

 voltage overcoming the impedance of the armature. This 

 depends only upon the strength of the armature current. 



The phase of the reactance voltage must always be 

 such that the back electro-motive force and the impe- 

 tance voltage together exactly balance the applied voltage, 

 since the three voltages are the only ones acting. Hence, 

 if the back electro-motive force is less than the impressed 

 voltage, the impedance voltage must be so related in 

 phase as to assist the back electromotive force and 

 oppose the impressed electro-motive force, i.e., it must 

 have an energy component in phase with the back 

 electro-motive force, and the current producing the im- 

 pedance voltage will lag behind the impressed electro- 

 motive force in phase. If the excitation of the machine 

 is so great that the back 'electro-motive force becomes 

 greater than the impressed voltage, the impedance voltage 



