390 ELECTRICAL ENGINEERING 



speed of the converter is fixed by the frequency of the system and 

 remains constant. When changing from direct current to alter- 

 nating current the speed is not fixed but depends on the excitation 

 and varies as the field strength varies. When the load is inductive 

 the wattless lagging current demagnetizes the field and so raises 

 the speed of the converter and the frequency of the alternating 

 current. This may increase the lagging current and so raise the 

 speed more until it gets beyond safe limits. The inverted con- 

 verter has thus the two disadvantages, (1) that it tends to run 

 at dangerous speeds and (2) that it supplies a current of varying 

 frequency. It must, therefore, be provided with some means of 

 cutting off the load when the speed rises above a certain value or 

 with some means of limiting the speed. 



If the converter is excited by a direct-current generator mounted 

 on the same shaft any increase in speed raises the exciter voltage 

 at a higher rate and, therefore, the field of the converter is 

 strengthened and the speed is limited. 



Rotary converters to be used as inverted converters should not 

 be compound wound. 



246. Double-current Generator. If mechanical power is sup- 

 plied to drive a rotary converter it can be used as a double-current 

 generator to supply direct current from the commutator and alter- 

 nating current from the slip rings. 



The two currents in this case flow in the same direction in the 

 armature conductors and the losses are increased and the arma- 

 ture reaction is the sum of the reactions due to the two currents. 

 The voltage regulation is, therefore, poorer than in the converter. 



247. Three-wire Generator. The three-wire direct-current 

 generator is similar in construction to a single-phase or quprter- 

 phase rotary converter. It is used to supply a three-wire system 

 with from 220 to 280 volts between outer wires and from 110 to 140 

 volts between each of the outer wires and the neutral wire. 



In order to obtain a point at a potential midway between the 

 potentials of the direct-current brushes special transformers called 

 compensators are used. They have a single winding tapped at 

 the centre and are connected by means of slip rings across points 

 on the armature winding 180 electrical degrees apart. (Fig. 370.) 

 The neutral wire of the system is connected to the central point 

 of the compensator and its potential is maintained almost midway 

 between the potentials of the outer wires. When more than one 



