CONVERSION OF ALTERNATING CURRENT. 185 



control the direct voltage of a synchronous converter by means 

 of a field rheostat, an inductance must be connected in the alter- 

 nating-current circuit.* 



The compounding of the synchronous converter. If the field 

 magnet of a synchronous converter is excited in part by a shunt 

 winding and in part by a series winding, then an increase of 

 direct-current output gives an increased field excitation, and, if 

 /J M is kept at a constant value, this increase of field excitation 

 is counteracted by a leading wattless current in the armature, 

 and E Q remains sensibly unchanged in value. In order that a 

 series field winding may produce an increase of Q with increase of 

 direct-current output (over-compounded machine), an inductance 

 must be connected in the alternating-current circuit. 



91. Current relations of the synchronous converter. It is evi- 

 dent from the preceding article that there is a nearly fixed ratio 

 between E Q and JE n of a synchronous converter. The ratio of 

 the direct-current output of a converter (/ ) to the effective value 

 of the alternating current entering at a collector ring (/) is sub- 

 ject to greater variations than the voltage ratio. The voltage 

 ratio must be known with considerable accuracy if one is to be 

 able to design a transformer to deliver alternating current to a con- 

 verter which in its turn is to deliver direct current at a specified 

 voltage ; an exact knowledge of the current ratio, on the other 

 hand, is of no great importance. Indeed the current ratio is of 

 importance chiefly in connection with the discussion of armature 

 heating, which determines the power rating of the converter, and 

 for this purpose an approximate value of the current ratio is 

 sufficient. 



In the discussion of the current relations of a synchronous 

 converter we shall assume (a) that the efficiency of the machine 

 is 100 per cent., that is, we .shall assume that the intake of power 

 is equal to the output of power, and (&) that the alternating cur- 

 rent / n flowing in the armature between two collector rings is 



* See Appendix B for a description of the split-pole converter. 



