1/4 ELEMENTS OF ELECTRICAL ENGINEERING. 



efficiency than the motor-generator and its cost is very much less, 

 but there is a nearly fixed relation between the effective value of 

 the voltage of the alternating-current supplied to the synchronous 

 converter and the steady voltage of the direct current delivered 

 by the machine, so that the voltage of the direct-current side of 

 the machine cannot be changed through a wide range without 

 changing the effective voltage of the alternating-current supply. 

 See Art. 90. The motor-generator, on the other hand, is very 

 flexible in this respect, the voltage of the direct-current generator 

 can be varied independently of the voltage of the alternating- cur- 

 rent supply by varying the field excitation of the generator. 



85. The synchronous converter. Consider two opposite com- 

 mutator bars on an ordinary two-pole direct-current dynamo. 

 The electromotive force between these two bars alternates as the 

 armature rotates, and if these two commutator bars are connected 

 to two collector rings the direct-current dynamo may be used to 

 deliver alternating current ; the machine becomes an alternator 

 and, its commutator being undisturbed, it remains a direct-current 

 dynamo. Such a machine may be belt driven and deliver direct 

 current, or alternating current or both ; it may be driven as a 

 direct-current motor and deliver mechanical power, or alternating 

 current, or both ; or it may be driven as a synchronous alter- 

 nating-current motor and deliver mechanical power, or direct 

 current or both. The most important use of such a machine is 

 to convert alternating current into direct current, for which pur- 

 pose it is driven as a synchronous alternating-current motor and 

 loaded by delivering direct current ; and the machine is called a 

 synchronous converter * when used in this way. 



Single-phase and polyphase synchronous converters, The ma- 

 chine above described, having two collector-rings, is called a two- 

 ring or single-phase converter. The three-ring or three-phase 

 converter has three collector rings connected to three commutator 

 bars 1 20 apart (on a two-pole machine), the four-ring, or so-called 



* Also often called a rotary converter. 



