174 TKEATISE ON ALTERNATING CURRENTS. 



A rotary converter has less heating and higher efficiency than 

 a motor generator of the same output, one reason for this being 

 that in the converter no mechanical transfer of energy takes place, 

 since the torque required for the generation of the continuous 

 currents and that produced by the alternating current both act on 

 the same armature. In the motor generator, on the other hand, 

 power is transmitted mechanically from the motor to the 

 generator. 



A rotary converter is thus a machine with a commutator on 

 one side of the armature, and two, three, or four collector rings on 

 the other side, according as it is intended for mono-phase, tri- 

 phase, or quarter-phase currents. The field magnets are generally 

 excited by a shunt or compound winding on the direct-current 

 side. When the machine is running synchronously with the 

 supply current, the counter-electromotive force in the armature 

 will approximately balance the applied alternating P.D., although 

 they may differ in phase. The E.M.F. developed on the direct- 

 current side will have a steady value equal to the maximum value 

 of the counter E.M.F., and will therefore approximately equal the 

 maximum value of the applied P.D. 



119. To find the ratio of the E.M.F.s and currents on the two 

 sides of a rotary, we will follow the method employed by Mr. C. P. 

 Steinmetz. 1 



Let Fig. 74 represent diagrammatically the commutator of a 

 direct-current machine, with the armature coils shown connected 

 "to adjacent commutator bars. The brushes are represented by BI 

 and j 2 , and the field magnets by FI, F 2 ; a\ and # 2 are two opposite 

 points of the commutator, and are connected to two collector-rings, 

 DI and Z> 2 . 



It is obvious that between the collector-rings there is an alter- 

 nating E.M.F. e, whose maximum value is equal to the continuous 

 E.M.F. E, and which makes p periods per revolution of the arma- 

 ture, where 2p is the number of poles. In the diagram p is 

 unity. 



Hence we have 



e = E sin 2irnt 



where n is the frequency, and E the E.M.F. between the brushes 

 on the commutator. 



1 "The Converter," by C. P. Steinmetz, Electrical World, vol. xxxii.pp. 650-652 

 (1898). 



