CHAP. IV.] INDUCED E.M.F. 77 



Prob. 41. Prove that if in the preceding problem the frequency of the 

 rotation of the poles is / and the frequency of the alternating current in 

 the exciting winding is/ 2 , that the voltage induced in the armature is a com- 

 bination of two waves having frequencies of/i +/, and/, /, respectively. 



32. The Ratio of A.C. to D.C. Voltage in a Rotary Converter. 

 A rotary converter resembles in its general construction a direct - 

 current machine, except that the armature winding is connected 

 not only to the commutator, but also to two or more slip rings. 1 

 \Vhen such a machine is driven mechanically it can supply a direct 

 current through its commutator, and at the same time an alter- 

 nating current through its slip rings. It is then called a double- 

 current generator. But if the same machine is connected to a 

 source of alternating voltage and brought up to synchronous 

 speed it runs as a synchronous motor and can supply direct current 

 through its commutator. It is then called a rotary converter. It 

 is also sometimes used for converting direct current into alter- 

 nating current, and is then called an inverted rotary. 



Both the direct and the alternating voltages are induced in the 

 armature of a rotary converter by the same field, and our problem 

 is to find the ratio between the two voltages for a given arrange- 

 ment of the slip rings. Consider first the simplest case of a sin^le- 

 phase converter with two collector rings connected to the arma- 

 ture winding, at some two points 180 electrical degrees apart. If 

 the armature has a multiple winding each slip ring is connected 

 to the armature in as many places as there are pairs of polos. In 

 the case of a two-circuit winding each collector ring is connected 

 to the armature in one place only. 



If the machine has p poles then p times during each revolut ion 

 the direct-current brushes make a connection with the same arma- 

 ture conductors to which the slip rings are connected. At the>e 

 moments the alternating voltage is a maximum, because the d i 

 current brushes are placed in the position where the induced volt- 

 age in the armature is a maximum. Thus, with two slip rings, 

 connected 180 electrical degrees apart, the maximum value of the 

 alternating voltage is equal to the voltage on the direct-current 

 side. If the pole shoes are shaped so that the alternating voltage 

 is approximately sinusoidal, the effective value of the v< 

 between the slip rings is 1/V2-70.7 per cent of t hat between the 



1 See the author's Experimental Electrical Engineering, Vol. 2, Chapter 28. 



