176 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



absolutely at the neutral point, and, consequently, only the direct 

 flux is cut by the armature conductors; 



2. The alternating current E.M.F. is affected only to a slight 

 extent, because the field resulting from the active current I w , and 

 from a direct current /2, which is 10 to 15 per cent weaker, consti- 

 tutes only 10 to 15 per dent of the transverse reaction field of the 

 direct-current armature, which field, as a rule, would not exceed half 

 the direct field. But, even assuming that this resultant transverse 

 field may amount to 20 per cent of the direct field, the alternating 

 E.M.F. would be thereby increased only in the ratio, 



\/I+(0.2o) 2 



=1.02, 



or, only about two per cent, which is negligible. We are therefore 

 warranted in treating the rotary converter as a synchronous motor 

 having no transverse reaction, as has been done by other authors. 

 There might possibly be an exception in the case of rotary converters 

 having no field-excitation; but these are used very little, and their 

 theory, in any case, can be established much more easily by con- 

 sidering them as transformers. 



As in the case of every alternator, the armature, in a rotary con- 

 verter, is subject to magnetic leakage, that is to say a portion of the 

 magnetic flux produced by the alternating currents in the armature 

 winding is shunted by the air or by the armature-teeth. This leakage 

 is offset in part by the leakage which the secondary current tends to 

 produce, in the contrary direction (and which offsets, especially, the 

 effect of the active current); hence, the resultant leakage is much 

 smaller than in a Synchronous motor. Nevertheless, in order to 

 make the theory general, we will suppose that this magnetic leakage 

 exists; and it will be represented, as usual, by an inductance, s, giving 

 rise to a reactance, a>s. 



Factors Determining the Practical Conditions of Operations. 

 It is very important to note that the operation of rotary converters 

 does not depend on the characteristics of the machine only, but also 

 depends on the constants of the electric supply-circuit. It is, in 

 fact, the self-induction of the supply-circuit and the difference between 

 its E.M.F. and the E.M.F. induced in the converter, which govern 

 the ratio of reactive to active currents. 



Every active current lagging behind the E.M.F. produces a 

 demagnetizing effect, whereas every reactive current leading the 



