RECTIFYING DEVICES 373 



Therefore, the alternating voltage of a synchronous 

 converter is equal to 61.2 % of the continuous voltage and 

 this ratio is independent of the size of the converter, etc. 



The voltage ratio for any other number of taps can be 

 found by substituting the proper angle for a/2. 



The actual ratio of voltages on a synchronous converter 

 are somewhat different from those obtained from this 

 theoretically derived formula. There is an impedance drop 

 in the armature, which, of course, varies with the load 

 and this makes the ratio vary slightly as the machine is 

 loaded. A three-phase converter having a ratio of .615 

 at no load might have a ratio of perhaps .63 or more when 

 carrying full load. If the wave of alternating e.m.f . impressed 

 is not a true sine curve of course the ratio will be different 

 from that obtained on the assumption of sine curve of 

 e.m.f. 



114. Current Forms in the Coils of a Synchronous 

 Converter. The rated capacity of a synchronous con- 

 verter is generally fixed by the safe rise in temperature, 

 the same as for any other electrical machine. Of course, 

 the heating increases with increase of load due to the in- 

 creased PR loss in the armature coils. 



Now the currents flowing in the different coils of a 

 synchronous converter armature are of very peculiar shape. 

 An alternating current is flowing into the armature at the 

 a-c. taps and a continuous current is flowing away from 

 the c-c. brushes. The actual current in any coil is the 

 difference of these two currents. 



Current Due to the C-C. Load. Consider the armature 

 shown in Fig. 241, connected to a commutator and supply- 

 ing a c-c. load, i.e., acting merely as a c-c. generator. The 

 currents in the different coils would be rectangular waves, 

 as shown in Fig. 245. If the load current were 10 amperes, 

 the current in each coil would be 5 amperes because the arma- 

 ture has two paths. 



As each coil moves under a brush its current is changed 



