244 ELECTRICAL MACHINERY 



Fig. 149 shows the end connections of a four-pole turbo- 

 generator; the long end connections are evident and some 

 of the bracing may be seen. Such a machine, if it has 

 insufficient bracing at the end connections, may have its 

 coils so twisted and bent in case of a short-circuit that the 

 whole winding is ruined. 



60. Armature Reaction. A-C. Machine compared to 

 a C-C. Machine. The study of armature reaction in alter- 

 nators is more difficult than that of continuous current 

 generators. The conductors of a continuous current machine 

 carry a current of constant value during one-half a revolution 

 (180 electrical degrees) and the current has the same value 

 during the next half revolution but is reversed in direction. 

 In Chapter III it was shown that this resulted in a constant 

 magnetizing or demagnetizing effect, combined with a 

 constant cross-magnetizing effect on the field. 



Now the conductors of an alternating current armature 

 carry a current which is continually varying, so that each 

 coil by itself produces a periodic effect on the strength of the 

 field. In the case of a single -phase machine the entire 

 armature winding produces a pulsating effect on the main 

 field, while in the case of a polyphase machine the instan- 

 taneous effects of all the separate coils so combine as to 

 produce on the main field an effect which is constant in 

 magnitude and direction, the same as the continuous current 

 armature. 



Analysis of the Action of a Single-coil Alternator with a 

 Non-inductive Load. A fair idea of the action of a single- 

 coil alternator may be obtained by studying Fig. 150. A 

 two-pole machine is considered and it is supposed that the 

 current in the coil is in phase with the generated e.m.f. so that 

 the current is a maximum when the coil is in the axis of 

 the main field. This position of the coil is numbered 

 4-4' in Fig. 150. The m.m.f. of the coil is given by the 

 vector Od, at right angles to the plane of the coil. 



When the coil is in the position 1-1' the current is zero 



