358 



ELECTRICAL MACHINERY 



pensating coils as in the armature. In Fig. 231 the com- 

 pensating coils are indicated by dotted lines and the main 

 field coils by full lines. 



106. Commutation. Sparkless commutation is a very 

 difficult thing to obtain on the a-c. series motor; of course, 

 in the c-c. motor commutation is the most difficult problem 

 for the designer to solve but in the a-c. motor it is a much 

 harder problem. 



The difficulties met in the c-c. motor are present in the 

 a-c. motor and, in addition, there is a transformer effect of 



Fia. 231. Sketch of the Winding of a Series Single-phase Motor, 

 Showing Compensating Winding. 



the field on the armature coils which may produce a large 

 current in those coils short circuited by the brushes. In Fig. 

 232 is shown a diagram of the field and armature coils, a 

 ring armature being shown for simplicity of representation. 

 Short-circuited Coils have Transformer E.M.F. The 

 coils marked X and Y are short circuited by the brushes 

 and these coils are threaded by the field flux, which is 

 alternating. Hence an e.m.f. will be induced in coils 

 X and Y just as though they were the secondary winding 

 of a transformer of which the field coils represent the 



