34 ELECTRICAL MACHINERY 



in which the armature conductors may turn. Now all 

 of the magnetic lines generated by the field coils will not 

 pass through the armature core; part of them will go by 

 various so called leakage paths and this is called the leakage 

 flux. In Fig. 11 is shown the possible distribution of the 

 leakage flux between two poles of a multipolar generator. 

 The normal path of the flux is shown by the full lines and 

 the dotted lines show some of the leakage paths. In a 

 well designed machine the proportion of leakage flux to 

 total flux is kept low; perhaps it may be 20% on a small 

 machine. 



FIG. 11. Part of a Multipolar Machine Showing Distribution of 

 Magnetic Flux. Useful flux in solid lines, leakage flux in dotted 

 lines. 



Advantage of Multipolar Frame. The reason for using 

 a multipolar, instead of a bipolar, field frame on a large 

 machine may be seen from an examination of the length 

 of the magnetic path in two machines of the same capacity, 

 one bipolar and the other multipolar. In the bipolar 

 machine a much longer path exists than in the multipolar 

 and the longer the magnetic path the more leakage there 

 will be; also a larger and more expensive field coil is required 

 for the machine having the long magnetic path. Difficulties 

 from armature reaction (explained in Chapter III) occur 



