DIRECT-CURRENT MACHINERY 



163 



amount depending both on the mechanical construction of the 

 machine and on the load. 



The dispersion coefficient or leakage factor is the ratio of the 

 flux through the field poles to the flux crossing the gap into the 

 armature, that is, the ratio of the total flux to the useful flux; thus 

 the leakage factor is 



v = 



v varies from about 1.1 to 1.5 depending on the construction of 

 the machine but usually has a value of from 1.15 to 1.2 at no load. 



FIGS. 128 and 129. Leakage flux. 



Under load the armature also exerts a m.m.f. which in part is 

 demagnetizing and opposes the field m.m.f. and in part is cross 

 magnetizing and increases the reluctance of the magnetic circuit. 

 (See Art. 109.) Thus under load a greater proportion of the total 

 m.m.f. of the field is required for the air gaps, teeth and armature 

 than at no load and therefore the leakage flux is increased and at 

 the same time the main flux is decreased. The leakage factor is 

 therefore greater under load than at no load. 



107. Determination of the No-load Saturation Curve of a 

 Dynamo. Fig. 130 shows the dimensions of the magnetic cir- 

 cuit of a 6-pole, 150-kilowatt, 150-volt, 280 r.p.m. generator. The 

 armature winding has 512 conductors and is multiple wound. 



The voltage generated in the armature between brushes is 



& = 





volts, 



