CHAPTER VII 



THE MAGNETOMOTIVE FORCE OF DISTRIBUTED 



WINDINGS 



42. The M.M.F. of a Direct-current or Single-phase Dis- 

 tributed Winding. In the two preceding chapters it is shown how 

 to calculate the ampere-turns required for a given flux in an elec- 

 tric machine. When the exciting winding is concentrated, that is, 

 when all the turns per pole embrace the whole flux, the number of 

 ampere-turns is equal to the product of the actual amperes flowing 

 through the winding times the number of turns. Such is the case 

 in a transformer, in a direct-current machine, and in a synchronous 

 machine with salient poles. In some cases, however, the exciting 

 windings are distributed along the air-gap, so that only a part of the 

 flux is linked with all the turns, and the actual ampere-turns have 

 to be multiplied by a factor in order to obtain the effective m.m.f. 

 Such is the case in an induction motor, and in an alternator with 

 non-salient poles. Moreover, one has to consider the m.m.f. of 

 distributed armature windings when calculating the performance 

 of a machine under load, because the armature currents modify the 

 no-load flux. In this chapter the m.m.fs. of distributed windings 

 are treated mainly in application to the performance of the induc- 

 tion motor; in particular, to the calculation of the no-load current 

 and the reaction of the secondary currents. The armature reaction 

 in synchronous and in direct-current machines is analyzed in the 

 next two chapters. 



Distributed Winding for Alternator Field. A cross-section of a 

 four-pole field structure with non-salient poles for a turbo-alterna- 

 tor is shown in Fig. 33a. The flux is graded (Fig. 336) in spite of 

 a constant air-gap, because the total ampere-turns act only upon 

 the part a of a pole; two-thirds of the ampere-turns act upon the 

 parts 6, 6 and one-third upon the parts c, c. The m.m.f. and the 

 flu\ in t he parts d, d are equal to Bero. Tims, theoretically, the 

 flux density in the air-gap should vary according to a " stepped " 



U! 



