AIR-GAP FLUX DISTRIBUTION 275 



at all times in all the armature conductors, and equation 65 

 (page 136) shows how the armature m.m.f . follows a straight-line 

 law over a zone equal to the pole pitch, this being the distance 

 between brushes referred to armature surface. In alternating- 

 current and polyphase generators the curve of armature m.m.f. 

 can no longer be represented graphically by straight lines as in 

 Fig. 53, because the value of the current will not be the same in 

 all the conductors included in the space of a pole pitch. 



Considering first the polyphase synchronous generator, and 

 assuming a sinusoidal current wave, it is an easy matter to draw 

 a curve representing the armature m.m.f. at any particular in- 

 stant of time, provided the phase displacement or position of 

 the conductors carrying the maximum current relatively to 

 center line of pole is known. If this be done for different time 

 values, a number of curves will be obtained, all consisting of 

 straight lines of varying slopes, the length of which relatively 

 to the pole pitch will depend on the number of phases for which 

 the machine is wound. The average of all these curves will be 

 a sine curve of which the position in space relatively to the 

 poles is constant, and exactly 90 electrical space degrees behind 

 the position of maximum current. 



The method of drawing the curve of armature m.m.f. for any 

 instant of time, is illustrated in Fig. 107, where the upper diagram 

 shows the distribution of m.m.f. over the armature periphery 

 of a three-phase generator at the instant when the current in 

 phase (2) has reached its maximum value. If the power factor 

 is unity (load non-inductive), the current maximum will occur 

 simultaneously with the voltage maximum, i.e., when the belt of 

 conductors is under the center of the pole face, as shown in the 

 diagram. A low power factor would cause the current to attain 

 its maximum value only after the center of the pole has travelled 

 an appreciable distance beyond the center of the belt of conduc- 

 tors, and this effect will be explained later; at present we are 

 concerned merely with the distribution, and magnitude, of the 

 armature m.m.f. The vector diagram on the right-hand side 

 of the (upper) figure shows how the value of the current in 

 phases (1) and (3), at the instant considered, will be exactly half 

 the maximum value; and the magnetizing effect of phase (1) 

 or (3) is therefore exactly half that of phase (2). The angle of 

 60 degrees between vectors representing three-phase currents 

 with a phase displacement at terminals of 120 degrees is 



