328 ELECTRICAL ENGINEERING 



202. Revolving M. M. F. and Flux of the Stator. In Fig. 312 

 OX is the direction of the m.m.f. of phase 1 of the two-phase motor 



in Fig. 308 and its value at 

 any instant is 



mi = UI!Q sin (0+90) = nJ Q cos 0, 



where tti is the number of 

 turns per phase and ii = I 

 sin (6 + 90) is the current in 

 phase 1. 



y 



At the same instant the 

 FIG. 312. m.m.f. of phase 2 is 



W2 = tti/o sin 6 in direction OY, 



where i% = I Q sin 6 is the current in phase 2. 



The resultant m.m.f. of the two phases is 



m = Vrai 2 + ra 2 2 = nJ Vcos 2 + sin 2 = ni/ 



and makes an angle with the OX axis. 



The resultant m.m.f. is, therefore, constant in value, being 

 equal to the maximum m.m.f. of one phase, and it revolves at 

 synchronous speed in the anti-clockwise direction. 



This constant m.m.f. acting on a path of constant reluctance 

 produces a field of constant strength revolving with the m.m.f. 

 and, therefore, revolving at synchronous speed relative to the 

 winding of the stator. The flux linking with each phase of the 

 stator is an alternating flux which reaches its maximum when 

 the current in the phase is maximum and is therefore in phase 

 with it. 



Figs. 309(a), (&) and (c) represent respectively the winding of a 

 three-phase, two-pole stator, the currents supplied and the m.m.fs. 

 produced. Fig. 313 shows the m.m.fs. of the three phases as 

 vectors. 



The currents are 



ii = IQ cos 0, in phase 1, 



iz = IQ cos (0 120), in phase 2, 



is = IQ cos (0 240), in phase 3. 



