170 



ELECTRICAL ENGINEERING 



If there is no residual magnetism, the e.m.f. cannot build up 

 until power is supplied to the winding from some outside source to 

 start the flux. 



120 



8000 16000 24000 32000 



Ampere Turns per Pair of Poles 



FIG. 135. No-load saturation curve. 



109. Distribution of Magnetic Flux. Fig. 136 shows approx- 

 imately the distribution of flux in a two-pole machine with the 

 fields excited but without current in the armature winding. Curve 

 1 shows the rn.m.f. acting at each point of the armature circum- 

 ference; under the north pole it is positive and has a constant 

 value; under the south pole it is negative but of the same mag- 

 nitude; beyond the pole tips its value may be represented by the 

 straight line which passes through zero midway between the poles. 

 The m.m.f. is expressed in ampere turns and is denoted by l\ff. 



The flux density produced by this m.m.f. is at every point di- 

 rectly proportional to the m.m.f. and is inversely proportional to 

 the reluctance of the patl:. It is of constant value over the pole 



