94 ELECTRICAL MACHINERY 



be of the same size and would be placed side by side on the 

 armature shaft. As the coil moves from A to B it is 

 cutting the magnetic field and, as the field is supposed to 

 be of uniform density under the pole face, the e.m.f. 

 generated will be constant during this time. In moving 

 from B to C no flux is cut, hence no e.m.f. is generated. 

 From C to D the coil is again cutting flux, but now the 

 e.m.f. will be in the opposite direction as compared to the 

 e.m.f. when the coil was moving from A to B, because 

 the flux is in the same direction as it was before, but the 

 motion is in the opposite direction. Then, as the coil 

 moves from D to A, no e.m.f. is generated. 



IT /,:TT 



w 



Time 



FIG. 54. E.M.F. Wave Form, Without Commutator. 



If the e.m.f. wave be represented by using the position 

 of the coil for the X axis and the magnitude of the generated 

 e.m.f. for the Y axis, a curve will be obtained, such as is 

 shown in Fig. 54. Calling the position zero when the coil 

 is at A, it is evident that when the coil has revolved 180 

 the negative part of the e.m.f. begins; and, of course, when 

 the coil has rotated 360 (i.e., back to A) the cycle of events 

 begins over again. 



Such a machine gives, then, an e.m.f. which alternates 

 in direction once every time the coil moves by a pole and, 

 if an external circuit were connected to the brushes (a) 

 and (6), an alternating current of the same shape as the 

 e.m.f. wave of Fig. 54, would flow in it. 



