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ELECTRICAL MACHINERY 



76; in case the machine is a motor the twist of the main 

 field is in the opposite direction. The twist shown in Fig. 

 76 would be produced by a motor running in the direction 

 opposite to that indicated by the arrow. 



Distortion Proportional to Load. As this twisting action 

 is produced by the m.m.f. of the armature windings, it 

 must be proportional to the current flowing through the 

 armature, i.e., to the load on the machine. At no load 

 there is no current in the armature windings and hence no 

 armature reaction. The twisting effect is a maximum when 

 the machine is carrying full load or overload. 



FIG. 76. Actual Field through Armature is Result of both Field 

 m.m.f. and Armature m.m.f. 



Effect of Distortion on Commutating Plane. The most 

 important effect of this armature reaction is the shift it 

 produces in the commutating plane. If no armature reac- 

 tion were present, the coil which is short circuited by the 

 brushes should be in the plane AB, Fig. 76 (provided that 

 the necessity of moving the short-circuited coil under 

 the pole fringe for e.m.f. commutation is temporarily 

 neglected). Now the coil in the plane AB, Fig. 76, lies in 

 a magnetic field due to the distortion produced by the 

 armature reaction and so has an e.m.f. induced in it. This 



