290 



ALTERNATING CURRENTS 



as to develop torque. Figure 266 (c) shows the direction of the 

 induced emfs. in the armature, neglecting the distorting effect 

 of the armature mmf. on the field flux. The emfs. in each half 

 of the armature act in conjunction, as shown in Fig. 264(6). 

 Assume for the time being that angle equals angle a, Fig. 266 

 (c). The current paths through the winding are abed and afed. 

 In path abed the emfs. E cd and E cb included in angles a and /3 

 respectively, each equal to the brush displacement angle, are 

 equal and act in opposition. Therefore, they cancel each other, 

 leaving Eai as the net emf . through path abed. Likewise in path 

 afed, the emfs. E fa and E fe cancel, leaving E ed as the net emf. 

 through this path. The net emfs., E a b and E t d are effective in 

 sending the current through the armature. 



(a) 



FIG. 266. Brush position 



in a repulsion motor which gives both current 

 and torque. 



The foregoing is not a rigorous analysis of repulsion motor 

 operation, but rather a statement of the general principles on 

 which the operation depends. A rigorous analysis involves 

 vector diagrams of considerable complexity and is beyond the 

 scope of this book. 1 



In this type of motor, the direction of rotation depends on 

 the brush position. For example, in Fig. 266 (c), the direction 

 of rotation may be reversed by moving the brushes so that they 

 cross the pol6 axis, the brush axis then making an angle ]8 with 

 the pole axis. Angle |8 must be less than 90. 



In the foregoing discussion a gramme-ring winding has been 

 considered, as it is a simple matter to follow the winding since 



!For more detailed analysis of single-phase motors see "Principles of 

 Alternating Current Machinery," by Prof. R. R. Lawrence; McGraw-Hill 

 Book Co. 



