366 



ELECTRICAL ENGINEERING 



both coils C and F. There is a large drop of voltage across F 

 since its reactance is high, but only a very small drop across C 

 since its reactance is low due to the presence of the short-cir- 

 cuited armature winding and thus at standstill a large flux 

 passes through F and a small flux through C. 



The flux in F is in time phase with the field current; the cur- 

 rent in the armature is in phase opposition to the field current 

 and therefore reaches its maximum at the same instant as the 

 flux in F, and the torque which is proportional to their product 

 retains its sign as they reverse together. 



When the armature rotates an e.m.f. is generated between the 

 brushes by the armature conductors cutting the flux from F. 

 This e.m.f. is at every instant proportional to the product of the 

 flux and the speed and is in phase with the flux and is therefore 

 Ec F 90 degrees behind the e.m.f. 



across F. The armature now 

 acts at the primary of a trans- 

 former with the compensat- 

 ing coil as secondary and it 

 produces a flux which trans- 

 fers the speed e.m.f. to the 

 compensating coil and the 

 coil C therefore consumes a 

 large component of the im- 

 pressed e.m.f. 



F E c 



FIG. 348. E.m.f. and current in a 

 repulsion motor. 



In Fig. 348 7 is the line current which flows in the coils F and C. 

 &F is the flux in F, which is in phase with the current. 

 Ep is the component of impressed e.m.f. across the terminals 



ofF. 

 ES is the e.m.f. generated in the armature by rotation and 



transferred to the compensating coil. (Shown as EC-) 

 EC is the component of impressed e.m.f. across the terminals 

 of C; it is equal to E s ii the coil C has the same number 

 of turns as the armature, and it is in phase with it. 

 E VE F * -f Ec 2 is the constant line voltage impressed on 



the motor. 



The e.m.f. consumed by the impedance of the armature and com- 

 pensating winding is neglected. As the speed increases the e.m.f. 

 EC increases and E F decreases, the flux in the main field F decreases 

 and the current and torque decrease. 



