THE GONTINUOUS CURRENT MOTOR 151 



sists of conductors carrying current, in a magnetic field) 

 and this torque opposes the motion of the armature. The 

 current in the armature flows with the e.m.f. generated 

 in the moving armature. We may say, therefore, that so 

 far as mechanical power is concerned the generator is 

 absorbing energy and so far as electrical power is concerned 

 the machine is giving out energy. 



Torque Acting in a Motor. In the case of a motor the 

 armature turns in the same direction as that in which it 

 is urged by the force acting on the armature conductors; 

 the mechanical power must therefore be positive, or output. 

 When the armature revolves it must generate an e.m.f. 

 (conductors moving in a magnetic field generate an e.m.f.) 

 and the current in the motor armature flows in a direction 

 opposite to this e.m.f.; the e.m.f. generated in a motor 

 armature is therefore called a counter e.m.f . As the current 

 flows against the armature e.m.f., the electrical power of 

 a motor must be negative, i.e., input. 



30. Types of Continuous-current Motors. The classifi- 

 cation of motors is generally made according to the kind of 

 field winding they have. The three types are the shunt, 

 series, and compound. In the shunt wound machine the 

 field consists of many turns of fine wire and is connected in 

 parallel with the motor armature. The series motor has 

 a field winding consisting of a few turns of heavy wire 

 connected in series with the armature. The compound 

 motor has two sets of field coils; one of many turns of fine 

 wire in parallel with the armature and another of a few 

 turns of heavy wire in series with the armature, 

 i The series winding of a compound motor may be so 

 connected that it assists the shunt winding in magnetizing 

 the field, or it may be so connected that the m.m.fs. of the 

 two windings oppose one another. The first is called a 

 cumulative-compound motor and the second a differential- 

 compound motor; the latter type is of so little use and of 

 so little practical importance that it is seldom met in practice. 



