COMMUTATING ALTERNATING CURRENT MOTORS 355 



of the poles and yoke would result from the excessive 

 hysteresis and eddy current losses due to the alternating 

 magnetic field through this part of the magnetic circuit; 

 and the sparking would result from certain causes taken 

 up in a succeeding paragraph. 



104. Construction. In a series c-c. motor the flux 

 through the field frame is uni-directional and constant, and 

 therefore this part of the magnetic circuit may be made 

 solid, of cast steel. It is only the armature, in which the 

 flux reverses, due to the rotation of the armature, which 

 is necessarily made of laminated iron. But in the series 

 a-c. motor the current through the field coils is alternating 

 and hence the flux through all of the magnetic circuit is 

 alternating, therefore, the entire field structure, as well as 

 the armature core, must be made of laminated iron. Be- 

 cause of this fact the series a-c. motor cannot be con- 

 structed as cheaply nor of such rigid mechanical design 

 as the c-c. motor. 



Windings. The field coils of the a-c. series motor con- 

 sist of but very few turns as compared to those of the 

 c-c. motor and it is found that the field frame is best made 

 without projecting poles, such as are used on the c-c. 

 motor. The field frame resembles very much the stator 

 of an induction motor, the field coils being imbedded in 

 slots. 



It would not be well to fill all the slots with field coils 

 as such a design would not be efficient. But it is found 

 necessary to put on the field frame an additional set of 

 coils called the compensating winding. The two sets of 

 coils (main field and compensating field) completely fill 

 the slots of the field so that it resembles very closely the 

 stator of an induction motor. The field of a series a-c. 

 motor is shown in Fig. 230; this field is completely assembled 

 and ready for the armature. 



The armature of an a-c. motor resembles that of a c-c. 

 motor very closely; the only difference is due to the manner 



