SINGLK-PHASE .Uoy 285 



is much greater than that of either the armature or the com- 

 ting field. The armature cross-magnetizing flux is not 

 ' ial to the operation of the motor and as far as possible can 

 be neutralized by the compensating field ampere-turns. In fact, 

 the motor operation is improved by the reduction of this cross- 

 mairnetiziiig flux. On the other hand, the main flux is essential 

 to the operation of the motor and cannot be reduced in value 

 without reducing the torque per am pen*. Hence, it is not prac- 

 ticable to neutralize the main flux and consequently the series- 

 field reactance drop must be large, even after the turns per 

 pole, etc., have been reduced to a minimum. 



When the alternating-field flux is at its maximum value, the 

 armature conductors are cutting the maximum flux, and the 

 back emf. is therefore a maximum. When the field flux is at its 

 /em value, the back emf. is zero. Therefore, the back electro- 

 motive force is in time-phase with the flux, and practically in 

 time-phase with the current, as shown in Fig. 262. 



The terminal voltage, V, is the vector sum of the back emf., 

 E, and the IR and IX voltage drops in the series field, the com- 

 pensating field and the armature. The product of the back 

 emf., E, and the current, 7, is the power developed in the arma- 

 ture. The power at the pulley is less than this by the amount 

 of the rotational losses. The cosine of the angle is the power- 

 factor of the motor. In order to have high power-factor, the 

 reactance drops must be low and the back emf. high. The 

 reactance drops are lowest and the back emf. is hiirhe<t at li^ht 

 loads, and therefore the power-factor of the single-phase series 

 motor is highest at light loads, as shown in Fig. 263. This is 

 the reverse of the power-fad <>r relations which exist in the in- 

 duction motor and in the transformer. 



The -inirlr-phase series motor has practically the same operat- 

 liaracteri>t jes as the direct-current series motor. This is 

 illustrated in I-'jir. WM, which uive> the operat iim characteri-t ir> 

 of a typical railway motor. The tor<pie or tractive effort \ 

 Dearly afl the square* Of the current and the speed varies inversely 

 as the curreni. or nearly SO. 



inductively rnmpen^at e. 1, the motor operates satisfactorily 

 with direct current and at increased output and etlicii tic\ . 

 When the motor is operated with alternating current, the speed 



