THE l\l>rcTION MOTOR 267 



does not change in sign and changes in magnitude by only a 

 small amount, if at all, when the machine passes from motor to 

 generator action through the speeding up of its armature. 

 Therefore, the exciting current I OJ which produces the flux, re- 

 mains substantially constant in both magnitude and phase, just 

 as the exciting current of a shunt motor does not change much 

 when the machine is speeded up by mechanical means so that 

 it U'comes a generator. 



However, as the rotor speeds up, it cuts the flux in a direction 

 which is opposite to that occurring when the machine operates 

 as a motor; that is, the slip becomes negative. Therefore, the 

 induced electromotive force in the rotor conductors reverses in 

 sign, as has already been pointed out. As the rotor reactance is 

 very low at these low values of slip, the currents produced in the 

 rotor are nearly in phase with their induced electromotive forces 

 and they flow in a direction opposite to that which they had as 

 motor currents. These induced currents react on the primary in 



ame manner that the secondary current of a transformer 

 reacts on the primary. As a result, an energy current l' t , 180 

 from the motor current /, is induced in the primary. The 

 currents in the rotor are nearly in phase with the induced elec- 

 tromotive force of the rotor. The induced electromotive force 

 of the stator is in phase with the rotor electromotive force. The 

 > tat or ampere-turns, excluding the effect of the in:gneti/ing cur- 

 rent, are e<jual and opposite to the secondary or rotor amper<>- 

 turns. These stator ami >ere-t urns are represented by the \. 

 l' in Fig. 248. Since the rotor ami>ere-turns are nearly in phase 

 with the secondary-induced electromotive force, tho oppite 

 and equal primary component l' a of t lie genera tor current must 

 be nearly in phase with the primary-induced electromotive force. 

 I'., and the generator terminal voltage V. In the vector dia- 

 gram /', actually leads E by a small angle which, for simplicity. 



jlerted in Fig. 248. Therefore. /' is practically all 61 

 current. For any particular kilowatt load, corresponding to the 



v current /'.. the total generator current ifl / . tin 



sum -id /,,. / :; is a leading current, as the generator 



terminal voltage F' i- nearly in phase wit h its induced el. 

 moii- i I rater phax -mied 



by the load, therefore, but by the ueiierator r 



