CHAPTER IX 

 SINGLE-PHASE MOTORS 



114. The Series Motor. It will be remembered that the 

 direction of rotation of either the direct-current shunt motor or 

 the direct -current series motor is the same invspeetive of the 

 polarity of the line voltage. If the line terminals he reversed, 

 hoth the field current and the armature current are reversed and 

 the direction of rotation remains unchanged. If such motors 

 he ^upplied with alternating current, the net torque developed 

 acts in one direction only. 



With alternating current, the shunt motor develops but little 

 torque. The high inductance of the shunt field causes the field 

 current and therefore the main flux to lag nearly 90 in time-phase 

 with rc>pect to the line voltage. The armature current cannot lag 

 the line voltage by a large angle if the motor is to operate at a 

 iable power-factor. Therefore, there will be considerable 

 phase difference between the main flux and the armature current, 

 quently, such a motor will develop but little torque per 

 ampere (see Par. 103, page 237). This particular type of alter- 

 nating-current shunt motor is therefore not practicable. 



In the series motor, the armature current and the field current 

 are in phase with each other. The main flux is practically in 

 phase with the field current. Therefore, the armature current 

 is substantially in phase with the flux, and the torque curve has 

 no negative loops (see Fig. 227. page 238). Consequently, the 

 series motor develops approximately the same torque per ampere 

 with alternating current M it dOM with direct current. Funda- 

 mentally, the sen >r has possibilities as an alternat ing- 

 cutTent motor. 



Tin- ordinary direct-current series motor does not operate 

 satisfactorily with alternating current for t he following reasons: 



(a) The alter t ml inti-fn H Jlujr .sr/x >//; /x in the 



, such as the yoke, cores, etc., causing 

 excessive heating and a lowering of efficiency. 



27'.) 



