INDUCTION MOTOR 



355 



The slip single-phase is less than two-phase since the whole 

 rotor corresponds to one phase of the stator and thus the rotor 

 current and rotor copper loss are decreased. 



The efficiency is lower because the output decreases more than 

 the losses. For a given impressed e.m.f. and frequency the iron 

 and friction losses remain practically constant. 



The power factor is lower because the magnetizing current is 

 approximately doubled. 



20 40 60 80 100 120 140 160 180 200 

 Torque in Percent of Full Load 



FIG. 336. Speed-torque characteristics of a single-phase induction 

 motor with various resistances inserted in the rotor. 



A given motor wound single-phase can be operated at higher 

 densities than when wound polyphase since the losses are less 

 and its ventilation is the same, and in this way its output may be 

 made from 65 to 75 per cent of its output polyphase. 



The torque at any speed can be increased by introducing re- 

 sistance into the rotor windings, but this changes the maximum 

 torque since the torque is proportional to 1 s. 



Fig. 336 shows typical speed-torque curves of a single-phase 

 induction motor with various external resistances connected in 

 the rotor windings. 



