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ELECTRICAL ENGINEERING 



resistance leads since without them the large short-circuit current 

 would weaken the main field and decrease the torque. 



Fig. 344 shows the characteristic curves of a 150-horse-power, 

 single-phase series motor. The torque and speed curves are very 

 much the same shape as those of the direct-current series motor. 



100 200 800 400 500 



Amperes 



700 800 900 1000 



FIG. 344. Characteristic curves of a 25-cycle, 250-volt, 150- 

 horse-power, single-phase series motor. 



The power factor approaches unity at light load when the 

 speed is high as explained above, but at full load it is still very 

 good, reaching 90 per cent in some cases. At start and at low 

 speeds it is low because the reactance of the motor is constant. 



Efficiencies up to 85 per cent can be obtained but the motors 

 must be designed more liberally than the corresponding direct- 

 current motors and are therefore heavier and more expensive. 



On account of unsatisfactory commutation alternating-current 

 series motors are only built for voltages of 250 volts and under. 



231. Repulsion Motor. In construction the repulsion motor 

 resembles the single-phase series motor with conductive com- 

 pensation. The armature is not connected in series with the 

 field but is short circuited and receives its current by induction. 

 (Figs. 345 to 347.) 



The principal of its operation can be understood by reference 

 to Figs. 345 to 347. In Fig. 345 the armature is shown short 



