254 



ALTERNATING CURRENTS 



duction of resistance in the rotor circuit. This method of speed 

 control is very similar to the armature-resistance method of 

 speed control in the direct-current motor (see Vol. I, page 339, 

 Par. 222). The lowering of the speed is accompanied by a 

 material lowering of the efficiency and by poor speed regulation. 



Slip 1.0 



FIG. 240. Effect on trfie slip-torque curve of inserting resistance in rotor circuit. 



The electrical efficiency of the rotor is equal to the ratio of actual 

 speed to synchronous speed. For example, at 25 per cent, slip, 

 the rotor efficiency is 75 per cent. That is, of the power trans- 

 mitted across the air-gap, 25 per cent, is lost as heat in the rotor 

 resistance. The other 75 per cent, is converted into mechanical 



FIG. 241. Wound rotor of 100 hp., 440-volt induction motor. 



power, although this is not all available at the pulley, because of 

 rotor friction and core losses. 



If sufficient resistance be introduced in the rotor circuit, maxi- 

 mum torque may be made to occur at standstill, as shown by 

 curve (3) Fig. 240. That is, break-down torque is obtained at 



