INDUCTION MOTORS 75 



with full impressed voltage, the current is 3.6 times the full-load 

 value and the torque is 0.62 of the full-load torque. What will 

 be the starting current and the starting torque with full impr 

 voltage if resistances of 0.2 ohm are inserted in each phase of the 

 rotor winding? 



22. With full-load current the slip and brake torque of a 500- 

 h.p., 3-phase, 60-cycle induction motor are 1.82 per cent, and 

 16,300 pound-feet. The ohmic resistance and leakage inductance 

 of the rotor winding are 0.24 ohm and 3.1 mil-henrys per phase. 

 What will be the slip, the brake torque and the output when the 

 current has its full-load value if resistances of 2 ohms are insrrt < 1 

 in each phase of the rotor winding? Estimate the copper loss 

 in each of these additional resistance units. 



23. The resistance and leakage inductance of the rotor wind- 

 ings of a 3-phase, 38-cycle induction motor are 0.013 ohm and 

 0.27 mil-henry per phase. The full-load torque and slip are 

 2,190 Ib.-ft. and 2.6 per cent. 



(a) To what per cent, of its normal full-load value should the 

 air-gap flux be reduced so that the motor will deliver its full- 

 load torque at one-half the full-load speed? 



(b) What resistance should be inserted in each phase of the 

 rotor so that the motor will deliver its full-load torque at one- 

 half the full-load speed? Assume that the air-gap flux has 

 its normal value. 



(c) Compare the rotor currents in (a) and (b). 



24. The full-load distribution of losses in a 10-h.p., 220-volt, 

 3-phase induction motor is: 



Stator copper loss 3. 9 per rent 



Rotor copper loss 4 . 8 per ccn t , 



Core loss 3.1 per cent. 



ion and windage loss. 4.0 per cent. 



(a) What are the slip and efficiency at full load ' 



(b) What are the slip and efficiency at one-hull" of full load'.' 



25. At full-load the >lip of a335-h.p., 2000-volt, 50- \ vie. ii-pole 

 induction motor is 1.8 per cent, The ohmic resistances of (he 

 stator and rotor windings are 0.165 ohm and 0.0127 ohm P<T 

 phase, and the leakage inductances are 2.1 mil-limn- ami 0.22 

 mil-henry PT phase. Both of the windings are conn. t. ,1 in V. 



III. rtator bat ~~ slots with 10 inductors per slot, and the rotor 

 has 90 slots with 2 inductors per slot. Th- ratio ctive 



