82 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



a current of 610 amperes when the impressed voltage is 500 volts. 

 Assume that the leakage reactances are in the same ratio as the 

 effective resistances at the rated frequency. 



What are the slip, the power factor and the efficiency when the 

 motor delivers a torque of 1950 pound-feet? Assume that the 

 voltage generated in the stator winding by the air-gap flux is the 

 same as at no load. 



48. l At no load when the line voltage has its rated value of 1900 

 volts, a 3-phase, 570-h.p. induction motor takes a line current of 

 57.2 ampers at a power factor of 0.091. The speed of the rotor is 

 74.6 rev. per min., and the slip is one revolution in 23 minutes. 

 The ohmic resistances of the stator and rotor winding are 0.488 

 ohm and 0.0138 ohm per phase. The motor is wound for 36 

 poles and both the stator and the rotor windings are connected in 

 delta, and have a ratio of transformation of 6 to 1. With the 

 rotor blocked the line current is 200 amperes and the power sup- 

 plied is 59.2 kw. when the impressed voltage is 600 volts. Assume 

 that the ratios of effective to ohmic resistance are the same 

 for the stator and rotor, and that the ratios of leakage reactance 

 to effective resistance are also the same for each winding. 



What is the friction and windage loss? What is the core loss? 



49. l A 3-phase, 2000-volt, 60-cycle induction motor is rated 

 to deliver 500 h.p. at full load. The ohmic resistances of the 

 stator and rotor windings are respectively 0.24 ohm and 0.0153 

 ohm per phase, and the ratios of effective to ohmic resistance are 

 respectively 1.5 and 1.6. With the rotor blocked the power 

 factor is 0.29. Assume* that the ratios of leakage reactance to 

 effective resistance at the rated frequency are the same for the 

 stator and rotor. The motor is wound for 44 poles, and both 

 the stator and rotor windings are connected in Y and have a 

 ratio of transformation of 16 to 5. The friction and windage loss 

 is 11.0 kw. and the core loss is 2470 watts at the rated voltage. 



With the impressed voltage adjusted so that the voltage gen- 

 erated in the stator winding by the air gap flux is 1120 volts per 

 phase, what is the slip in revolutions per minute at no load? 

 What are the impressed voltage, the current and the power for 

 this condition? 



50. 1 The full-load capacity of a 3-phase, 980- volt, 20-cycle 

 induction motor is 200 h.p. The effective resistance and leakage 



^se either the transformer diagram or the equivalent circuit in the 

 solution of this problem. 



