76 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



n-iManre to ohmic resistance at 50 cycles are 1.45 and 1.75 for 

 the stator and rotor windings. At full load the voltage gener- 

 ated in the stator winding by the mutual flux is 93 per cent, of 

 the impressed voltage. 



To what per cent, of its full-load value should the air-gap 

 flux be reduced so that the starting current will be 175 amperes? 

 What is the starting torque for this condition? What are the 

 full-load torque and rotor current? 



26. In problem 25 what resistance should be inserted in each 

 phase of the rotor winding so that when starting with full im- 

 pressed voltage the rotor current will be twice its full-load 

 value? What is the starting torque for this condition? What 

 is the running torque with full-load current? 



27. In problem 25 what reactance should be inserted in each 

 phase of the rotor winding so that when starting with full im- 

 pressed voltage the rotor current will be twice its full-load 

 value? What is the starting torque for this condition? What 

 is the running torque with full-load current? 



28. At the instant of starting on a reduced voltage of 500 volts 

 a 500-h.p., 3-phase, 60-cycle induction motor takes a line current 

 of 152 amperes at 0.31 power factor. The starting torque is 

 790 pound-feet. The motor is wound for 44 poles. The 

 ohmic resistance of the rotor winding is 0.0306 ohm measured 

 between terminals when the winding is not short-circuited. 

 The effective resistance is 1.7 times as great at the rated fre- 

 quency. The stator winding has 704 inductors per phase 

 and the rotor winding has 220 inductors per phase. The fric- 

 tion and windage loss is 11. kw. With an impressed voltage 

 of 2000 volts what brake torque would be delivered when the 

 slip is 1.8 per cent? Assume that the voltage generated in the 

 stator winding by the air-gap flux is 94 per cent, of the im- 

 pressed voltage. 



29. A 570-h.p., 3-phase induction motor is arranged so that 

 it may be connected to the line through a compensator at start- 

 ing. The motor is wound for 36 poles and a line voltage of 1900 

 volts at 22.5 cycles. When the compensator reduces the im- 

 pressed to 600 volts the line current is 200 amperes and the power 

 is 58.8 kw. at the instant of starting. The stator and rotor ohmic 

 resistances are equal when reduced to the same side and the ef- 

 fective resistances are 1.5 times as great as the ohmic. The 

 friction and windage loss at full load is 12. kw. Assume that 



