72 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



at no load and the line current is 46 amperes. The friction and 

 windage loss is 3.9 kw. The effective resistance of the stator 

 winding is 0.082 ohm per phase. The effective resistance of the 

 stator winding of the 8-pole motor is 1 1 per cent, less than this. 

 The windings are connected in Y. What current and power 

 will the 8-pole motor take at no load from the same circuit? 

 Assume that the friction and windage loss varies as the speed 

 and neglect the leakage reactance. Where approximations are 

 made state what they are. 



12. At no load a 335-h.p., 3-phase induction motor takes a 

 line current of 15.5 amperes and absorbs 10.1 kw. from a 2000- 

 volt, 50-cycle circuit. The effective resistance and the leakage 

 reactance of the stator winding are respectively, 0.248 and 0.76 

 ohm per phase. The windings are connected in Y. The friction 

 and windage loss is 2.5 kw. What current and power will this 

 motor take at no load from a 2200-volt, 60-cycle circuit? As- 

 sume that the core losses vary as B 1 ' 7 / 1-3 and that the generated 

 voltage due to the air-gap flux is equal to the impressed voltage 

 less the magnetizing current multiplied by the leakage reactance. 



13. At no load a 150-h.p., 3-phase induction motor takes a 

 line current of 46 amperes and absorbs 5.65 kw. from a 500-volt, 

 38-cycle circuit. The friction and windage loss is 4.1 kw. The 

 stator has 12 slots per pole and the winding pitch is 12 slots. 

 If this motor is rewound with the same number of turns using 

 a pitch of 10 slots, what current and power will it take at no 

 load from the same circuit? Neglect the resistance and the 

 leakage reactance, and assume that the core losses vary as J5 1 7 / J 3 



14. The stator of a 150-h.p., 3-phase, 500-volt induction 

 motor has 108 slots, and in each of the following cases it is wound 

 for 6 poles with 4 inductors per slot. With a winding pitch 

 of 18 slots the core losses are 3.9 kw. and the magnetizing cur- 

 rent is 45 amperes. If a winding pitch of 14 slots is used what 

 will be the core losses and the magnetizing current at the same 

 voltage and frequency? 



15. The stator of a 25-h.p., 3-phase, 250-volt induction 

 motor has 72 slots, and in each of the following cases it is wound 

 with 8 inductors per slot. If this motor is wound for 8 poles 

 with a winding pitch of 8 slots the core losses are 590 watts, 

 the friction and windage loss is 220 watts, and the magnetizing 

 current is 11 amperes. What current and power will this motor 

 take from the same circuit at no load if it is wound for 6 poles 



