INDUCTION MOTORS 73 



with ii winding pitch of 10 slots? Neglect the resistance and 

 leakage reactance and assume that the friction and windage loss 

 varies as the speed and that the core losses vary as B l ' 1 f 1<s . 



16. The stator of a 25-h.p., 3-phase, 50-cycle induction motor 

 is wound for 250 volts. With an impressed voltage of 110 volts 

 the starting current is 61 amperes at 0.336 power factor. The ef- 

 tVctive resistance and leakage reactance of the stator winding are 

 respectively 0.159 ohm and 0.46 ohm per phase. The stator 

 has 72 slots with 9 inductors per slot and the rotor has 120 slots 

 wit h 2 inducters per slot. Both the stator and rotor windings 

 connected in Y. The ohmic resistance of the rotor winding is 

 0.015 ohm per phase. 



What is the leakage inductance of the rotor winding per phase? 

 What is the ratio of effective to ohmic resistance for the rotor 

 winding at 50 cycles? 



If the stator is rewound for 500 volts by using twice as 

 many turns of wire of one-half the size, what voltage should 

 lc impressed to have a starting current of 30 amperes? 



17. The full-load line current taken by a 200-h.p., 3-phase, 980- 

 volt induction motor is 101 amperes. The stator has 216 slots 

 with 5 inductors per slot, and the ohmic resistance of the winding 

 is 0.246 ohm per phase. The rotor has 288 slots with 1 inductor 

 per slot and the ohmic resistance of the winding is 0.010 ohm per 

 phase. Both the stator and rotor windings are connected in delta. 

 The leakage inductances of the stator and rotor windings are 

 respectively 3.9 mil-henrys and 0.19 mil-henry per phase. At 

 the rated frequency of 50 cycles the ratios of effective to ohmic 

 resistance are respectively 1.6 and 1.9 for the stator and rotor. 



(a) What voltage should be impressed on this motor in order 

 that the starting current will lie twice the full-load current ? 



(b) If the stator winding is reconnected in V what voltage 

 should l>e impressed on the motor in order that tin- st art ing cur- 

 rent will be twice the full-load current'.' In this case tin- rated volt- 



>mes 1700 volts. 



18. At full load the stator and rotor copper loe- of a ."iOO-h.p.. 

 ii-phase, 2000-volt induction motor are respectively 1. 11 per 

 cent, and l.S'J per cent. The core loss i- 'J 17(1 watts and the fric- 

 tion and windage loss is 11.0 kw. The magnetizing component 

 of the line current U 23 amperes. At the rated frequency of 60 

 cycle* the ratio of effective resistance to ohmic resistance is 1.55 

 for the stator winding and 1.85 for the rotor winding. At the 



