INDUCTION MOTORS 71 



and leads the current in phase three by the same amount. Place 

 the beginning of phase one at the extreme left of the paper and 

 draw the zero lines to allow for a maximum ordinate of 3 in. As 

 on li nates let 1 in. equal the maximum ampere turns per pole per 

 phase, and as abscissae let 3 in. equal the pole pitch. 



(a) Plot the distribution of the magnetomotive force in the 

 air-gap at the time that the current in phase one is a maximum. 



(b) Plot the distribution of the magnetomotive force in the 

 air-gap at one-eighth of a period later than in (a). 



(c) Plot the distribution of the magnetomotive force in the 

 air-gap at a time one-quarter of a period later than in (a). 



8. At no load a 25 h.p. induction motor takes 865 watts at 

 0.18 power factor from a 250- volt, 3-phase circuit. The effective 

 resistance of the stator winding is 0.15 ohms per phase. Neg- 

 lect the leakage reactance. What would have been the no-load 

 power and power factor if the motor had been designed with 

 an air-gap which would have made the reluctance of the magnetic 

 circuit 25 per cent, greater? 



9. A 200-h.p. induction motor is designed to operate from a 

 3-phase, 980- volt, 20-cycle circuit. Under this condition the 

 no-load power and power factor are 6.62 kw. and 0.086. The 

 friction and windage loss is 4.0 kw. The stator winding is delta- 

 connected and has an effective resistance of 0.387 ohms per 

 phase. What will be the no-load power and power factor if 

 thi< motor N operated from a 3-phase, 1100-volt, 25-cycle cir- 

 cuit? Neglect the leakage reactance and assume that the 

 core losses vary as B l V 1 ' 3 



10. At no load a 570-h.p., 3-pha>e induction motor i 

 15.1 kw. at 0.081 power factor. The friction and windage 

 loss is 12.0 kw. The effective resistance of the stator winding 

 is 0.757 ohm per phase. The windings are connected in delta. 



are 8 inductors in series per slot. It t he motor is rewound 

 with the same size wire but with (i inductor- in series per 

 slot, what no-load current and po\\<r will it take from the same 

 circuit? Neglect the leakage reactance and assume that tli< 

 losses vary as B 1 ' 7 f 1 '*. 



11. Both the stators and rotor- <.t two l.")()-h.p.. :{-pha-r in- 

 duction motors are of the same design except that one i- \\ound 

 for 6 poles and the ..ther for 8 poles. Each stator has the same 

 number of >lot> with the same number of inductors per slot. 

 The six-pole motor takes 5.7 kw. from a 500 volt, 38-cycle circuit 



