THREE-PHASE CURRENT MOTOR. 



least the author first saw it some years ago in a short paper of this 

 gentleman in the German Elektrotechnische Zeitschrift. 



67. The motor must be wound for 16 poles, which gives a synchro- 

 nous speed of 450 r. p. m. 



68. The circumferential speed of the revolving armature should not 

 exceed 7000 ft. p. m. A high speed is necessary in order to get a 

 large pole-pitch upon which, as we have seen, the leakage factor is 

 greatly dependent So high a speed as this is not always favorable 

 to a cheap, commercial design. Indeed, it may be urged that by 

 choosing so high a speed as 7000 ft. p. m. we have overstepped the 

 most favorable limit. Had I, in this work, set myself the task of de- 

 signing a cheap motor, as the market may here and there require, I 

 should have yielded to this objection from economy and adopted a 

 slower speed. It is still a much vexed question whether, in the long 

 run, the greatest commercial economy is not identical with excel- 

 lence of quality. I therefore prefer to make the motor as good as 

 possible without giving undue regard to cheapness. 



69. The motor receives a diameter of 150 cm, corresponding to a cir- 

 cumferential speed of 6960 ft. p. m. 



The pole-pitch is equal to ^7 = 29.5 cm. 

 The air-gap will be made equal to 0.15 cm. 



70. The coefficient c in formula (12) may be estimated for the shape 

 of the slots which we are going to adopt at 12 ; we then have for the 

 leakage factor according to formula (12) : 



0.15 



a = 12 . - - = 0.061 

 29.5 



71. With this leakage factor a maximum power factor of cos0o = 

 I 



= 0.89 is attainable. 



20 



72. The motor will take from the mains a current of about 54 am- 

 peres per phase at an output of 200 horse-power. 



43 



