PROBLEMS. 465 



motor specified in problem 78 ; and (&) calculate the torque in 

 pound-inches developed by the motor when its armature current 

 is 50 amperes, assuming 4> to have the same value at zero-load 

 and at full-load. Ans. (a) 6.6 ; (b) 465.3 pound-inches. 



83. A four-pole motor has a four-path armature 30 cm. in 

 diameter and 3 5 cm. long, that is to say, the armature core and 

 pole faces are 35 cm. long in a direction parallel to the shaft ; 0.7 

 of the periphery of the armature is under the four pole faces, and 

 there are 250 conductors on the surface of the armature. The 

 mean intensity of the magnetic field in the air gap is 4,500 

 gausses, and the total current flowing through the armature is 

 100 amperes, (a) Calculate the side push in dynes acting on 

 each wire under the pole faces, from this calculate the total 

 torque in dyne-centimeters and reduce the result to pound-inches. 

 () Calculate the value of <1> and the value of Z' from the above 

 data, and then calculate the torque in pound-inches by equation 

 (25). Ans. (a) 393,750 dynes and 915 pound-inches; (b) 915 

 pound-inches. 



84. Find the current which would flow through the armature 

 of the motor specified in problem 78 if the armature were to be 

 connected directly to the I lO-volt supply mains at starting. Ans. 

 1,000 amperes. 



85. The resistance of the field circuit of the shunt motor spe- 

 cified in problem 78 is doubled by adjusting a field rheostat, the 

 actual zero-load speed of the motor is observed to rise to 1,500 

 revolutions per minute, and the armature current to 3.3 amperes. 

 In what ratio has the armature flux <1> been changed by the 

 doubling of the resistance of the field circuit? Ans. I : 0.664. 



86. One terminal of a voltmeter is connected by a lead to the 

 positive brush (which is in the neutral axis) of the motor speci- 

 fied in problem 78, the other voltmeter lead is touched to the 

 commutator at an angular distance of 15 ahead of the positive 

 brush, and the voltmeter indicates 9.2 volts. What would be 

 the ideal zero-load speed of the motor if its main brushes were 

 given a forward lead of 1 5 ? Ans. 1,200.9 revolutions per minute. 



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