PROBLEMS 293 



given change of M decreases as the resistance in the armature 

 circuit increases. 



39. A crane has to lift a weight of 5 tons from rest 

 through a distance of 22 feet. M is constant, and equal to 

 16'4 ; R = 1*3 ohm. The diameter of the rope drum is 

 36 inches, and v = 65. The friction amounts to 380 inch- 

 pounds of torque on the motor shaft. The tension of the 

 line is 225 volts. Find the time required to cover the given 

 distance if the maximum current is limited to 160 amperes. 



86 seconds. 



40. A weight of 2 tons has to be lifted by a motor 

 working on the principle described in p. 157. E = 120 volts, 

 E = 0-15 ohm, M = 18, v = 95, and d = 24 inches. The 

 torque due to friction is 254 inch-pounds on the motor shaft. 

 Find the weight of a fly-wheel, of radius of gyration 6 inches, 

 that must be placed on the motor shaft, so that on connecting 

 the clutch the current drawn shall not exceed 50 amperes. 



18-1 pounds. 



41. Same data as in Problem 40. Find the weight of the 

 fly-wheel, of radius of gyration equal to 6 inches, so that the 

 speed shall not fall below that at which the motor runs when 

 raising the weight at a uniform speed. 36 Ibs. 



42. A crane has to lift a weight of 20 tons through 

 15 feet in 35 seconds from rest. E = 500, v = 80, M = 62 

 and is constant. The drop at full speed is to be 20 volts. 

 The frictional resistances amount to 10 per cent, of the load. 

 Find the diameter of the chain drum and the resistance of 

 the motor. 25'4 inches and 0'228 ohm. 



43. A motor with an induction factor of 75 and an in- 

 ternal resistance of 0-02 ohm is running at full speed on a 

 500-volt line, with a torque on the shaft equal to 68,500 inch- 

 pounds. A fly-wheel weighing 1'6 ton, with radius of gyra- 

 tion 8 feet, is mounted on the motor shaft. If the tension 

 of the line drops 10 per cent., find in how many seconds the 

 speed will have dropped the same per cent. Neglect the 

 moment of inertia of the armature. 4'1 seconds. 



