294 THE DIRECT-CURRENT MOTOR 



44. A turret weighing 20 tons, with a radius of gyration 

 of 7 feet 6 inches, is rotated by a motor with a gear ratio of 

 HOO, the induction factor being constant and equal to 8, and 

 the resistance being 0'2 ohm. The frictional torque is 338 

 inch-pounds on the motor shaft. The tension of the line is 

 80 volts. What is the shortest time in which the turret can 

 be turned through 240 degrees if the maximum current does 

 not exceed 40 amperes ? 24-4 seconds. 



45. The bascules of the Tower Bridge each weigh 1,070 

 tons, and have a radius of gyration of 40 feet. The arc of 

 rotation is 82 degrees, and the time 90 seconds. Motion is 

 derived from a motor shaft with a velocity ratio of 360. The 

 friction may be estimated at 200 inch-pounds of torque per 

 ton on the main bearings. The motion can be stopped in 12 

 degrees in twenty seconds. If the tension of the line is 100 

 volts, and the drop at full speed 4 volts, find the value of M 

 and the maximum current, the motor being shunt wound. 



M = 64 and C = 31-4 amperes. 



46. The trains on the Central London Railway weigh 

 145 tons, and are each hauled by a locomotive equipped with 

 four gearless motors, with driving wheels 42 inches in dia- 

 meter. The tension of the line is 500 volts. The frictional 

 retardation may be taken at 7'5 Ibs. per 1,000. The drop at 

 full speed is not to exceed 5 volts, and the mechanical 

 efficiency is to average 95 per cent. The motors are started 

 two in parallel, and changed over to four in parallel for full 

 speed. The time schedule requires that 2,000 feet shall be 

 covered in 75 seconds from rest. Find the current and 

 induction factor at full speed, and the force factor required to 

 start. 



Each motor must have lf=136 for 70 amperes, and a 

 maximum force factor of 40'8 kilodynes. 



ATORY, ! 



FACULTY OF APPLIED SCI-NCE. 



Date.. 



