292 THE DIRECT-CURRENT MOTOR 



inch-pounds. Find the most efficient current and the 

 efficiency for that current. 116 amperes and 85 per cent. 



84. The armature of an eight-pole motor contains 646,000 

 cubic centimetres of iron. If the hysteresis loss is 8,600 ergs 

 per cubic centimetre per cycle, find the current required to 

 turn the armature against the hysteresis torque, the induction 

 factor being 68. 38'2 amperes. 



85. A train weighing 40 tons is driven by two gearless 

 motors designed to run at 20 miles an hour in parallel on a 

 tension of 500 volts when the resistance to motion is 495 

 inch-pounds of torque per ton. The driving wheels are 

 88 inches in diameter. The resistance of the motors is 

 0-8 ohm each. The maximum current per motor is to be 

 200 amperes. If the acceleration is uniform up to full speed, 

 how many seconds will be spent in covering the first 200 

 yards from rest ? Assume M to be constant and the parallel 

 method of control. 31*5 seconds. 



36. A train weighing 600 tons is drawn by four gearless 

 motors at 10 miles an hour up a grade of 0'7 per cent., the 

 frictional resistance being 12 Ibs. per ton and the tension of 

 the line 500 volts. Find the total current required to start 

 up on the grade with an acceleration of 0-5 f.p.s. per second, 

 if the motors are connected permanently in series and M is 

 constant. 1,845 amperes. 



87. A motor has M 8'1, II = 2-4 ohms. The armature, 

 with an attached fly-wheel, weighs 1,500 Ibs., the radius of 

 gyration being 9'2 inches. The frictional and other resistance 

 to motion amounts to 280 inch-pounds, and is constant. 

 The motor is switched on to a line of 100 volts tension and 

 left to run. How many revolutions will it make in two 

 minutes ? 365. 



38. A motor is running on a line of constant tension with 

 a constant load. The speed has to be varied by varying M 

 with a rheostat in the magnet circuit, which is a shunt on 

 the main circuit. Show that the change of speed for any 



