290 TIIK D1KECT-CTKKEXT MOTOR 



is TO ohm. The driving wheels are 33 inches in diameter. 

 If the frictional resistances remain the same, find the speed 

 up a grade of 1 in 20. 12'7 miles an hour. 



28. A car weighing 30,000 Ibs. is driven by two geared 

 motors designed to run in parallel at 15 miles an hour on 

 a level when the frictional resistance is 600 Ibs. per motor. 

 If E = 500 volts, v = 4-78, d = 33 inches, and E = 1-5 ohm 

 per motor, find the steepest grade the car can ascend with 

 the motors in parallel. 12'4 per cent. 



24. A motor car weighing 20,000 Ibs. is driven by two 

 geared motors, connected in parallel, at 13 miles an hour on 

 a level when the total horizontal pull is 800 Ibs. per motor, 

 the tension of the line being 500 volts. If the load is in- 

 creased by that due to a grade of 1 in 10, find the speed 

 when the motors are connected (1) in parallel, (2) in series. 

 B = 1-3 ohm per motor, d = 33 inches, v = 4'78. 



10-7 and 3-3. 



25. A motor car weighing 40,000 Ibs. has to be driven by 

 two bipolar motors at 12 miles an hour up a grade of 1 in 15. 

 The frictional resistances amount to 10 Ibs. per 1,000, in- 

 cluding the motor losses. E = 500 volts, It = 1*0 ohm per 

 motor, v = 5. The poles each have a gap area of 967 square 

 centimetres. Find the magnetisation in the gap if the number 

 of surface conductors is 520. 8,000 per sq. cm. 



26. Two shunt-wound motors are mechanically coupled 

 and connected in parallel on a 500-volt line. If their in- 

 duction factors are 71 and 70, and the resistance of each is 

 - 4 ohm, find the total mechanical horse-power when one 

 motor is doing all the work. 11'7. 



27. Two shunt-wound motors are mechanically coupled 

 and connected in parallel on a tension of 140 volts. One 

 motor has an induction factor of 7'5, and a resistance of - 3 

 ohm ; the other has an induction factor of 8'7, and a 

 resistance of 0'5 ohm. Find the total torque on the shaft 

 when the motors are working at the same rate. 



5,720 inch-pounds. 



