226 THE DIRECT-CURRENT .M<>T<>K 



These results are tabulated below : 



cil. X 



Example 53. A crane has to lift a weight of 

 half a ton from rest to a height of 20 feet. The induction 

 factor of the motor is 60 ; the velocity ratio is 5 ; the 

 frictional torque on the drum axle is 5,000 inch-pounds ; 

 the tension of the line is 500 volts ; the accelerating 

 current must not exceed 40 amperes ; the drop at full 

 speed is to be 50 volts. To find the diameter of the drum, 

 and the value of R, so that the weight may be lifted in the 

 shortest time. 



Equation 102 gives at once 50 inches as the best 

 diameter. We then find the acceleration to be 19*4 f.p.s. 



-I? T> 



per second. Full speed is given by equation n } -= ' 



since c 7 ft=50 volts, we can deduce the value of the 

 final speed ; it is 19'7 feet per second ; the time of 

 acceleration is I'Ol seconds ; the distance covered in that 

 time is ] feet ; the remainder of the distance is covered 

 in 0'51 second, making the whole time l - 52 seconds. 



The torque on the drum axle, due to the weight, is 

 28,000 inch-pounds, which added to the frictional torque 

 makes the total torque 33,000 inch-pounds; the final 

 current is 78 amperes, so that the resistance of the motor 

 has to be 0*64 ohm. 



