158 T1IK DIRECT-CURRENT MOTOR CH. VII 



proper moment of inertia of the motor shaft is then given 

 by the equation 



where n 2 is given by 



E:, // 



.(90). 



Example 41. The moving mass of a lift is perfectly 

 balanced and weighs 1,500 pounds. This weight is moved 

 by ropes wound on a rope drum 48 inches in diameter. 

 The drum is driven by a motor connected to it with 

 velocity ratio of 70, connection being made by a clutch 

 which we may suppose to act without slipping, but with a 

 certain amount of elastic giving. The induction factor 

 of the motor is 5, and the resistance 0*1 ohm. The 

 frictional current is 10 amperes, and the motor runs at 

 24*8 r.p.s. on a line having a tension of 125 volts. We 

 have to design an equipment by which we can throw 

 on the clutch and start the lift without drawing more than 

 40 amperes from the line. When the motor is drawing 

 40 amperes, the speed will be 24'2 r.p.s., hence by Equation 

 89 the moment of inertia of the motor shaft must be 0'004 

 of the moment of inertia of the main shaft. If we neglect 

 the moment of inertia of the rope drum and wheel, in 

 comparison with that of the moving weight, we have 



Wr 2 

 J 2 = ; W is here the weight of 1,500 pounds, <j is 



/ 



32 "2, and r the radius of gyration of the moving mass is 

 two feet. Hence I 9 = 186'5. The moment of inertia of the 



