54 THE DIRECT-CURRENT MOT<> I ; (II. Ill 



Example 18. A motor is to be designed to work 

 a lift under the following conditions. The tension 

 of the line is 125 volts. The diameter of the rope 

 drum is 36 inches. Worm gearing is used with a 

 velocity ratio of GO. An unbalanced weight of 1,200 

 pounds has to be raised at speed of 180 feet per minute. 

 The resistance of the motor is to be 0'06 ohm. A 

 torque of 90 inch-pounds is required on the motor shaft to 

 overcome friction. Find the induction factor of the motor. 

 The frictional torque may be expressed by an equiva- 

 lent pull at the rim of the rope drum. Using equation 



2vt 

 T = we find that the friction maybe represented by a 



pull of 300 pounds, so that the value of T in the equation 

 for M is 1,500 pounds. We find the induction factor 



Td 

 to be 6'37. The current, given by c = --- is 50 



u*& .Ml' 



amperes. 



If the two shafts are free to move in a straight line, 

 while the driving wheel rolls without slipping 011 a rail 

 parallel to Al$, as shown in Fig. 14, the torque produced 

 by the motor tends to drive B in a direction parallel to the 

 rail. For the effort exerted by the motor can, as before, be 

 represented by a force of T pounds at the rim of the 

 driving wheel. Since p, the point of contact between the 

 wheel and the rail, is at rest at any instant, the moment 

 of the force T acting at p, taken about the centre of the 

 wheel, is equal to the moment of a force F acting at the 

 centre of the wheel taken about the point p ; it follows 

 that F is equal to T, in. other words, the effort exerted at 

 the centre of the driving wheel, in a direction parallel to AB, 

 is equal to the force T exerted at the rim of the wheel. 



