52 THE DIRECT-CURRENT MOTOR CH. Ill 



maximum rate of working is 25 horse-power, if I Ill- 

 resistance is reduced to 1'67 ohms, the maximum possible 

 horse-power is double. 



We must not infer that a motor would be designed to 

 work at its maximum possible output, for as we shall see 

 later on, the efficiency would then be very low ; we here 

 simply state what are the conditions under which a solution 

 of the equations for the induction factor is possible. 



For a given resistance the maximum output varies as 

 the square cf the tension of the line. Thus if the tension 

 is halved, the maximum possible horse-power is reduced 

 to one-fourth of its former value. 



If the main shaft is provided with a driving wheel of 

 diameter d inches, the whole resistance to be overcome 

 can be represented by an equivalent force of T pounds at 

 the circumference of the driving wheel, and we may then 



Td 

 substitute for t, the torque on the main shaft. 



a 



First, suppose that the two shafts are fixed in space, as 

 in Fig. 13, where A is the motor shaft, and B is the 

 main shaft. If n is the speed of the main shaft, we may 

 write 



*= 1571 nd .................. (19). 



where s is the velocity in feet per minute of a point on 

 the rim of the driving wheel. Equation 18 may then be 

 written in the form : 



-. 



V9 \ V V H-07 E 2 ) ) 



This equation is applicable to all kinds of hoisting 

 machinery. 



