134 THE DIRECT-CURRENT MOTOR CH. VI 



Rc 



this value, the efficiency is nothing, since -=/ is unity. 



Hence we see that 2 */~^L must be greater than ' 



Rc 



and ~-i must be less than unity. 

 E 



The expression ' total electrical watt input ' does not 

 here include the power used in exciting the magnets. 

 In a shunt-wound machine this can be made anything we 

 please by adjusting the number of turns. To find the 

 maximum efficiency including the C*R loss in the magnets, 

 we must add the magnetising current c in to the current 

 from the line in Equation 56, and we shall find that the 

 current for greatest efficiency is then given by 



9 / I \ / I \ ff*C\\ 



where c v is the whole current from the line including the 



magnetising current, and c x = --. The maximum efficiency 



K 



is then : 



Thus, if in Example 34 the magnetising current c m is 

 2 amperes, the maximum efficiency is 86'8 and the 

 current for this efficiency 86'6 amperes. 



If M is kept constant and the tension of the line 

 altered, the maximum efficiency and the most efficient 

 current will increase with the tension. 



Example 35. If the motor in Example 34 be placed 

 on a line of 250 volts tension, the torque losses and the 

 resistance remaining the same, the maximum efficiency 



