CfJ THE DIRECT-CrUKKNT .M<iT>l; dl. IV 



order to reduce the speed to of n will be given by 



(24). 



Example 23. A motor of resistance 0-3 ohm is 

 running on a line of 120 volts tension, the current being 

 40 amperes. What must be the resistance of a rheostat 

 in the circuit in order that the speed may be halved ? 

 From the equation we find at once that 1*65 ohms is 

 required, of which 0-3 ohm is in the motor itself, so that 

 the resistance of the rheostat must be 1*35 ohms. The 

 result may be checked by showing that in the first case 

 the induced tension was 108 volts and in the second ~> \ 

 volts. The current remains unaltered. 



When the load on the motor is constant, the speed may 

 be altered by changing the induction factor, the 



tension of the line and the resistance remaining the same. 



jii 



From Equation 23 we see that when .!/ = -71 = the speed 



B 



is nothing. This represents the condition when the greatest 



current that can flow from the line is only just sufficient 



/,< 



to balance the torque ; the current is then equal to . 



H 



If we differentiate Equation (23) with respect to W, 

 and equate to nothing, we shall find that the speed is a 



maximum when 3/=2 =^ , the speed then being 



j 4 /// 



f 1 



and the current - . 

 &H 



In Fig. 15 values of M are plotted parallel to the 

 horizontal axis and values of n and c parallel to the 

 vertical axis. The speed is nothing at a. From a to b 



