  
 Example 2.18   
E=220V 
 when extra resistance is added in the armature circuit,the speed is halved   
 
 The load torque is conatant   
extra resistance in the armature circui,R=5.500000ohms 
 The load torque directly proportional to square of speed   
 
 if N is halfed, Iais one-fourthed   
extra resistance in the armature circui,R=23.500000ohms 