A 230 V DC series motor has an armature circuit resistance of 0.2 and a eld circuit resistance of 0.1. At rated voltage, the motor draws a line current of 40A and runs at a speed of 1000 rpm. Find the speed of the motor for a line current of 20 A at 230 V. Assume that the ux at 20 A line current is 60% of the ux at 40 A line current.