Thread: How to find resistance to be inserted in the field circuit?

A 220V shunt motor has Ra=0.1ohm and Rsh=75ohm which drives a constant load torque & armature current of 10A at 400rpm.If the speed is to be increased from 600rpm to 700 rpm then find resistance to be inserted in the field circuit by assumption of magnetization linear curve

The speed is inversely proportional to speed. In a running motor as I did accidentally once, if field side is opened, the motor can accelerate to very high speeds. Fortunately after some max speed, the speed reduces due to inadequate power..
Anyway, with Rsh as 75 ohms if you get 600 rpm, you require a total of 75*700/600=87.5 for speed of 700 rpm. Place a resistance of 12.5 ohms additional to 75 in field circuit.