Electrical Circuit Theory and Technology 6th Edition by John Bird

Exercise 5

A 400 V shunt motor runs at it's normal speed of 20 rev/s when the armature current is 100 A. The armature resistance is 0.25 . Calculate the speed, in rev/min, when the current is 50 A and a resistance of 0.40  is connected in series with the armature, the shunt field remaining constant.