Exam Question

A 105V, 3kW d.c. shunt motor has a full-load efficiency of 82 percent. The armature and field resistances are 0.25Ω and 90Ω respectively. The full-load speed of the motor is 1000 rev/min. Neglecting armature reaction and brush drop, calculate the speed at which the motor will run at no load if the line current at no load is 3.5A. Calculate the resistance to be added to the armature circuit, in order to reduce the speed to 800 rev/min, the torque remaining constant at full-load value. (16)