A long-shunt differentially compounded DC motor is rated at 250 V, which has an armature resistance of 0.2071 ohm. The magnetization curve obtained at a speed of 1400 rpm is given in the figure. Long-shunt connection means that the shunt field is connected to the motor terminals in parallel. The resistance of the shunt and series field windings is 20 ohm and 0.15 ohm, respectively. The shunt field circuit has a variable rheostat whose value changes between 0 and 900 ohm. The number of turns in the shunt and series field windings is 1500 turns and 13 turns, respectively. What must be the value of the rheostat for an armature current of 100 A and a speed of 1000 rpm? Note that the armature reaction is expressed as FAR = 5IAR.