A 15 hp, 230-V, 1750 rpm shunt motor with draws 56.2A when operating at rated conditions. The motor parameters are Ra=0.28 ohm and R 137ohm. Determine: a) The rated torque b) Armature current at locked rotor if no starting resistance is used c) d) The external resistance required in the armature circuit that would limit the current and developed 200% rated torque when starting. Assuming constant flux Assuming the system voltage drops to 215 V, determine the locked-rotor torque using the external resistance in c)