A 220 V, 1500 rpm, 50 A, separately excited dc motor with armature resistance of 0.5 Ω Is fed from a 3-phase half-controlled rectifier. The available ac source is 440 V, 50 Hz. A star delta connected transformer is used to feed the armature so that the motor terminal voltage equals rated voltage when converter firing angle is zero. Find the firing angle in degrees when motor is running at 1200 rpm and rated torque.