The armature may burn if the back e.m.f. of a D.C. motor vanishes suddenly.
Answer : The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is 0.5.
Answer : In case of D.C. shunt motors the speed is dependent on back e.m.f. only because flux is practically constant in D:C. shunt motors.
Description : What is the back EMF of a dc motor?
Answer : If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens this will make armature to take heavy current, possibly burning it.
Description : What is back EMF of synchronous motor?