(a) What is meant by the term 'back e.m.f' as applied to an electric motor? (6)

(b) A 40kW, 220V shunt motor has a full-load efficiency of 90 per cent, an armature resistance of 0.075 ohms and a shunt-field resistance of 55 ohms. When 'at starting', the starter handle is moved onto the first stud, it is desired to limit the current, through the armature to 1.5 times the value which it has when the motor is on full load. What must be the total value of the starting resistance? If, on overload, the speed falls to 90 per cent of its normal full-load value, what would be the armature current? Neglect the effect of armature reaction. (10)