Class 4 (4th Engineer) MET 📅 Dec 2023

Exam Question

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

(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 55ohms. 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)

Reference Answer

### (a) Back E.M.F. (Counter-Electromotive Force)
**1. Definition and Principle:**
Back E.M.F., also known as Counter-Electromotive Force (C.E.M.F.), is a voltage that is induced in the rotating armature conductors of a DC motor. When the armature rotates within the magnetic field produced by the field windings, the armature conductors cut the magnetic flux. According to Faraday's Law of Electromagnetic Induction, an electromotive force (e.m.f.) is induced in these conductors. By Lenz's Law, the direction of this induced e.m.f. is such that it opposes the very cause producing it. The cause is the supply voltage applied to the armature. Therefore, this induced e.m.f. is termed 'back' or 'counter' e.m.f. ($E_b$).
**2. Governing Equation:**

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