Class 4 (4th Engineer) MET 📅 Apr 2023

Exam Question

(a) Explain what is meant by the back e.m.f. of a motor. (6)

(b) A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2 Ω. The machine has 6 poles and the armature is lap connected with 864 conductors. The flux per pole is 0.05 wb. Calculate (10)

(a) speed,

(b) the gross torque developed by the armature.

Reference Answer

### (a) Explanation of Back E.M.F. (6 Marks)
**1. Definition and Principle:**
Back Electro-Motive Force (Back E.M.F. or $$E_b$$) is the voltage induced in the armature conductors of a DC motor as they rotate and cut the magnetic field lines produced by the field poles. This phenomenon is a direct application of Faraday's Law of Electromagnetic Induction, which states that an E.M.F. is induced in any conductor that cuts magnetic flux.
**2. Opposition to Supply Voltage (Lenz's Law):**

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