Class 4 (4th Engineer) MET 📅 Dec 2022

Exam Question

(a) Explain the effect of changing current and its associated magnetic flux on the induced e.m.f (6)

(b) A d.c. generator gave the following O.C.C. when driven at 1000 rev/min.

Field Current (A) 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6

Armature Voltage (V) 32, 58, 78, 93, 104, 113, 120, 125

If the machine is run as a shunt generator at 1000 rev/min, the shunt-field resistance being 100Ω, find

(i) the O.C. voltage

(ii) the critical value of the shunt-field resistance,

(iii) the O.C. voltage if the speed was raised to 1100 rev/min, the field resistance being kept constant at 100Ω.

Reference Answer

### Part (a): Effect of Changing Current and Magnetic Flux on Induced E.M.F.
According to Faraday's Law of Electromagnetic Induction, the magnitude of the electromotive force (e.m.f.) induced in a conductor or coil is directly proportional to the rate of change of magnetic flux linkage with that conductor or coil.
The relationship is mathematically expressed as:
$$ e = -N \frac{d\Phi}{dt} $$

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