Class 2 (2nd Engineer) MET 📅 Jul 2025

Exam Question

(a) In a.c. generators, voltage dip occurs in two stages.

(i) Sketch a voltage-time graph showing the pattern of voltage dip.

(ii) Referring to this graph, state with reasons the effect on the electrical system of a small power installation when a large load is suddenly switched.

(b) Explain EACH of the following categories of voltage control:

(i) Error operated;

(ii) Functional.

Reference Answer

(a) (i) Voltage time graph showing voltage dip:
The regulation state that the voltage must recover within 1.5 seconds. However an acceptable recovery time would be 0.5 seconds for a brushless and 0.2 seconds or less for a compounded machine.
(ii) Effect on small power installation:
When a large load is suddenly applied, the electrical system experiences a significant voltage dip. Initially, there is a sharp drop in voltage due to the high inrush current drawn by the load. This is followed by a slower decrease as the alternator's reactance and power factor affect the voltage. During this period, the alternator’s excitation system, AVR (Automatic Voltage Regulator), and prime mover governor work to restore the voltage.

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