Marine Electrical Oral Exam Questions with Answers — Function 5

AC vs DC in one breath?

AC reverses at 50/60 Hz through impedance with power factor below 1 — travels far, transforms easily. DC holds one direction through resistance only at unity power factor — dies with distance, rules batteries and electronics.

Active vs reactive power?

In-step volts and amps work as heat and motion (active); 90°-shifted volts and amps merely magnetise and return (reactive) — motors need both, bills charge the first.

State the three laws functionally?

Ohm: current tracks voltage over resistance. Faraday: changing linked flux induces EMF at its rate. Kirchhoff: junctions sum currents to zero (charge), loops sum IR drops to EMFs (energy).

Preferential vs reverse power — whose safety?

Preferential sheds hotel load to save the bus (system safety) — galley, AC, ventilation in staged 5/10/15 s steps. Reverse power trips a motoring set at 5–15% over 0.5–3 s to save its prime mover — proved by load-shifting into the trip.

Shunt, UV and thermal trips do what?

Shunt trips on command voltage, undervoltage trips when control volts die (35–70%, reclose past 85%), thermal bends with overload heat like the cable it guards.

ABT logic and steering overload bands?

ABT feeds the emergency board from live mains, throws to the emergency set on failure, and mechanically forbids paralleling sources. Steering feeders ride 300–375% (DC) or 175–200% locked-rotor (AC) — last to trip by design.

Why 8–10× starting current, and the starter ladder?

Shorted low-resistance rotor at maximum slip gulps current within a 15% bus dip. DOL for small sets; star-delta (⅓ current and torque, interlocked changeover); tapped auto-transformer for tunable starts; soft starter ramping volts (never speed — that needs hertz).

Single phasing — signs and hunt?

Loud hum with shaking shaft, excess current, rapid heat toward burnout (running) or no start at all (stationary). Meter all three phases loaded at the terminal box — phase-to-ground ÷1.7 isolates the dead leg; pitted contacts pass meters yet fail under load.

Dead overhauled motor — your order?

Terminal bars and star/delta links, supply volts, phase-to-phase continuity with matched ohms, then megger phase-to-phase and to earth at 1 MΩ minimum — full fail-to-start drill lives in the generator-operation lesson.

Brushless excitation without brushes — how?

Stationary exciter field feeds a spinning exciter armature, shaft rectifier turns it DC into the main rotating field — residual magnetism bootstraps the loop, AVR trims the stationary current to steer it all.

Parallel a set by the book?

Meters proved, set stable a minute, volts matched, synchroscope coaxed to slow clockwise (~4 s/rev, incoming slightly fast so it takes load), breaker just before 12, load shared across governors — full lamp-method backup lives in the alternators lesson.

Transformer kVA, DC, auto vs isolation?

kVA because current heats copper regardless of power factor. No DC: steady flux induces nothing. Auto shares one tapped winding (small, cheap, unisolated); isolation separates windings for shock and noise immunity.

Synchroscope 6 vs 12? Why close at 11?

12 is matched volts, frequency and phase — the only closable instant. 6 is maximum opposition — never close. Ordered at 11 because mechanism delay walks the contacts onto 12.

Slip significance? R/L/C uses?

1–5% sag from no-load to full-load marks healthy torque response; wider means heavier pull. Resistors limit and sense, inductors smooth and block AC change, capacitors store, time, filter and start motors.

Series vs parallel banks? Quick vs trickle?

Series adds volts, parallel adds amp-hours (matched cells only). Constant-voltage quick fills fastest under controller watch; slightly-above-rated trickle holds full banks against self-leakage.

Lead-acid vs alkaline in one line each?

Lead-acid: 2 V per cell, cheap surge power, sulphates and self-discharges. Alkaline: 1.2 V per cell, holds charge for months and shrugs off neglect, costlier with more cells per volt.

Why 440 V motors, 220 V lights? 440→220 how?

440 halves current, quartering losses and copper; 220 keeps fittings and humans safe. Step-down transformer preferred, or single-phase-plus-neutral tap (~254 V, lossy, unpreferred).

Megger vs multimeter? IR procedure and limits?

500 V DC stresses what 3 V cannot — torque-balance pointer, infinity open, zero shorted. Prove meter, ground twice, probe conductors: 1 MΩ motors, (kV+1) MΩ HV (1.44 at 440 V, 12 at 11 kV).

Thermocouple in a line? Earth fault hunt?

Dissimilar junction makes microvolts with heat — K general, E loud, J to iron's Curie. Earth: alarm first, sectionalise live to one feeder, then dead-test and repair.

ICCP in a line? Emergency generator tests?

Converter current off anodes makes hull the cathode, reference cell steering. EG rhythm: weekly unloaded battery start, monthly second-mode start, monthly auto-changeover proved on load — two independent starting means, always; full procedures live in the emergency-generator lesson.

Tube light strike? Tester safety? Stroboscopic fix?

Choke kick ionises argon-mercury, phosphor turns UV white, starter drops out running. Neon testers pass micro-amps through body resistance — glow without harm, within rating only. Flicker freezing machinery dies with three-phase or high-frequency lighting.

Power factor value, benefit, improvement?

Shipboard ~0.8, best 1.0 resistive. Nearer 1 means less current for the same kW — lower I²R losses, stiffer volts. Lagging plant corrects with capacitor banks.

Generator start and on-load order?

Pre-lube 2 minutes, bar two dry turns, blow through, start to speed, prove pressures and firing — then auto-close or manual synchroscope routine just before 12 with load shared by governor.