Module 22 • Marine Engineering Oral Preparation
A high-detail TST crib sheet for marine alternators, generator control, excitation, AVR response, synchronising, power factor, kW/kVAr/kVA sharing, PMS logic and switchboard protection.
Golden rule
Governor = Hz/kW.
AVR = volts/kVAr. Keep this separation clear in every oral answer.
Power triangle
kVA² = kW² + kVAr².
Power factor is kW divided by kVA.
Synchronising
V • f • sequence • angle.
Never close if phase sequence or angle is wrong.
Use this infographic to convert electrical theory into MCA oral answers. Start with the principle, link it to the shipboard system, then state the fault consequence and safe operating action.
Mark each section complete as you revise. Progress saves locally in this browser.
01 • 3-Phase Generation
DC excitation makes the rotor an electromagnet. As it rotates inside the stator, it cuts the stator conductors and induces AC voltage.
Three stator winding groups are physically spaced 120 electrical degrees apart, giving R-Y-B phase voltages separated by 120°.
Fleming’s right-hand rule: thumb = motion, first finger = magnetic field, middle finger = induced current.
02 • Governor vs AVR
| Controller | Acts on | Controls | Parallel operation |
|---|---|---|---|
| Governor | Prime mover fuel / speed | Frequency and active power kW | kW load sharing and droop |
| AVR | DC excitation to rotor field | Terminal voltage and reactive power kVAr | kVAr sharing and voltage matching |
| PMS | Generator start/stop/load decisions | Reserve power and automatic sequencing | Auto synchronising, load shedding and unload/stop |
“The governor is mechanical speed and kW; the AVR is magnetic field strength, voltage and kVAr. If I mix those up, the rest of the generator answer becomes wrong.”
03 • Frequency Formula
f = P × N / 120
f = frequency in Hz, P = number of poles, N = rotor speed in rpm. Example: 4-pole alternator at 1800 rpm gives 60 Hz.
When a heavy load starts, engine speed dips before the governor adds fuel. A strong oral answer links frequency recovery to governor response, fuel rack movement and prime-mover load acceptance.
04 • Excitation and AVR
Controlled DC supplied to the rotor field. More field current strengthens magnetic flux and raises induced stator EMF.
Sensing transformer measures terminal voltage; comparator compares it to reference; AVR adjusts firing/exciter output to correct the error.
A PMG gives the AVR an independent power source, so voltage can recover during heavy motor starts or large transient dips.
05 • Synchronising
Incoming generator voltage must match busbar voltage; trim with AVR.
Incoming set should be very slightly fast; trim with governor.
R-Y-B must match the busbar. Rotating lamp pattern means do not close.
Close just before 12 o’clock with synchroscope moving slowly fast.
06 • Power Factor
PF = kW / kVA
Active power does useful work. Reactive power creates magnetic fields. Apparent power is the total current burden that heats the alternator windings.
07 • Load Sharing
Increase fuel/governor setting on one machine and reduce the other. Active load follows mechanical torque and prime-mover fuel input.
Adjust AVR voltage trim/excitation. Over-excited machines carry more lagging kVAr; under-excited machines can move toward leading PF.
Heavy under-excitation weakens rotor flux. The generator can lose synchronism, slip a pole and trip or damage equipment.
“Fuel shares kW. Field shares kVAr. Current heats the alternator, so the machine is rated in kVA.”
08 • PMS and Blackout Recovery
PMS monitors reserve power. If reserve falls below limit, it starts, synchronises and closes a standby generator before allowing heavy consumers.
PMS gradually shifts kW off a surplus set, opens the breaker near zero load, runs the engine on idle to cool, then stops it.
Non-essential consumers are shed in priority order to prevent overload and preserve essential services such as steering and propulsion support.
09 • Protection and Practical Faults
| Protection / fault | What it detects | Why it matters | Oral action |
|---|---|---|---|
| Reverse power | kW flowing from busbar into generator. | Alternator motors the failed diesel engine. | Trip breaker, investigate fuel/governor/prime mover. |
| Earth fault on IT system | First insulation fault to hull. | First fault alarms; second fault can short-circuit. | Trace and clear without unnecessary blackout. |
| CT secondary open | Live current transformer with open secondary. | Can generate lethal high voltage. | Short CT links before opening circuit. |
| Differential protection | Current entering and leaving stator differs. | Internal alternator fault. | Trip immediately and do not reclose until tested. |
| Short circuit vs overload | Instant massive fault vs delayed high load. | Different trip time and protection purpose. | Identify whether fault or load management issue. |
| Switchboard fire | Live electrical fire. | Electrocution/arcing/re-ignition risk. | Isolate power before firefighting medium. |
10 • Oral Exam Matrix