Diesel Generator Starting Procedure & Alternator On-Load | Marine Guide
How a diesel set goes from dead iron to sharing bus load, what slip says about motors, and why breakers refuse fools.
Key Principles at a Glance 6 points
- Start proves fluids, freedoms and barring first: 2-minute pre-lube, two dry revolutions, blow-through, then start with cocks shut.
- On-load auto closes and shares by itself; manual earns its breaker at slow clockwise drift just before 12, then shares by governor.
- Slip is the working margin: no-load to full-load sag of 1–5% only — wider slip means heavier torque, and zero slip means synchronous.
- An idle breaker refuses to close without synchronism (undervoltage interlock), and 6 o’clock means out while 11 anticipates breaker delay to land on 12.
- On blackout the tie breaker opens, the emergency set starts itself and feeds the emergency board alone — the link interlock forbids paralleling main and emergency sources; full test procedures live in the emergency-generator lesson.
- A dead overhauled motor is hunted in order: terminal bars and links, supply volts, phase continuity with matched ohms, then insulation phase-to-phase and to earth.
1. Local Start That Respects the Iron
Idea in one line: the engine must prove oil, freedom and breath before fuel — every step here buys evidence that starting will not break iron.
Local control set — proved in three checks:
- Fuel, jacket-water and cooling systems lined up.
- Sump, governor and alternator bearing levels checked.
- Starting-air receiver pressure proved.
Pre-lube pump to auto with pressure proved for at least 2 minutes — bearings wet before they turn; check pressure drops across lube-oil filters.
Bar at least one full turn (two with ratchet spanner through the starter pinion, cocks open) — water, oil or fuel in a cylinder would hydraulic-lock it. Remove the turning gear before going further.
Blow through from local, shut cocks, LOCAL selected, START to rated speed — then leaks, firing on all units, lube-oil and fuel pressures and temperatures proved before changeover to remote.
2. Taking Load — Auto and Manual
Idea in one line: auto trusts the panel to close and share, manual makes the operator earn the breaker at the one instant two live sources agree.
Auto — panel decides
Mode selector to AUTO, ACB control to close — breaker shuts and load shares itself across governors.
Manual — operator earns it
Selector to MANU, synchroscope to the incoming set, governor trimmed to slow clockwise drift, breaker ordered just before 12, load balanced by hand.
Trim for incoming slightly fast (~4 s/rev drift) so the set takes load on closing — a slow incoming set would motorise and add load to the bus instead. Then synchroscope off and load balanced by hand across governors. Lamp backup (top-dark, two-bright) and the full four-condition ritual live in the alternators lesson.
3. Slip, Sync and Skewed Bars
Idea in one line: an induction rotor must always trail the field — that lag is the only thing letting its bars cut flux and make torque.
Slip is synchronous speed minus rotor speed over synchronous speed — the 1–5% sag between no-load and full-load that lets rotor bars cut flux harder as torque demand rises. Near-constant speed is why induction motors rule auxiliaries. Synchronous machines lock rotors to field speed (constant speed all loads, needs DC excitation, can correct power factor); asynchronous (induction) ones always trail it — no trail, no induction, no torque.
Rotor bars short through end rings so induced current has a circulating path; skewing staggers each bar's field-cutting so torque flows continuously instead of jerking bar-by-bar — while defeating magnetic cogging lock between stator and rotor slots and silencing running hum.
4. Breakers and Clock Faces
Idea in one line: the breaker is smarter than the finger on its button — it refuses any closure that would parallel two disagreeing sources.
ACB (air circuit breaker) rules generator duty at 400 V-plus currents. Stab CLOSE on an idle, unsynchronised set and nothing happens — the undervoltage interlock bars closure without control volts (reclose only past ~85%), guarding the bus against blind paralleling and riding through voltage loss.
5. Emergency Changeover — Tie Breaker, Link Interlock, ESB Loads
Idea in one line: on blackout the emergency board divorces the main bus and marries its own generator — and the interlock makes bigamy mechanically impossible.
Normally the auto-bus-transfer (tie) breaker feeds the emergency switchboard from the live main bus. On falling mains voltage or frequency a start-up relay fires the emergency set from its own independent source (battery or air/hydraulic), the tie breaker opens, and the emergency breaker closes onto the emergency board alone. On restoration the relay drops out, the set goes off-load, and the operator returns the selector to manual and stops it. The interlock forbids paralleling main with emergency or shore with emergency — never both breakers at once.
| Needs its own supply | Why it sits on the ESB |
|---|---|
| Emergency lighting, alarms, communications | Evacuation and muster need light and orders with mains dead |
| Watertight doors | Subdivision must still close on blackout |
| Steering gear motor | Second widely separated supply; sequential restart within seconds |
| Emergency fire pump, bilge pump | Fire and flooding do not wait for the bus |
| Emergency air-start compressor | Recharges the second starting means |
Test rhythm (full drill in the emergency-generator lesson): weekly unloaded battery start, monthly second-mode (hydraulic/air) start, monthly automatic start proved on load — fuel kept full, coolant and stored-energy sources kept ready. On power return, essential kit like steering restarts itself in sequence rather than all at once.
6. Fail-to-Start Drill — the Overhauled Motor That Sits Dead
Idea in one line: hunt from the terminals inward — connections, supply, windings, insulation — and never skip a layer.
Terminal box: connection bars sound, star/delta links match the design — never run a delta motor in star on load (√3 overcurrent per phase, burnout unless the overload relay saves it).
Supply volts proved at the terminals; no power, blown fuse, tripped overload, burnt contactor coil, bad contacts, faulty control relay, open stop circuit — each gets a meter, not a guess.
Phase-to-phase continuity (U–V, V–W, W–U) with matched ohms — open bars show here; direction of rotation confirmed before coupling to load.
Megger phase-to-phase and phase-to-earth at 500 V: 1 MΩ minimum on motors — run it or rewind it on the evidence, and log the reading.
Do not confuse the victims: open circuit refuses to start (or burns the two healthy phases if one leg opens running); short circuit gulps fault current; earth fault leaks to hull. Each has its own meter proof — continuity, volts, insulation.