In ship what conclusion can be drawn when the the generator frequency drops while the load remains constant?
Here’s an explanation of what it means when generator frequency drops while the load (kW) stays constant on a ship.
- Fundamental relationship: frequency = generator speed
On a ship’s AC generator (typically a 4‑pole or 6‑pole synchronous machine), frequency is directly proportional to rotor speed:
f = \frac{N \times P}{120}
where N = RPM, P = number of poles.
So a drop in frequency means the generator rotor is slowing down – the prime mover (diesel engine or steam turbine) is losing rotational speed. - Why constant load makes this significant
Electrical load (kW) is the real power demanded by the ship’s systems (pumps, fans, thrusters, lighting, etc.).
Therefore, if frequency drops with constant load, the prime mover is failing to produce the torque it previously produced – even though the load hasn’t changed. - Detailed conclusion: the prime mover is in a state of torque deficiency
The generator’s rotor is a massive flywheel. To maintain constant speed (and thus constant frequency), the engine’s mechanical torque must exactly balance the generator’s opposing electrical torque (from load).
When electrical torque is unchanged (constant load), but engine torque decreases:
The rotor decelerates → frequency falls. - Possible causes on a ship
Fuel system problems
· Clogged fuel filters (common on ships with heavy fuel oil)
· Leaking or stuck fuel injection pump plunger
· Worn injectors causing poor atomisation or dribble
· Air in the fuel system
· Incorrect fuel viscosity (temperature too low for HFO)
Governor or speed control issues
· Governor not responding to droop signal (in parallel operation)
· Incorrect droop setting causing unnecessary speed reduction
Air induction / turbocharger problems
· Dirty air filter or restricted air intake
· Turbocharger fouling (soot or salt deposits) reducing boost pressure
· Exhaust gas bypass (wastegate) stuck open
· Charge air cooler clogged (high air temperature reduces density)
Mechanical engine faults
· Low compression in one or more cylinders (worn rings, valve leakage)
· Sticking or burnt exhaust valve
· High exhaust back pressure (restricted silencer or exhaust boiler)
· Bearing seizure (early stage – increasing friction)
· Cylinder liner or piston scoring increasing frictional drag
Cooling / lubrication
· High jacket water temperature (reducing volumetric efficienc - Consequences if not corrected
· Underfrequency trips – ship’s protection systems (e.g., 47 Hz for a 60 Hz system) will trip the generator off the bus.
· Load shedding – if multiple generators run in parallel, the slower generator may reverse power or be shed.
· Damage – prolonged low‑speed operation with high fuel rack can cause turbocharger surge, exhaust overheating, or cylinder washout. - Troubleshooting steps for ship engineers
- Confirm load is truly constant – check both kW and kVAR meters.
- Check governor – observe fuel rack position. Is it increasing (attempting to compensate) or stuck?
- Check fuel system – change filters, vent air, check injection pressures.
- Check exhaust temperatures per cylinder – a cold cylinder indicates no combustion.
- Check turbocharger speed and scavenge air pressure – low boost points to air side issues.
- If in parallel operation – check load sharing: is this generator picking up less load than others? Could be governor droop mismatch.
Summary conclusion
A frequency drop with constant electrical load on a ship’s generator conclusively indicates a loss of prime mover torque – not an increase in electrical demand. The root cause lies in the engine, its fuel system, air supply, or governor. Immediate investigation is required to prevent underfrequency trips or a blackout.