What to check on a Shaft Generator / PTO.
A shaft generator is driven mechanically off the main engine's shaft line, through a step-up gear or a direct power take-off (PTO), converting propulsion power into electricity instead of burning separate fuel in an auxiliary diesel generator. It only supplies useful power once the main engine is running at a steady sea speed; during manoeuvring, in port, or at reduced main engine load, the ship still needs its diesel generators, so a shaft generator is fitted alongside them, not instead of them.
What sets a shaft generator apart
A shaft generator is driven mechanically off the main engine's shaft line, through a step-up gear or a direct power take-off (PTO), converting propulsion power into electricity instead of burning separate fuel in an auxiliary diesel generator. It only supplies useful power once the main engine is running at a steady sea speed; during manoeuvring, in port, or at reduced main engine load, the ship still needs its diesel generators, so a shaft generator is fitted alongside them, not instead of them.
Main components
Power take-off and gearbox
A gearbox taps power from the main engine shaft, most commonly through a step-up gear ahead of the propeller, sized to bring the shaft's low, load-dependent RPM up to a speed the generator can use.
Generator and frequency control
Because main engine RPM varies with load and slip, a shaft generator either runs through a frequency converter that decouples generator frequency from shaft speed (a shaft generator/motor, or SGM, system), or is limited to operating only in the narrow RPM band where the main engine holds a near-constant speed.
Clutch and synchronizing gear
A clutch lets the shaft generator disengage without affecting the main engine, and a synchronizing panel matches its voltage, frequency and phase to the switchboard before it is paralleled with the running diesel generators.
Cooling and monitoring
Air- or water-cooled depending on rating, with winding temperature and vibration monitoring since the generator is mechanically coupled to a large rotating shaft line rather than isolated on its own bedplate.
Selection and sizing
- Rated output is set against the main engine's available shaft power margin at normal sea speed, not against its maximum continuous rating.
- Fixed-ratio gearing suits ships that run at a narrow, near-constant RPM; a frequency converter is needed where main engine speed varies with weather, draught or speed orders.
- Space and weight in way of the shaft line, and the length of shaft available for the PTO gearbox.
- Redundancy: the auxiliary diesel generators must still be able to carry full electrical load alone if the shaft generator is disengaged or faulty.
Regulations and class
Class rules on electrical installations require at least two independent means of generating electrical power, so a shaft generator is always counted as one source among several, never as the sole source. Protection relays for reverse power, unbalance and loss of excitation are required before paralleling with the switchboard, and the paralleling and load-sharing arrangement is proven during sea trials together with the diesel generators.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Bearing wear | Misalignment with the shaft line or vibration from the main engine | Progressive vibration, eventual insulation damage in the windings |
| Converter cooling fan failure | Dust ingress or bearing seizure in the fan motor | Frequency converter trips under load, generator drops off the board |
| Out-of-phase paralleling attempt | Faulty synchroscope or operator error | Severe mechanical and electrical shock, potential damage to generator and switchboard |
| Clutch slip | Worn friction plates or low engagement pressure | Loss of drive to the generator without an obvious alarm, output silently drops |
What to look for in a supplier
- Generator rating matched to the actual shaft power available across the ship's real operating range, not just at MCR.
- Frequency converter harmonic performance, since a poorly filtered converter can distort the whole switchboard.
- Spares availability for converter electronics, which age faster than the generator itself.
- Integration with the existing power management system for automatic load sharing and fallback to diesel generators.
Never parallel a shaft generator manually without confirming phase sequence and synchroscope behaviour first — an out-of-phase connection is one of the few faults on a switchboard that can destroy a generator in seconds.