Generators

How a marine alternator works

The engine turns a wound rotor inside the stator. A rotating exciter on the same shaft feeds the rotor through a rotating diode bridge, so there are no brushes. A permanent-magnet pilot exciter on many machines powers the automatic voltage regulator (AVR) independently of the output. The AVR holds terminal voltage and shares reactive load with the other sets. During a fault the machine must keep supplying short-circuit current long enough for breakers downstream to trip selectively.

Configurations

• Single-bearing machines bolt to the engine flywheel housing through an SAE adaptor. They are common on high-speed sets.
• Two-bearing machines stand on their own pedestals or end-shield bearings with a flexible coupling. Medium-speed sets and large machines use them.
• Open air-cooled machines (often IP23) draw room air. Closed machines with air-to-water coolers (IP44 or higher) suit dusty or salty engine rooms.
• Medium-voltage machines usually run with high-resistance neutral earthing, which shapes your protection scheme.

Key specifications

Standards and what they require

IEC 60034-1 sets ratings and temperature limits for rotating machines. IEC 60092-301 and IEC 61892-3 add marine and offshore requirements for generators. Class issues a product certificate after witnessed factory tests, and a used machine without one usually needs testing again before class accepts it. A machine in a hazardous area needs certification under IEC 60079 or ATEX. Repairs and rewinds of Ex machines must follow IEC 60079-19, or the Ex certificate lapses.

Failure modes and inspection

The stator winding decides most of the value.

• Insulation resistance and polarisation index per IEEE 43. IEEE 43 recommends a polarisation index of at least 2.0 for class B, F and H windings.
• Winding resistance on each phase, to find loose or cracked connections.
• Partial discharge and tan delta tests on medium-voltage stators, to show voids and contamination in the groundwall insulation.
• Visual checks of end-windings for oil, carbon dust, cracked bracing and loose slot wedges.
• Rotor insulation, exciter winding and each rotating diode and surge suppressor.
• Bearings, shaft runout, air gap at several points and the insulation of the non-drive-end bearing that blocks shaft currents.
• Cooler tubes and the leak detector on water-cooled machines.

A machine stored without heaters may read low insulation resistance until it is dried out. Never apply a high-voltage test to a damp winding.

Documentation

• Class product certificate and factory test report
• Data sheet with reactances and decrement curve, needed for your short-circuit study
• Recent insulation, polarisation index and winding resistance results
• Bearing and temperature detector records from the last service
• Rewind or repair reports, with post-repair test results
• Storage history, including heater operation

Integration and compatibility

Pole count and speed must match the engine, and the shaft height and coupling must match the base frame. Check that the AVR accepts your power management system's signals and that its droop settings match the other sets. The machine's reactances change the switchboard fault level, so redo the short-circuit and protection study. Any new engine and alternator pair needs a torsional vibration calculation approved by class. On medium-voltage boards, confirm the neutral earthing resistor and differential protection current transformers suit the new machine.

Rewinds, repairs and spares

A rewind to the original insulation class with vacuum pressure impregnation gives a sound frame a new winding life. Ask for surge comparison, high-potential and polarisation index results after the rewind. An unused surplus machine from a cancelled project usually has its factory test report, which makes class acceptance simpler. The engine that drives it is under Main diesel engines, and the board it feeds under Switchboards and power distribution.