Marine generators and How They Work

Onboard AC generators can be separated into two broad categories: alternator types, which produce AC by spinning a magnet (rotor) inside a set of coils; and armature types, which spin the coils inside magnets.

Alternator-type marine generators.

Just as with an automotive alternator, a DC current is passed through a set of field windings on the rotor, creating a magnetic field that induces output in the stator windings. But unlike an alternator, when a generator is shut down, the rotor retains a degree of residual magnetism. This is sufficient to induce a low-level output in the stator when the generator is restarted. This stator output is then used to supply the field current necessary to produce full generator output.

The generator is said to be self-exciting. Paguro Marine Generators are of this type.

In operation, some alternator-type generators tap one of the stator windings for the field current, using a bridge rectifier to convert part of the AC output of the stator to the DC current required by the field windings. This current is then fed to the rotor via brushes and slip rings. Just as with an alternator, a voltage regulator is used to control the field current, and thus the generator's output voltage.

Most alternator-type generators, however, are brushless. On start-up the residual magnetism in the rotor stimulates a separate exciter winding in the stator which in turn induces AC output in a winding on the rotor. Diodes built into the rotor rectify this output to DC, which is used to power the field windings. No brushes or slip rings are neeeded to supply the field current to the rotor.

In addition to the exciter windings and the main stator windings, many brushless generators include another auxiliary winding in the stator. The output from this winding is rectified to DC and controlled by a voltage regulator. On larger generators (8.0 kW and up) this output is normally used to power and control the exciter windings. On smaller generators it will charge the generator engine's starting battery (Figure 6-3A). In the former case, failure of the auxiliary winding, its rectifier, or the regulation circuit will disable the output of the generator. In the latter case, a failure of the auxiliary winding circuit will have no effect on the main AC output: It can safely be ignored when troubleshooting the generator's AC output.

Armature type marine generators. 

Multiple coils are wound around the rotor or armature, and two or more electromagnets are mounted in a fixed position inside the generator case. The armature is spun inside these magnets, producing alternating current in the armature coils. This AC output is fed to slip rings on the end of the armature shaft, where it is picked up by spring-loaded brushes and conveyed to the boat's AC distribution panel.

The generator field windings are once again designed to retain a degree of magnetism when the generator is at rest. This is sufficient to produce a low level of output in the armature when the armature is first spun. This output is tapped for field current. Since generator output is AC, and field windings require DC, the field current must first be rectified via a bridge rectifier.

(A few generators, notably AutoGen generators, supply current to the field windings by a direct feed from the boat's batteries, thus eliminating the need for a rectifier.)

This type of generator may have two, three, or four slip rings and brushes, depending on internal configuration and power output. The simplest (small, 120-volt AC generators in the USA; 240 volts in the UK) have two slip rings and brushes: one hot, the other neutral (which is grounded to the generator frame, thus preserving the requirement to ground the neutral at the power source).