How a marine generator works is something I taught to students at South Shields Marine School many times.
The photo is of a marine generator from an old ship.
The end has been removed to allow easy access, and for demonstration and test purposes.
The marine generator in the photo was original attached to the ‘Prime Mover’ (ships engine) by a coupling at the other side of the generator.
The coupling is connected to a shaft which goes into the generator casing.
Inside the generator casing the shaft is connected to a Rotor.
Attached to the Rotor are electromagnetic Poles.
The Poles are supplied with DC (Direct Current) electricity, and act as electro-magnets.
Theory states that electricity can be generated by moving a magnet through a coil of wire.
This is why the Poles attached to the rotor, are turned into electro magnets.
As the rotor, and hence the poles rotate, they are surrounded by large coils of wire.
The large coils of wire that surround the poles is called the Stator.
The Stator coil in a marine generator, consists of three sets of copper wire coils.
There are three sets because the generator is a three-phase generator.
The three coils are connected in a star configuration as shown on the screen.
Each of the phase connections, which I have labelled ‘phase 1’, ‘phase 2’, ‘phase 3’, are connected to the generator ‘Bus Bar’.
The Bus Bar is the output connection from the generator, which connects to the ships electrical system.
Generator Exciter
I mentioned earlier that the poles which are attached to the generators rotor, are supplied with DC (Direct Current).
The device that generates the DC voltage is called an Exciter.
The Exciter is attached to the same rotating shaft as the main generator (which is driven by the Prime Mover).
The difference with the Exciter compared with the main generator, is that the poles are fixed & do not rotate with the rotor.
Instead the rotor, which contains coils of wire, rotates between the poles.
Therefore like the main generator, the exciter produces electricity.
The poles in the Exciter differ slightly from those in the generator.
The difference is that they retain magnetism, even when the generator is not being used.
Without this residual magnetism, the generator would not be able to start.
This is because there would be no magnetic field for the coil of wire (in the stator) to move through.
Therefore no electricity generated.
Just like the main generator, the Exciter produces AC, or Alternating Current.
Therefore to produce the DC needed to supply the generator poles, the AC needs to be connected to DC.
This is done using a rectifier circuit, which is incorporated into the Exciter.
A rectifier circuit uses diodes to chop off half of the alternating current, so that only DC is produced at the rectifier circuits output.
This DC is then fed via wires, into the Poles of the main generator, creating magnetism in the Poles.
If we didn’t change the original AC produced by the Exciter, into DC, then there would not be a stable magnetic field produced in the generator Poles.
Fault Finding
If the generator has been idle for a period of time, and you try to start it, it may not work.
This is due to the loss of magnetism in the Exciter Poles.
The Poles are designed to maintain a residual magnetism, even when the generator is off.
This magnetism can however ‘leak away’.
This happens over a period of time, due to the fact that the Exciter is encased in a metal casing, which can absorb the magnetism.
If the generator will not start, and it has not been used for a while, this could be the generator starting problem.
The solution is to put the lost magnetism, back into the Exciter Poles.
This is done by what is known as ‘field flashing’.
You can field flash the Exciter Poles by attaching a battery to the Poles wiring connections, for a short period of time.
This will re-magnetise the Poles, and hopefully allow the generator to start.
Generator Maintenance Testing
A marine generator is both mechanical & electrical.
Mechanical Checks
Include bearing lubrication, and wear measurements, using Feeler Guages.
Electrical checks are mainly focused on the continuity & Insulation resistance values of the generator Stator.
Continuity Checks
As previously stated the three coil windings in a marine generator Stator are connected at one end, to form a Star connection.
Continuity checks test that the coils are not broken, and have a low electrical resistance, from one end of the coil to the other end.
The only slight problem you may face is that the ‘Star Point’, which is the point at which the three coils are connected together, is not accessible, on your generator.
This is because the Star Point is often buried in the Stator windings.
If this is the case, what you need to do is measure the continuity through two sets of windings at a time.
This is done via the three Bus Bars, using a low range Ohmmeter.
The resistance should be low, and very similar, between the different coil combinations tested.
Insulation Resistance Checks
The three separate coils of wire in the three-phase generator Stator should have a high resistance between them.
If there was no or little resistance between the coils, then a short circuit would occur, and the generator would not run.
An insulation resistance meter tests the windings resistance under realistic working conditions, by supplying a high voltage to the coils.
For a 440 Volt marine generator, you would normally set the insulation meter to double its normal operating voltage.
Insulation testers typically offer 250, 500 & 1000 Volts ranges.
Therefore for a 440 Volt marine generator you would test at 1000 Volts.
If you are regularly testing, you may wish to reduce the meter setting to 500 Volts, so not to unduly put stress on the Stator winding’s.
The minimum insulation resistance figure under SOLAS regulations is 0.5 Mega Ohms.
Though really you would not want to see anything below 2 Mega Ohms in a healthy marine generator Stator.