What is an inverter?

[Nowhere-Man]

Could be.  I haven't seen "dynamo" used here in decades.

The alternator on my Lister SR2 air cooled diesel boat engine is called a "dynamo" in the service manual. It's a Lucas.

A few weeks ago I was getting a bit frustrated trying to find a proper explanation of the so-called "dummy light" or "idiot light". I went through numerous automotive explanations and was left baffled because all were stating that "12 volts each side" of the light causes it to go out. Made no sense to me at all.

So, on the subject of charging and generating, here is how it works:

Inside the alternator there's a fixed delta winding (3 coils called the stator).

The rotor (a spinning coil) is an electromagnet and works just like the old HT chokes in vintage radio that smoothed the A.C. ripple while making a magnet around the speaker. With the rotor you need slip rings and carbon brushes so the AC output can be taken to the rectifier.

Here's the important bit: When ignition is turned the rotor field coil is energised by the 12 volt battery and thus becomes an electromagnet. At this point the lamp is wired between battery + and a negative potential terminal called D+ on the alternator. Therefore it's at 12 volts and lights up.

When the engine too is turned over the energised rotor spins between the stator windings and the electric field is produced. At this point mechanical generation makes the battery 12 volt supply to the rotor redundant and the field is now generated by the alternator itself. The D+ then becomes positive and with no opposite potential across the lamp, it simply goes out. Hence, dummy lamp - the alternator is charging.

Most people will already know the 3 stages of AC taken from the stator are rectified by six diodes. The D+ dummy lamp stage is rectified by 3 other diodes and provides the supply to the field rotor winding.

I don't know much about dynamos but assume you don't need a rectifier for a cycle lamp dynamo so maybe it's just an small AC generator.