Most Japanese manufacturers call them alternators, but they also called generators which is in some sense the correct term. The alternator alternates the current, it’s as simple as that: it spins on one end of the crank, a variable based on rpm by a magnet passing a fixed point with a coil on it and generates electricity. The role of an alternator is to generate an electrical current in order to recharge your battery, so you could say it’s a vital part of your motorcycle.
With an alternator, there are other items required in order to take that alternating currency (AC power). We used to have dynamos, which were DC and therefore wan no need for such luxuries as a regulator rectifier. Instead a more basic regulator controlled the current to the battery: a much simpler system and far less effective. We now have the advent of ‘smart’ rectifiers that click in and out and work only when called upon, rather than a free-flow job.
Nowadays, when you turn your motorcycle’s ignition on, there’s fuel injection, lights, alarms and starter button all drawing current, so you need a decent input. Years ago we used to see car alternators on motorcycles – I think MV Agusta was the last motorcycle manufacturer to use a car item – but those days are gone, and it’s all about trying to reduce inertia, and ultimately give the engine less work to do.
All batteries on a motorcycle are DC – Direct Current. It’s to do with the waves, man. AC coming from your alternator is, nine times out of ten, unregulated, so it then has to go via a regulator unit, or a regulator-rectifier. So it’s controlled and converted into DC at a value that the battery can accept as a charge. The excess electricity that isn’t required is converted into heat and dispersed via the heat sink, which is why they have fins on.
Most motorcycles are three phase, so you’ve got three banks of coils and three wires coming out of the alternator. When we build a trackday or race motorcycle, we only need a two phase, as there’s no need to cater for headlights or other ancillary bits. When some uneducated folk work out of their remit and convert road legal motorcycles into race bikes, there can be catastrophic consequences.
When you’ve got each phase producing 30Volt AC across the three phases, it means you end up with 90 volt unregulated. If you’re riding your motorcycle on a normal road it’s perfectly fine, as the motorcycle is drawing current with lights and all sorts of other electric demanding components. On a race bike, you don’t need all that current, so the regulator-rectifier is trying to dump that excess electricity. This is why you see so many failures on trackdays and racing motorcycles; it’s just too much current to dump. The early Triumph Daytona 675s were renowned for regulator-rectifier issues, as they weren’t mounted properly and/or installed in a place where air could reach the unit.
If you get an alternator that’s chucking out an open current, it’s producing way too much electricity that’s not being regulated and therefore badgering the battery. Or, at worst, frying your battery alive under sweating gusset. It’s a given that you battery should be cared for but so should your alternator.
If you’re ever going to check your alternator / regulator-rectifier, you’ll need a multimeter but it’s not a job you should just have a crack at. We’re talking about dealing with electricity, albeit 12 volt electricity, but it still can kill if you’re not careful.