By: Abu Hafss
A motorcycle's charging system is different from that on cars. The voltage alternator or generator on cars are electro-magnet type which are quite easy to regulate. Whereas, the generators on motorcycles are permanent magnet type. The voltage output of an alternator is directly proportional to the RPM i.e. at high RPM the alternator will produce high voltages more than 50V hence, a regulator becomes essential to protect the entire electrical system and the battery too.
Some small bikes and 3-wheelers which do not run at high speeds, only have 6 diodes (D6-D11) to perform full-wave rectification. They don't need regulation but those diodes are high ampere rated and dissipate a lot of heat during operation.
In bikes with proper regulated charging systems, normally shunt-type regulation is used. This is done by shorting out the alternator's windings for one cycle of the AC waveform. An SCR or sometimes a transistor is used as shunting device in each phase.
For the rectifier, there are three options:
a) Six automotive diodes
b) One 3-phase rectifier
c) Two bridge rectifiers
All must be rated at least 15A and heat-sinked.
The automotive diodes are two types positive body or negative body hence, should be used accordingly. But they might be little difficult to contact to heat-sink.
If using two bridge rectifiers, they may be used as shown.