Why Do Motorcycle Charging Systems Use Shunt Regulators Instead of Open Circuit?

Hi all,

Motorcycles generally use a 3 phase alternator and a reg/rectifier. By basic understanding of this has a few holes in it. The faster the engine revs , the greater the output from the alternator and the more energy the "shunt" type regulator has to dissipate. All of this unused power is lost as heat. My question is simply , Why does it need to be shunted. Why not just open circuit the alternator?


Thanks for any response

Avi