What happens when shunt regulating a 3 phase stator?

Discussion in 'Automotive Electronics' started by Rich:-), Oct 14, 2016.

  1. Rich:-)

    Thread Starter New Member

    Oct 14, 2016
    When looking at a motorcycle power system (commonly referred to as charging system), the basic setup is to have a permanent magnet generator composed of magnets in the flywheel revolving around a stator, often 3 phase. This then feeds to a Reg/ReC (regulator & rectifier in one package) before powering the bike.

    Here is diagram of the setup.


    The simplistic understanding of what's happening here is that the stator produces the max power is can at a given revs, the load from the bike will hold the voltage level down, at some point the stator will start to output more than is used and the voltage peaks will start to get too high. This is when the regulator steps in and starts to fire the SCR's which short each phase of the stator to the other phases.

    It's easy to assume that it is just grounding each phase and thus dumping the unwanted power back into itself, turning it into heat.
    Another thought is that it is actually dumping back through the other 2 phases and thus using them a dump resistors..
    I'm not sure either of these are correct.

    I came across a post on a bike forum where it was suggested that shorting the stator back to itself has a magnetic effect, and that doing so actually prevents the stator from generating - thus no excess heat / wasted power.

    My electronics understanding is ok but I'm not so good on magnetism & generators.
    What do you think about this please?

  2. tedstruk

    New Member

    Oct 8, 2016
    First, back in the days when motorcycles were run by magnetos that didn't need a battery or regulator or rectifier, when the engine turned over it induced current which was pumped through a coil that stored up current until it fired the plug and ran the motor. A good one took about 15 strong stomps to build up enough current in the coil.
    Today, capacitors are used to store the current and some motorcycles don't have batteries anymore. Regulators were installed in vintage bikes because the engine generators made to much power that blew up the battery. A while after that, designers decided that a motorcycle that had lots of lights and accessories would require a better powerplant and installed an AC generator that required a rectifier to fire the coils(ac has to short a phase to build high voltage coil power.) The only other way to make an AC system charge a coil, is to shunt the generator directly into the coil. This burns up coils, and was proven to be to much of a hassle to do...to hard to fix when it broke down and caused alot of other problems when it failed. So, rectifiers take the AC (short phase) in one end, and run it through a set of diodes that are each.... part of a circuit cycle that grounds, increasng the phase as it runs around in there... Each time the diodes keep the ground from running directly back into the system, it gains a bit of phase and pretty soon... walla.. DC! So I guess what you want to know is whether the shunt is capable of handling the high voltage??? Well capacitors can handle the DC, but AC s generally controlled by specially made capacitors that handle a certain voltage. If you can stabilize the volts....install a fixed cap in the starting system that is the right size, and regulate the wiring volts in the system so it doesn't melt... Why not !!!!!?!