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 !!!!!?!
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
  4. crutschow


    Mar 14, 2008
    It's quite simple, notwithstanding ts's explanation. :rolleyes:
    The SCR's short-circuit the windings (through their respective diodes) when the voltage gets too high.
    The current, under these conditions, is intentionally limited by the magnetic saturation of the iron.
    The power dissipated than equals the square of this short circuit current times the winding resistance of the coils.
    This dissipated power will heat up the coils some but it is much less than the maximum output power the generator can provide at it's normal 12v output.

    This does also generate a fair amount of heat in the SCR's and diodes, so the regulator case is typically finned to help get rid of the heat, although they can still get very hot.
    (I've had three different units fail on my motorcycle due apparently to their overheating.)

    To minimize this, some of the new regulators use MOSFETs rather than SCR's to provide the short circuit, since MOSFETs can have a fraction of a volt drop when ON as compared to the nearly 2V of an SCR.
    The MOSFETs can also do double duty as synchronous rectifiers, replacing three of the diodes, which further reduces dissipation.
    Here's a short article I wrote on such a MOSFET design.
    Last edited: Oct 25, 2016
    MrSoftware likes this.
  5. MrSoftware

    Active Member

    Oct 29, 2013
    My understanding is more high level than @crutschow, but to over simplify my understanding; shunt type regulators limit output power by basically shunting (shorting) the coils as necessary. It's very electrically noisy, wasteful and creates heat. But I'm guessing they're cheap and easy to produce and reasonably reliable, though the heat doesn't do any favors for the coils. On my personal bike, after a cold start the cover over the stator gets warm faster than the rest of the motor, I'm assuming it's due to the stator heating up.

    In this thread (click) I posted some oscilloscope screen shots from my motorcycle while kick starting, with and without a battery. You can see how noisy the power output is without the battery to clean it up. Some bikes use capacitors in place of batteries.