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
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    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.

    [​IMG]

    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?

    Rich.:)
     
  2. tedstruk

    New Member

    Oct 8, 2016
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    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
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    HUH?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    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
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  5. MrSoftware

    Active Member

    Oct 29, 2013
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    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.
     
  6. RadicalTireSkid

    New Member

    Apr 21, 2017
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    So I'm new to this forum. I'm poking around a bit trying to figure out how a stator works and what not. I have a mechanical engineering background... not electric. So anyway, I have a motorcycle that is known to burn up stators. There are mixed opinions on how to limit this on our bikes. Some people say to keep ALL electronics on to limit the heat output of the stator. Others in the thread say limiting electronic usage will help keep the stator cool. There is no data supporting either assumption, just theory.

    Based on this post, it seems like pulling more power/current from the stator reduces the short circuit current, which would reduce the power/amperage running back into the stator wires, thus reducing the heat. So I should leave many electronics on.

    Anyway.. you guys seem smart. Any thoughts? Should I keep my lights on?
     
  7. Rich:-)

    Thread Starter New Member

    Oct 14, 2016
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    I would say that the thinking of "use the power rather than shunting the power" is still the best assumption.

    I use to ride without my front light on when off tarmac (trail riding), after thinking about this more I decided I'd prefer that 55 Watts to be dissipated in my head light rather than in my stator. You can be smart about this too, like if your doing lots of low speed offroad riding (and low engine revs), then switching the headlight off might maximise battery charge, especially if your doing frequent re-starts. If your doing lots of continuous medium to high revs riding then switch everything on (where possible).:)

    Another thought on premature stator death is oil quality. Some think that premium fully synth engine oil does a better job of moving heat from the stator, or maybe just runs cooler in the engine. Either way it was noted that for my bikes engine type, most of the stator failures were from bikes running cheaper semi-synth oils, and this had burnt onto some of the stator winding thus heat insulating those coils.
     
  8. Rich:-)

    Thread Starter New Member

    Oct 14, 2016
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    So, from the above responses I think we all agree that there is no weird magnetic self balancing effects going on.
    The stator just makes as much power as it can all the time, once it goes over a predetermined voltage level the short circuit SCR for each phase is fired and then part of each sign wave gets shorted back across the stator cut the output to the bike.
     
  9. RadicalTireSkid

    New Member

    Apr 21, 2017
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    Yea, so the change in magnetic field is what's confusing me. People explain that is why electric motors heat up so much when running at low RPM. There is some kind of mag field flux that happens at lower RPMs which increases heat. I was wondering if that was somehow connected to the power demand of the stator when electronics are on/off assuming constant RPM (or maybe should not assume constant RPM?).

    Also, I've measured the AC voltage output from the stator when my lights are on/off at constant RPM. The voltage increased when I turned the lights on. I couldn't get a good amperage reading though.
     
  10. Rich:-)

    Thread Starter New Member

    Oct 14, 2016
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    I think it depends on what type of motor it is, but generally speaking if a motor is forced to run slower it will push harder & draw more current, this is not what a stator is doing btw. The stator runs at full demand all the time.

    To see what is happening with the output of a stator you really need to view it with an oscilloscope. The pulses are a mess as it is 3 phases, full wave rectified (so 6 pulses) overlaid on top of the DC from the battery, and as the revs climb they get partially chopped by the regulator.
    That is until you switch the lights on which increases the load, thus lowers the voltage output which can in turn stop the regulator from chopping the pulses.
    A DMM will try to average out what it's "seeing", which will be increased voltage output voltage from the stator.
     
  11. RadicalTireSkid

    New Member

    Apr 21, 2017
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    Yea. its basically a dirtbike stator.

    I measured the AC voltage from all 3 stator output wires. 2/3 wires increased voltage. Don't have an oscilloscope.
     
  12. MrSoftware

    Active Member

    Oct 29, 2013
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    Keep the lights on to stay alive. Look for an aftermarket stator that is more durable, or have the stocker rewound by someone who does that kind of work. These guys have been around a long time and have a good reputation in the off-road crowd, maybe they can help with your bike too:

    https://www.bajadesigns.com/products/Motorcycle-Stators.asp
     
  13. RadicalTireSkid

    New Member

    Apr 21, 2017
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    Much appreciated. I'll keep the lights on I think. I already have a spare ricky stator for the bike. My stocker still works, just have the spare for when the stocker blows out.
     
  14. ian field

    AAC Fanatic!

    Oct 27, 2012
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    The generator makes at least some pretence of being constant current.

    At low RPM there just isn't enough energy to damage anything - as RPM increases; inductive reactance becomes significant.

    Most Honda generators peak at 5000RPM, output current tails off above that.
     
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