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Motorcycle charging system

  1. Oct 25, 2015 #1
    Hello, I am please to meet your acquaintances! I registered for the following problem:

    The system at hand is a motorcycle charging system with a permanent magnet alternator and a shunt rectifier. The common belief is that the regulator/rectifier (R/R) dissipates all surplus power. I think that it does not, that all induced current does not flow but instead some is stored in the stator, which would then provide some back-torque.

    I have devised an experiment for this. Run the bike at a fixed rpm. Put an ammeter on the stator output and measure the current. Now turn on additional electrical accessories. Re-measure the stator output current. If the current increases, then current is stored in the stator and the R/R is only dissipating some or a fixed amount of surplus power. If however the current out of the stator remains flat with additional system load, than the stator is always supplying 100% of its current (and power), and the rectifier is always dissipating all excess power, as is the common belief.

    Lastly something that is foggy to me: I think that the induced current is only a property of the stator physical makeup and rpm, and any control from the regulator/rectifier does not affect the induced current. If loading the electrical system alters the regulation, which alters the induced current, please tell me. If I am missing something, I suspect it may lie here.
    Last edited: Oct 25, 2015
  2. jcsd
  3. Oct 25, 2015 #2
    Let me correct some bad phrasing:

    I understand that the R/R fully loads the alternator 100% of the time, which if true would cause the induction of the full current the alternator can produce. My theory is all of this current is not allowed to flow, but some remains stored (as emf) in the stator.
  4. Aug 20, 2016 #3
  5. Aug 20, 2016 #4


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    Staff: Mentor

    Hi Circle Current. Welcome back to PF!

    You have been very patient.

    I don't know how a bike's electrics is regulated, have you tried google? But I can say that current cannot be stored (not for more than a fraction of a second, anyway, unless you have superconductors at very cold temperatures).
  6. Aug 20, 2016 #5

    jim hardy

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    Gold Member

    Without resorting to hand drawn pictures, let me try to paint a verbal one.
    First, a correction to terminology - it's not current that's induced but voltage. Current flows when it is allowed to , that is when the load accepts it.
    Second, alternators have a peculiar trait. Unlike a household wall plug they will not deliver a whole lot of current into a short circuit. They're innately self current-limiting .

    Otherwise you are on the right track with your thinking. Let's simplify it a little more as follows:

    Think of the rotor as just a permanent magnet that spins inside stator. That's exactly what it is as you well know, you can feel that when you have the primary chaincase open and can access the rotor just bring a steel tool near it...... as you have doubtless done.

    Now here's where we'll depart from conventional thinking:
    Think of the stator as an electromagnet, actually an array of them surrounding the rotor. Really it is just that, but it's not usual to describe it so.
    The array of electromagnets is wound such that current flowing in them cancels out the magnetism from the permanent magnet rotor. Somewhere around ten or twenty or thirty amps will cancel all of it.
    That's why alternators innately limit their own current.
    Designers can control maximum current by adjusting strength of rotor magnets and number of turns on stator. They size both to meet the electrical needs of the vehicle.
    Which means that at probably ten or twenty or thirty amps your alternator's terminal volts will fall to zero.
    Power (Watts) equals Volts times Amps , so at zero volts alternator power equals zero! Regardless of current.
    And that ten or twenty or thirty amps is what the alternator will deliver into a short circuit.

    >>>>Here's the key to understanding motorcycle shunt regulators:::<<<<
    So if the alternator is allowed to deliver twenty or thirty amps into a short circuit its power output drops to zero !
    That's what the shunt regulator does, it throws a dead short across the alternator stator for a small part of every revolution , causing alternator power to drop to zero for that part of every revolution.
    It measures battery voltage and adjusts the fraction of time it applies the short to keep battery voltage somewhere around 14 volts .

    So the common belief is almost right - the regulator dumps not power but current.
    By dumping current it reduces alternator power output to zero for part of every revolution,
    making alternator power average just what the vehicle needs.

    Make that sequence of thoughts work in your mind and there's no need for myths like "storing power in the stator" .

    "Dump current not power" . That's why the heatsink in your regulator is really small for the amount of power it controls (not dissipates). Compare it to this one that you'd need to dissipate full alternator power.. size of a boot.

    technical term for alternator's self current-limiting is "Armature Reaction", closely related to "Synchronous Impedance". Both are simply the stator behaving as an electromagnet that opposes the magnetism of the rotor. Try a search on those terms, maybe somebody has explained it better than i .

    old jim
    Last edited: Aug 23, 2016
  7. Aug 22, 2016 #6
    Jim pretty much covered everything, just wanted to add a quick .02 (along with a minor gripe). If you read a motorcycle manual from the old days (70's early 80's) it's very in-depth and does a good job explaining the individual component functions, but the modern ones have been simplified to little more than some basic testing info. However, some of the modern service manuals are still decent, and you can get them for free (a good example would be a 2006 GSXR 600/750 manual). The charging system section will show an IC regulator being connected in parallel with the load and three SCRs with the anode connected to the AC inputs, cathode connected to the DC- (ground, since it's a vehicle it's entirely self-contained), and the gate connected to the IC regulator. Although you're not going to find the topology for the IC chip itself, it illustrates clearly how the shunt regulator's short-circuit works; when necessary it closes the gate of the SCR and connects the AC input to ground. As Jim already explained better than I could, no potential difference=no voltage=no power.
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